# -*- coding: utf-8 -*-
r"""
Isogenies

An isogeny `\varphi: E_1\to E_2` between two elliptic curves `E_1` and
`E_2` is a morphism of curves that sends the origin of `E_1` to the
origin of `E_2`. Such a morphism is automatically a morphism of group
schemes and the kernel is a finite subgroup scheme of `E_1`.  Such a
subscheme can either be given by a list of generators, which have to
be torsion points, or by a polynomial in the coordinate `x` of the
Weierstrass equation of `E_1`.

The usual way to create and work with isogenies is illustrated with
the following example::

    sage: k = GF(11)
    sage: E = EllipticCurve(k,[1,1])
    sage: Q = E(6,5)
    sage: phi = E.isogeny(Q)
    sage: phi
    Isogeny of degree 7 from Elliptic Curve defined by y^2 = x^3 + x + 1 over Finite Field of size 11 to Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 11
    sage: P = E(4,5)
    sage: phi(P)
    (10 : 0 : 1)
    sage: phi.codomain()
    Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 11
    sage: phi.rational_maps()
    ((x^7 + 4*x^6 - 3*x^5 - 2*x^4 - 3*x^3 + 3*x^2 + x - 2)/(x^6 + 4*x^5 - 4*x^4 - 5*x^3 + 5*x^2), (x^9*y - 5*x^8*y - x^7*y + x^5*y - x^4*y - 5*x^3*y - 5*x^2*y - 2*x*y - 5*y)/(x^9 - 5*x^8 + 4*x^6 - 3*x^4 + 2*x^3))

The functions directly accessible from an elliptic curve ``E`` over a
field are ``isogeny`` and ``isogeny_codomain``.

The most useful functions that apply to isogenies are

- ``codomain``
- ``degree``
- ``domain``
- ``dual``
- ``rational_maps``
- ``kernel_polynomial``

.. WARNING::

   Only cyclic, separable isogenies are implemented (except for [2]). Some
   algorithms may need the isogeny to be normalized.

AUTHORS:

- Daniel Shumow <shumow@gmail.com>: 2009-04-19: initial version

- Chris Wuthrich : 7/09: changes: add check of input, not the full list is needed.
  10/09: eliminating some bugs.

- John Cremona 2014-08-08: tidying of code and docstrings, systematic
  use of univariate vs. bivariate polynomials and rational functions.

"""

# ****************************************************************************
#       Copyright (C) 2009 Daniel Shumow <shumow@gmail.com>
#
#  Distributed under the terms of the GNU General Public License (GPL)
#                  https://www.gnu.org/licenses/
# ****************************************************************************

from copy import copy

from sage.categories import homset

from sage.categories.morphism import Morphism

from sage.rings.all import PolynomialRing, Integer, LaurentSeriesRing
from sage.rings.polynomial.polynomial_element import is_Polynomial
from sage.schemes.elliptic_curves.all import EllipticCurve
from sage.schemes.elliptic_curves.ell_generic import is_EllipticCurve

from sage.rings.number_field.number_field_base import is_NumberField

from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism, isomorphisms

from sage.sets.set import Set
from sage.structure.richcmp import richcmp_not_equal, richcmp

#
# Private function for parsing input to determine the type of
# algorithm
#
def isogeny_determine_algorithm(E, kernel):
    r"""
    Helper function that allows the various isogeny functions to infer
    the algorithm type from the parameters passed in.

    INPUT:

    - ``E`` (elliptic curve) -- an elliptic curve

    - ``kernel`` -- either a list of points on ``E``, or a univariate
      polynomial or list of coefficients of a univariate polynomial.

    OUTPUT:

    (string) either 'velu' or 'kohel'

    If ``kernel`` is a list of points on the EllipticCurve `E`, then
    we will try to use Velu's algorithm.

    If ``kernel`` is a list of coefficients or a univariate
    polynomial, we will try to use the Kohel's algorithms.

    EXAMPLES:

    This helper function will be implicitly called by the following examples::

        sage: R.<x> = GF(5)[]
        sage: E = EllipticCurve(GF(5), [0,0,0,1,0])

    We can construct the same isogeny from a kernel polynomial::

        sage: phi = EllipticCurveIsogeny(E, x+3)

    or from a list of coefficients of a kernel polynomial::

        sage: phi == EllipticCurveIsogeny(E, [3,1])
        True

    or from a rational point which generates the kernel::

        sage: phi == EllipticCurveIsogeny(E,  E((2,0)) )
        True

    In the first two cases, Kohel's algorithm will be used, while in
    the third case it is Velu::

        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import isogeny_determine_algorithm
        sage: isogeny_determine_algorithm(E, x+3)
        'kohel'
        sage: isogeny_determine_algorithm(E, [3, 1])
        'kohel'
        sage: isogeny_determine_algorithm(E, E((2,0)))
        'velu'
    """
    kernel_is_list = isinstance(kernel, list)

    if not kernel_is_list and kernel in E:
        kernel = [kernel]
        kernel_is_list = True

    if (is_Polynomial(kernel) or ( kernel_is_list) and (kernel[0] in E.base_ring()) ):
        algorithm = "kohel"
    elif (kernel_is_list) and (kernel[0] in E):
        # note that if kernel[0] is on an extension of E this
        # condition will be false
        algorithm = "velu"
    else:
        raise ValueError("Invalid Parameters to EllipticCurveIsogeny constructor.")
    return algorithm

def isogeny_codomain_from_kernel(E, kernel, degree=None):
    r"""
    Compute the isogeny codomain given a kernel.

    INPUT:

    - ``E`` - The domain elliptic curve.

    - ``kernel`` - Either a list of points in the kernel of the isogeny, or a
                   kernel polynomial (specified as a either a univariate
                   polynomial or a coefficient list.)

    - ``degree`` - an integer, (default:``None``)  optionally specified degree
                   of the kernel.

    OUTPUT:

    (elliptic curve) the codomain of the separable normalized isogeny
    from this kernel

    EXAMPLES::

        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import isogeny_codomain_from_kernel
        sage: E = EllipticCurve(GF(7), [1,0,1,0,1])
        sage: R.<x> = GF(7)[]
        sage: isogeny_codomain_from_kernel(E, [4,1], degree=3)
        Elliptic Curve defined by y^2 + x*y + y = x^3 + 4*x + 6 over Finite Field of size 7
        sage: EllipticCurveIsogeny(E, [4,1]).codomain() == isogeny_codomain_from_kernel(E, [4,1], degree=3)
        True
        sage: isogeny_codomain_from_kernel(E, x^3 + x^2 + 4*x + 3)
        Elliptic Curve defined by y^2 + x*y + y = x^3 + 4*x + 6 over Finite Field of size 7
        sage: isogeny_codomain_from_kernel(E, x^3 + 2*x^2 + 4*x + 3)
        Elliptic Curve defined by y^2 + x*y + y = x^3 + 5*x + 2 over Finite Field of size 7

        sage: E = EllipticCurve(GF(19), [1,2,3,4,5])
        sage: kernel_list = [E((15,10)), E((10,3)),E((6,5))]
        sage: isogeny_codomain_from_kernel(E, kernel_list)
        Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 3*x + 15 over Finite Field of size 19

    """

    algorithm = isogeny_determine_algorithm(E, kernel)

    if ("velu"==algorithm):
        # if we are using Velu's formula, just instantiate the isogeny
        # and return the codomain
        return EllipticCurveIsogeny(E, kernel).codomain()
    elif ("kohel"==algorithm):
        return compute_codomain_kohel(E, kernel, degree)

def compute_codomain_formula(E, v, w):
    r"""
    Compute the codomain curve given parameters `v` and `w` (as in
    Velu / Kohel / etc formulas).

    INPUT:

    - ``E`` -- an elliptic curve

    - ``v``, ``w`` -- elements of the base field of ``E``

    OUTPUT:

    The elliptic curve with invariants
    `[a_1,a_2,a_3,a_4-5v,a_6-(a_1^2+4a_2)v-7w]` where
    `E=[a_1,a_2,a_3,a_4,a_6]`.

    EXAMPLES:

    This formula is used by every Isogeny instantiation::

        sage: E = EllipticCurve(GF(19), [1,2,3,4,5])
        sage: phi = EllipticCurveIsogeny(E, E((1,2)) )
        sage: phi.codomain()
        Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 9*x + 13 over Finite Field of size 19
        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import compute_codomain_formula
        sage: v = phi._EllipticCurveIsogeny__v
        sage: w = phi._EllipticCurveIsogeny__w
        sage: compute_codomain_formula(E, v, w) == phi.codomain()
        True
    """
    a1,a2,a3,a4,a6 = E.ainvs()

    A4 = a4 - 5*v
    A6 = a6 - (a1**2 + 4*a2)*v - 7*w

    return EllipticCurve([a1, a2, a3, A4, A6])

def compute_vw_kohel_even_deg1(x0, y0, a1, a2, a4):
    r"""
    Compute Velu's (v,w) using Kohel's formulas for isogenies of
    degree exactly divisible by 2.

    INPUT:

    - ``x0``, ``y0`` -- coordinates of a 2-torsion point on an elliptic curve E

    - ``a1``, ``a2``, ``a4`` -- invariants of E

    OUTPUT:

    (tuple) Velu's isogeny parameters (v,w).

    EXAMPLES:

    This function will be implicitly called by the following example::

        sage: E = EllipticCurve(GF(19), [1,2,3,4,5])
        sage: phi = EllipticCurveIsogeny(E, [9,1]); phi
        Isogeny of degree 2 from Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 4*x + 5 over Finite Field of size 19 to Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 9*x + 8 over Finite Field of size 19
        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import compute_vw_kohel_even_deg1
        sage: a1,a2,a3,a4,a6 = E.ainvs()
        sage: x0 = -9
        sage: y0 = -(a1*x0 + a3)/2
        sage: compute_vw_kohel_even_deg1(x0, y0, a1, a2, a4)
        (18, 9)
    """
    v = (3*x0**2 + 2*a2*x0 + a4 - a1*y0)
    w = x0*v

    return (v,w)

def compute_vw_kohel_even_deg3(b2,b4,s1,s2,s3):
    r"""
    Compute Velu's (v,w) using Kohel's formulas for isogenies of
    degree divisible by 4.

    INPUT:

    - ``b2``, ``b4`` -- invariants of an elliptic curve E

    - ``s1``, ``s2``, ``s3`` -- signed coefficients of the 2-division
      polynomial of E

    OUTPUT:

    (tuple) Velu's isogeny parameters (v,w).

    EXAMPLES:

    This function will be implicitly called by the following example::

        sage: E = EllipticCurve(GF(19), [1,2,3,4,5])
        sage: R.<x> = GF(19)[]
        sage: phi = EllipticCurveIsogeny(E, x^3 + 7*x^2 + 15*x + 12); phi
        Isogeny of degree 4 from Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 4*x + 5 over Finite Field of size 19 to Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 3*x + 15 over Finite Field of size 19
        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import compute_vw_kohel_even_deg3
        sage: (b2,b4) = (E.b2(), E.b4())
        sage: (s1, s2, s3) = (-7, 15, -12)
        sage: compute_vw_kohel_even_deg3(b2, b4, s1, s2, s3)
        (4, 7)
    """
    temp1 = (s1**2 - 2*s2)
    v = 3*temp1 + b2*s1/2 + 3*b4/2
    w = 3*(s1**3 - 3*s1*s2 + 3*s3) + b2*temp1/2 + b4*s1/2

    return (v,w)


def compute_vw_kohel_odd(b2,b4,b6,s1,s2,s3,n):
    r"""
    Compute Velu's (v,w) using Kohel's formulas for isogenies of odd
    degree.

    INPUT:

    - ``b2``, ``b4``, ``b6`` -- invariants of an elliptic curve E

    - ``s1``, ``s2``, ``s3`` -- signed coefficients of lowest powers
      of x in the kernel polynomial.

    - ``n`` (int) -- the degree

    OUTPUT:

    (tuple) Velu's isogeny parameters (v,w).

    EXAMPLES:

    This function will be implicitly called by the following example::

        sage: E = EllipticCurve(GF(19), [18,17,16,15,14])
        sage: R.<x> = GF(19)[]
        sage: phi = EllipticCurveIsogeny(E, x^3 + 14*x^2 + 3*x + 11); phi
        Isogeny of degree 7 from Elliptic Curve defined by y^2 + 18*x*y + 16*y = x^3 + 17*x^2 + 15*x + 14 over Finite Field of size 19 to Elliptic Curve defined by y^2 + 18*x*y + 16*y = x^3 + 17*x^2 + 18*x + 18 over Finite Field of size 19
        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import compute_vw_kohel_odd
        sage: (b2,b4,b6) = (E.b2(), E.b4(), E.b6())
        sage: (s1,s2,s3) = (-14,3,-11)
        sage: compute_vw_kohel_odd(b2,b4,b6,s1,s2,s3,3)
        (7, 1)

    """
    v = 6*(s1**2 - 2*s2) + b2*s1 + n*b4
    w = 10*(s1**3 - 3*s1*s2 + 3*s3) + 2*b2*(s1**2 - 2*s2) + 3*b4*s1 + n*b6

    return (v,w)


def compute_codomain_kohel(E, kernel, degree):
    r"""
    Compute the codomain from the kernel polynomial using Kohel's
    formulas.

    INPUT:

    - ``E`` -- an elliptic curve

    - ``kernel`` (polynomial or list) -- the kernel polynomial, or a
      list of its coefficients

    - ``degree`` (int) -- degree of the isogeny

    OUTPUT:

    (elliptic curve) -- the codomain elliptic curve ``E``/``kernel``

    EXAMPLES::

        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import compute_codomain_kohel
        sage: E = EllipticCurve(GF(19), [1,2,3,4,5])
        sage: phi = EllipticCurveIsogeny(E, [9,1])
        sage: phi.codomain() == isogeny_codomain_from_kernel(E, [9,1])
        True
        sage: compute_codomain_kohel(E, [9,1], 2)
        Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 9*x + 8 over Finite Field of size 19
        sage: R.<x> = GF(19)[]
        sage: E = EllipticCurve(GF(19), [18,17,16,15,14])
        sage: phi = EllipticCurveIsogeny(E, x^3 + 14*x^2 + 3*x + 11)
        sage: phi.codomain() == isogeny_codomain_from_kernel(E, x^3 + 14*x^2 + 3*x + 11)
        True
        sage: compute_codomain_kohel(E, x^3 + 14*x^2 + 3*x + 11, 7)
        Elliptic Curve defined by y^2 + 18*x*y + 16*y = x^3 + 17*x^2 + 18*x + 18 over Finite Field of size 19
        sage: E = EllipticCurve(GF(19), [1,2,3,4,5])
        sage: phi = EllipticCurveIsogeny(E, x^3 + 7*x^2 + 15*x + 12)
        sage: isogeny_codomain_from_kernel(E, x^3 + 7*x^2 + 15*x + 12) == phi.codomain()
        True
        sage: compute_codomain_kohel(E, x^3 + 7*x^2 + 15*x + 12,4)
        Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 3*x + 15 over Finite Field of size 19

    .. NOTE::

       This function uses the formulas of Section 2.4 of [Koh1996]_.
    """
    # First set up the polynomial ring

    base_field = E.base_ring()
    poly_ring = PolynomialRing(base_field,'x')

    if (is_Polynomial(kernel)):
        psi = poly_ring(kernel)
        kernel_list = psi.list()
    elif isinstance(kernel, list) and (kernel[0] in base_field):
        kernel_list = kernel
        psi = poly_ring(kernel_list)
    else:
        raise ValueError("Invalid input to compute_codomain_kohel")

    # next determine the even / odd part of the isogeny
    psi_2tor = two_torsion_part(E, psi)

    if (0 != psi_2tor.degree()): # even degree case

        psi_quo = psi//psi_2tor

        if (0 != psi_quo.degree()):
            raise ArithmeticError("For basic Kohel's algorithm, if the kernel degree is even then the kernel must be contained in the two torsion.")

        n = psi_2tor.degree()

        if (1 == n): # degree divisible exactly by 2

            a1,a2,a3,a4,a6 = E.ainvs()

            x0 = -psi_2tor.constant_coefficient()

            # determine y0
            if (2 == base_field.characteristic()):
                y0 = (x0**3 + a2*x0**2 + a4*x0 + a6).sqrt()
            else:
                y0 = -(a1*x0 + a3)/2

            # now (x0,y0) is the 2-torsion point in the kernel

            (v,w) = compute_vw_kohel_even_deg1(x0,y0,a1,a2,a4)

        elif (3 == n): # psi_2tor is the full 2-division polynomial

            b2 = E.b2()
            b4 = E.b4()

            s = psi_2tor.list()
            s1 = -s[n-1]
            s2 = s[n-2]
            s3 = -s[n-3]

            (v,w) = compute_vw_kohel_even_deg3(b2,b4,s1,s2,s3)

    else:  # odd degree case

        n = psi.degree()

        b2 = E.b2()
        b4 = E.b4()
        b6 = E.b6()

        s1 = 0
        s2 = 0
        s3 = 0

        if 1 <= n:
            s1 = -kernel_list[n-1]

        if 2 <= n:
            s2 = kernel_list[n-2]

        if 3 <= n:
            s3 = -kernel_list[n-3]

        # initializing these allows us to calculate E2.
        (v,w) = compute_vw_kohel_odd(b2,b4,b6,s1,s2,s3,n)

    return compute_codomain_formula(E, v, w)


def two_torsion_part(E, psi):
    r"""
    Return the greatest common divisor of ``psi`` and the 2 torsion
    polynomial of `E`.

    INPUT:

    - ``E`` -- an elliptic curve

    - ``psi`` -- a univariate polynomial over the base field of ``E``

    OUTPUT:

    (polynomial) the gcd of psi and the 2-torsion polynomial of ``E``.

    EXAMPLES:

    Every function that computes the kernel polynomial via Kohel's
    formulas will call this function::

        sage: E = EllipticCurve(GF(19), [1,2,3,4,5])
        sage: R.<x> = GF(19)[]
        sage: phi = EllipticCurveIsogeny(E, x + 13)
        sage: isogeny_codomain_from_kernel(E, x + 13) == phi.codomain()
        True
        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import two_torsion_part
        sage: two_torsion_part(E, x+13)
        x + 13

    """
    x = psi.parent().gen() # NB psi is univariate but could be constant
    psi_2 = E.two_division_polynomial(x)
    return psi.gcd(psi_2)

class EllipticCurveIsogeny(Morphism):
    r"""
    Class Implementing Isogenies of Elliptic Curves

    This class implements cyclic, separable, normalized isogenies of
    elliptic curves.

    Several different algorithms for computing isogenies are
    available.  These include:

    - Velu's Formulas: Velu's original formulas for computing
      isogenies.  This algorithm is selected by giving as the
      ``kernel`` parameter a list of points which generate a finite
      subgroup.

    - Kohel's Formulas: Kohel's original formulas for computing
      isogenies.  This algorithm is selected by giving as the
      ``kernel`` parameter a monic polynomial (or a coefficient list
      (little endian)) which will define the kernel of the isogeny.

    INPUT:

    - ``E`` -- an elliptic curve, the domain of the isogeny to
      initialize.

    - ``kernel`` -- a kernel, either a point in ``E``, a list of
      points in ``E``, a monic kernel polynomial, or ``None``.  If
      initializing from a domain/codomain, this must be set to None.

    - ``codomain`` -- an elliptic curve (default:``None``).  If
      ``kernel`` is ``None``, then this must be the codomain of a
      cyclic, separable, normalized isogeny, furthermore, ``degree``
      must be the degree of the isogeny from ``E`` to ``codomain``. If
      ``kernel`` is not ``None``, then this must be isomorphic to the
      codomain of the cyclic normalized separable isogeny defined by
      ``kernel``, in this case, the isogeny is post composed with an
      isomorphism so that this parameter is the codomain.

    - ``degree`` -- an integer (default:``None``).  If ``kernel`` is
      ``None``, then this is the degree of the isogeny from ``E`` to
      ``codomain``.  If ``kernel`` is not ``None``, then this is used
      to determine whether or not to skip a gcd of the kernel
      polynomial with the two torsion polynomial of ``E``.

    - ``model`` -- a string (default:``None``).  Only supported
      variable is ``minimal``, in which case if ``E`` is a curve over
      the rationals or over a number field, then the codomain is a
      global minimum model where this exists.

    - ``check`` (default: ``True``) checks if the input is valid to
      define an isogeny

    EXAMPLES:

    A simple example of creating an isogeny of a field of small
    characteristic::

        sage: E = EllipticCurve(GF(7), [0,0,0,1,0])
        sage: phi = EllipticCurveIsogeny(E, E((0,0)) ); phi
        Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + x over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 3*x over Finite Field of size 7
        sage: phi.degree() == 2
        True
        sage: phi.kernel_polynomial()
        x
        sage: phi.rational_maps()
        ((x^2 + 1)/x, (x^2*y - y)/x^2)
        sage: phi == loads(dumps(phi))  # known bug
        True

    A more complicated example of a characteristic 2 field::

        sage: E = EllipticCurve(GF(2^4,'alpha'), [0,0,1,0,1])
        sage: P = E((1,1))
        sage: phi_v = EllipticCurveIsogeny(E, P); phi_v
        Isogeny of degree 3 from Elliptic Curve defined by y^2 + y = x^3 + 1 over Finite Field in alpha of size 2^4 to Elliptic Curve defined by y^2 + y = x^3 over Finite Field in alpha of size 2^4
        sage: phi_ker_poly = phi_v.kernel_polynomial()
        sage: phi_ker_poly
        x + 1
        sage: ker_poly_list = phi_ker_poly.list()
        sage: phi_k = EllipticCurveIsogeny(E, ker_poly_list)
        sage: phi_k == phi_v
        True
        sage: phi_k.rational_maps()
        ((x^3 + x + 1)/(x^2 + 1), (x^3*y + x^2*y + x*y + x + y)/(x^3 + x^2 + x + 1))
        sage: phi_v.rational_maps()
        ((x^3 + x + 1)/(x^2 + 1), (x^3*y + x^2*y + x*y + x + y)/(x^3 + x^2 + x + 1))
        sage: phi_k.degree() == phi_v.degree() == 3
        True
        sage: phi_k.is_separable()
        True
        sage: phi_v(E(0))
        (0 : 1 : 0)
        sage: alpha = E.base_field().gen()
        sage: Q = E((0, alpha*(alpha + 1)))
        sage: phi_v(Q)
        (1 : alpha^2 + alpha : 1)
        sage: phi_v(P) == phi_k(P)
        True
        sage: phi_k(P) == phi_v.codomain()(0)
        True

    We can create an isogeny that has kernel equal to the full 2
    torsion::

        sage: E = EllipticCurve(GF(3), [0,0,0,1,1])
        sage: ker_list = E.division_polynomial(2).list()
        sage: phi = EllipticCurveIsogeny(E, ker_list); phi
        Isogeny of degree 4 from Elliptic Curve defined by y^2 = x^3 + x + 1 over Finite Field of size 3 to Elliptic Curve defined by y^2 = x^3 + x + 1 over Finite Field of size 3
        sage: phi(E(0))
        (0 : 1 : 0)
        sage: phi(E((0,1)))
        (1 : 0 : 1)
        sage: phi(E((0,2)))
        (1 : 0 : 1)
        sage: phi(E((1,0)))
        (0 : 1 : 0)
        sage: phi.degree()
        4

    We can also create trivial isogenies with the trivial kernel::

        sage: E = EllipticCurve(GF(17), [11, 11, 4, 12, 10])
        sage: phi_v = EllipticCurveIsogeny(E, E(0))
        sage: phi_v.degree()
        1
        sage: phi_v.rational_maps()
        (x, y)
        sage: E == phi_v.codomain()
        True
        sage: P = E.random_point()
        sage: phi_v(P) == P
        True

        sage: E = EllipticCurve(GF(31), [23, 1, 22, 7, 18])
        sage: phi_k = EllipticCurveIsogeny(E, [1]); phi_k
        Isogeny of degree 1 from Elliptic Curve defined by y^2 + 23*x*y + 22*y = x^3 + x^2 + 7*x + 18 over Finite Field of size 31 to Elliptic Curve defined by y^2 + 23*x*y + 22*y = x^3 + x^2 + 7*x + 18 over Finite Field of size 31
        sage: phi_k.degree()
        1
        sage: phi_k.rational_maps()
        (x, y)
        sage: phi_k.codomain() == E
        True
        sage: phi_k.kernel_polynomial()
        1
        sage: P = E.random_point(); P == phi_k(P)
        True

    Velu and Kohel also work in characteristic 0::

        sage: E = EllipticCurve(QQ, [0,0,0,3,4])
        sage: P_list = E.torsion_points()
        sage: phi = EllipticCurveIsogeny(E, P_list); phi
        Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 3*x + 4 over Rational Field to Elliptic Curve defined by y^2 = x^3 - 27*x + 46 over Rational Field
        sage: P = E((0,2))
        sage: phi(P)
        (6 : -10 : 1)
        sage: phi_ker_poly = phi.kernel_polynomial()
        sage: phi_ker_poly
        x + 1
        sage: ker_poly_list = phi_ker_poly.list()
        sage: phi_k = EllipticCurveIsogeny(E, ker_poly_list); phi_k
        Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 3*x + 4 over Rational Field to Elliptic Curve defined by y^2 = x^3 - 27*x + 46 over Rational Field
        sage: phi_k(P) == phi(P)
        True
        sage: phi_k == phi
        True
        sage: phi_k.degree()
        2
        sage: phi_k.is_separable()
        True

    A more complicated example over the rationals (of odd degree)::

        sage: E = EllipticCurve('11a1')
        sage: P_list = E.torsion_points()
        sage: phi_v = EllipticCurveIsogeny(E, P_list); phi_v
        Isogeny of degree 5 from Elliptic Curve defined by y^2 + y = x^3 - x^2 - 10*x - 20 over Rational Field to Elliptic Curve defined by y^2 + y = x^3 - x^2 - 7820*x - 263580 over Rational Field
        sage: P = E((16,-61))
        sage: phi_v(P)
        (0 : 1 : 0)
        sage: ker_poly = phi_v.kernel_polynomial(); ker_poly
        x^2 - 21*x + 80
        sage: ker_poly_list = ker_poly.list()
        sage: phi_k = EllipticCurveIsogeny(E, ker_poly_list); phi_k
        Isogeny of degree 5 from Elliptic Curve defined by y^2 + y = x^3 - x^2 - 10*x - 20 over Rational Field to Elliptic Curve defined by y^2 + y = x^3 - x^2 - 7820*x - 263580 over Rational Field
        sage: phi_k == phi_v
        True
        sage: phi_v(P) == phi_k(P)
        True
        sage: phi_k.is_separable()
        True

    We can also do this same example over the number field defined by
    the irreducible two torsion polynomial of `E`::

        sage: E = EllipticCurve('11a1')
        sage: P_list = E.torsion_points()
        sage: K.<alpha> = NumberField(x^3 - 2* x^2 - 40*x - 158)
        sage: EK = E.change_ring(K)
        sage: P_list = [EK(P) for P in P_list]
        sage: phi_v = EllipticCurveIsogeny(EK, P_list); phi_v
        Isogeny of degree 5 from Elliptic Curve defined by y^2 + y = x^3 + (-1)*x^2 + (-10)*x + (-20) over Number Field in alpha with defining polynomial x^3 - 2*x^2 - 40*x - 158 to Elliptic Curve defined by y^2 + y = x^3 + (-1)*x^2 + (-7820)*x + (-263580) over Number Field in alpha with defining polynomial x^3 - 2*x^2 - 40*x - 158
        sage: P = EK((alpha/2,-1/2))
        sage: phi_v(P)
        (122/121*alpha^2 + 1633/242*alpha - 3920/121 : -1/2 : 1)
        sage: ker_poly = phi_v.kernel_polynomial()
        sage: ker_poly
        x^2 - 21*x + 80
        sage: ker_poly_list = ker_poly.list()
        sage: phi_k = EllipticCurveIsogeny(EK, ker_poly_list)
        sage: phi_k
        Isogeny of degree 5 from Elliptic Curve defined by y^2 + y = x^3 + (-1)*x^2 + (-10)*x + (-20) over Number Field in alpha with defining polynomial x^3 - 2*x^2 - 40*x - 158 to Elliptic Curve defined by y^2 + y = x^3 + (-1)*x^2 + (-7820)*x + (-263580) over Number Field in alpha with defining polynomial x^3 - 2*x^2 - 40*x - 158
        sage: phi_v == phi_k
        True
        sage: phi_k(P) == phi_v(P)
        True
        sage: phi_k == phi_v
        True
        sage: phi_k.degree()
        5
        sage: phi_v.is_separable()
        True

    The following example shows how to specify an isogeny from domain
    and codomain::

        sage: E = EllipticCurve('11a1')
        sage: R.<x> = QQ[]
        sage: f = x^2 - 21*x + 80
        sage: phi = E.isogeny(f)
        sage: E2 = phi.codomain()
        sage: phi_s = EllipticCurveIsogeny(E, None, E2, 5)
        sage: phi_s
        Isogeny of degree 5 from Elliptic Curve defined by y^2 + y = x^3 - x^2 - 10*x - 20 over Rational Field to Elliptic Curve defined by y^2 + y = x^3 - x^2 - 7820*x - 263580 over Rational Field
        sage: phi_s == phi
        True
        sage: phi_s.rational_maps() == phi.rational_maps()
        True

    However only cyclic normalized isogenies can be constructed this
    way. So it won't find the isogeny [3]::

        sage: E.isogeny(None, codomain=E,degree=9)
        Traceback (most recent call last):
        ...
        ValueError: The two curves are not linked by a cyclic normalized isogeny of degree 9

    Also the presumed isogeny between the domain and codomain must be
    normalized::

        sage: E2.isogeny(None,codomain=E,degree=5)
        Traceback (most recent call last):
        ...
        ValueError: The two curves are not linked by a cyclic normalized isogeny of degree 5
        sage: phihat = phi.dual(); phihat
        Isogeny of degree 5 from Elliptic Curve defined by y^2 + y = x^3 - x^2 - 7820*x - 263580 over Rational Field to Elliptic Curve defined by y^2 + y = x^3 - x^2 - 10*x - 20 over Rational Field
        sage: phihat.is_normalized()
        False

    Here an example of a construction of a endomorphisms with cyclic
    kernel on a CM-curve::

        sage: K.<i> = NumberField(x^2+1)
        sage: E = EllipticCurve(K, [1,0])
        sage: RK.<X> = K[]
        sage: f = X^2 - 2/5*i + 1/5
        sage: phi= E.isogeny(f)
        sage: isom = phi.codomain().isomorphism_to(E)
        sage: phi.set_post_isomorphism(isom)
        sage: phi.codomain() == phi.domain()
        True
        sage: phi.rational_maps()
        (((4/25*i + 3/25)*x^5 + (4/5*i - 2/5)*x^3 - x)/(x^4 + (-4/5*i + 2/5)*x^2 + (-4/25*i - 3/25)), ((11/125*i + 2/125)*x^6*y + (-23/125*i + 64/125)*x^4*y + (141/125*i + 162/125)*x^2*y + (3/25*i - 4/25)*y)/(x^6 + (-6/5*i + 3/5)*x^4 + (-12/25*i - 9/25)*x^2 + (2/125*i - 11/125)))

    Domain and codomain tests (see :trac:`12880`)::

        sage: E = EllipticCurve(QQ, [0,0,0,1,0])
        sage: phi = EllipticCurveIsogeny(E,  E(0,0))
        sage: phi.domain() == E
        True
        sage: phi.codomain()
        Elliptic Curve defined by y^2 = x^3 - 4*x over Rational Field

        sage: E = EllipticCurve(GF(31), [1,0,0,1,2])
        sage: phi = EllipticCurveIsogeny(E, [17, 1])
        sage: phi.domain()
        Elliptic Curve defined by y^2 + x*y = x^3 + x + 2 over Finite Field of size 31
        sage: phi.codomain()
        Elliptic Curve defined by y^2 + x*y = x^3 + 24*x + 6 over Finite Field of size 31

    Composition tests (see :trac:`16245`)::

        sage: E = EllipticCurve(j=GF(7)(0))
        sage: phi = E.isogeny([E(0), E((0,1)), E((0,-1))]); phi
        Isogeny of degree 3 from Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 7
        sage: phi2 = phi * phi; phi2
        Composite map:
          From: Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 7
          To:   Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 7
          Defn:   Isogeny of degree 3 from Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 7
                then
                  Isogeny of degree 3 from Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 7

    Examples over relative number fields used not to work (see :trac:`16779`)::

        sage: pol26 = hilbert_class_polynomial(-4*26)
        sage: pol = NumberField(pol26,'a').optimized_representation()[0].polynomial()
        sage: K.<a> = NumberField(pol)
        sage: j = pol26.roots(K)[0][0]
        sage: E = EllipticCurve(j=j)
        sage: L.<b> = K.extension(x^2+26)
        sage: EL = E.change_ring(L)
        sage: iso2 = EL.isogenies_prime_degree(2); len(iso2)
        1
        sage: iso3 = EL.isogenies_prime_degree(3); len(iso3)
        2

    Examples over function fields used not to work (see :trac:`11327`)::

        sage: F.<t> = FunctionField(QQ)
        sage: E = EllipticCurve([0,0,0,-t^2,0])
        sage: isogs = E.isogenies_prime_degree(2)
        sage: isogs[0]
        Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + (-t^2)*x over Rational function field in t over Rational Field to Elliptic Curve defined by y^2 = x^3 + 4*t^2*x over Rational function field in t over Rational Field
        sage: isogs[0].rational_maps()
        ((x^2 - t^2)/x, (x^2*y + t^2*y)/x^2)
        sage: duals = [phi.dual() for phi in isogs]
        sage: duals[0]
        Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 4*t^2*x over Rational function field in t over Rational Field to Elliptic Curve defined by y^2 = x^3 + (-t^2)*x over Rational function field in t over Rational Field
        sage: duals[0].rational_maps()
        ((1/4*x^2 + t^2)/x, (1/8*x^2*y + (-1/2*t^2)*y)/x^2)
        sage: duals[0]
        Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 4*t^2*x over Rational function field in t over Rational Field to Elliptic Curve defined by y^2 = x^3 + (-t^2)*x over Rational function field in t over Rational Field
    """

    ####################
    # member variables
    ####################

    __check = None
    #
    # variables common to all algorithms
    #
    __E1 = None  # domain curve
    __E2 = None  # codomain curve

    __degree = None

    __separable = True # This class only implements separable isogenies (for now.)

    __algorithm = None

    __this_hash = None

    __check = None
    #
    # pre isomorphism
    #
    __intermediate_domain = None
    __pre_isomorphism = None
    __prei_x_coord_ratl_map = None
    __prei_y_coord_ratl_map = None

    #
    # post isomorphism
    #

    __intermediate_codomain = None
    __post_isomorphism = None
    __posti_x_coord_ratl_map = None
    __posti_y_coord_ratl_map = None

    #
    # algebraic structs
    #
    __base_field = None
    __poly_ring = None # univariate in x over __base_field
    __mpoly_ring = None # bivariate in x, y over __base_field

    #
    # Rational Maps
    #
    __rational_maps_initialized = False
    __X_coord_rational_map = None
    __Y_coord_rational_map = None

    #
    # The dual
    #
    __dual = None

    #
    # Kernel Data
    #

    __kernel_list = None  # list of elements in the kernel

    __kernel_polynomial_list = None # polynomial stored as a little endian list of coefficients

    __kernel_polynomial = None # polynomial with roots at x values for x-coordinate of points in the kernel

    __inner_kernel_polynomial = None # the inner kernel polynomial (ignoring preisomorphism)

    __n = None


    #
    # member variables common to Velu's formula
    #

    # we keep track of the 2 torsion and non2torsion separately
    __kernel_2tor = None
    __kernel_non2tor = None

    # variables used in Velu's formula (as well as Kohel's variant)
    __v = None
    __w = None


    #
    # member variables specific to Kohel's algorithm.
    #

    __psi = None # psi polynomial
    __phi = None # phi polynomial
    __omega = None # omega polynomial


    #
    # Python Special Functions
    #

    def __init__(self, E, kernel, codomain=None, degree=None, model=None, check=True):
        r"""
        Constructor for EllipticCurveIsogeny class.

        EXAMPLES::

            sage: E = EllipticCurve(GF(2), [0,0,1,0,1])
            sage: phi = EllipticCurveIsogeny(E, [1,1]); phi
            Isogeny of degree 3 from Elliptic Curve defined by y^2 + y = x^3 + 1 over Finite Field of size 2 to Elliptic Curve defined by y^2 + y = x^3 over Finite Field of size 2

            sage: E = EllipticCurve(GF(31), [0,0,0,1,0])
            sage: P = E((2,17))
            sage: phi = EllipticCurveIsogeny(E, P); phi
            Isogeny of degree 8 from Elliptic Curve defined by y^2 = x^3 + x over Finite Field of size 31 to Elliptic Curve defined by y^2 = x^3 + 10*x + 28 over Finite Field of size 31

            sage: E = EllipticCurve('17a1')
            sage: phi = EllipticCurveIsogeny(E, [41/3, -55, -1, -1, 1]); phi
            Isogeny of degree 9 from Elliptic Curve defined by y^2 + x*y + y = x^3 - x^2 - x - 14 over Rational Field to Elliptic Curve defined by y^2 + x*y + y = x^3 - x^2 - 56*x - 10124 over Rational Field

            sage: E = EllipticCurve('37a1')
            sage: triv = EllipticCurveIsogeny(E, E(0)); triv
            Isogeny of degree 1 from Elliptic Curve defined by y^2 + y = x^3 - x over Rational Field to Elliptic Curve defined by y^2 + y = x^3 - x over Rational Field
            sage: triv.rational_maps()
            (x, y)

            sage: E = EllipticCurve('49a3')
            sage: R.<X> = QQ[]
            sage: EllipticCurveIsogeny(E,X^3-13*X^2-58*X+503,check=False)
            Isogeny of degree 7 from Elliptic Curve defined by y^2 + x*y = x^3 - x^2 - 107*x + 552 over Rational Field to Elliptic Curve defined by y^2 + x*y = x^3 - x^2 - 5252*x - 178837 over Rational Field

        """
        if not is_EllipticCurve(E):
            raise ValueError("E parameter must be an EllipticCurve.")

        if not isinstance(kernel, list) and kernel in E:
            # a single point was given, we put it in a list
            # the first condition assures that [1,1] is treated as x+1
            kernel = [kernel]

        # if the kernel is None and the codomain isn't
        # calculate the kernel polynomial
        pre_isom = None
        post_isom = None

        self.__check = check

        if (kernel is None) and (codomain is not None):

            if (degree is None):
                raise ValueError("If specifying isogeny by domain and codomain, degree parameter must be set.")

            # save the codomain: really used now (trac #7096)
            old_codomain = codomain

            (pre_isom, post_isom, E, codomain, kernel) = compute_sequence_of_maps(E, codomain, degree)

        self.__init_algebraic_structs(E)

        algorithm = isogeny_determine_algorithm(E, kernel)

        self.__algorithm = algorithm

        if ("velu"==algorithm):
            self.__init_from_kernel_list(kernel)
        elif ("kohel"==algorithm):
            self.__init_from_kernel_polynomial(kernel)

        self.__compute_E2()

        self.__setup_post_isomorphism(codomain, model)

        if (pre_isom is not None):
            self.set_pre_isomorphism(pre_isom)

        if (post_isom is not None):
            self.__set_post_isomorphism(old_codomain, post_isom)   #(trac #7096)

        # Inheritance house keeping

        self.__perform_inheritance_housekeeping()

    def _call_(self, P):
        r"""
        Function that implements the call-ability of elliptic curve
        isogenies.

        EXAMPLES::

            sage: E = EllipticCurve(GF(17), [1, 9, 5, 4, 3])
            sage: phi = EllipticCurveIsogeny(E, [6,13,1])
            sage: phi(E((1,0)))
            (15 : 13 : 1)

            sage: E = EllipticCurve(GF(23), [0,0,0,1,0])
            sage: phi = EllipticCurveIsogeny(E, E((0,0)))
            sage: phi(E((1,5)))
            (2 : 0 : 1)

            sage: E = EllipticCurve(QQ, [0,0,0,3,0])
            sage: P = E((1,2))
            sage: phi = EllipticCurveIsogeny(E, [0,1])
            sage: phi(P)
            (4 : -4 : 1)
            sage: phi(-P)
            (4 : 4 : 1)

            sage: E = EllipticCurve(GF(17), [0,-1,0,-3,-1])
            sage: Q = E((16,0))
            sage: tau = E.isogeny([Q],E)
            sage: tau(Q)
            (0 : 1 : 0)

        TESTS:

        Tests for :trac:`10888`::

            sage: K.<th> = NumberField(x^2+3)
            sage: E = EllipticCurve(K,[7,0])
            sage: phi = E.isogeny(E(0,0))
            sage: P = E(-3,4*th)
            sage: phi(P)
            (-16/3 : 8/9*th : 1)
            sage: Q = phi(P)
            sage: phihat = phi.dual()
            sage: phihat(Q)
            (-1/48 : 127/576*th : 1)

        Call a composed isogeny (added for :trac:`16238`)::

            sage: E = EllipticCurve(j=GF(7)(0))
            sage: phi = E.isogeny([E(0), E((0,1)), E((0,-1))])
            sage: phi(E.points()[0])
            (0 : 1 : 0)
            sage: phi2 = phi * phi
            sage: phi2(E.points()[0])
            (0 : 1 : 0)

        Coercion works fine with :meth:`_call_` (added for :trac:`16238`)::

            sage: K.<th> = NumberField(x^2+3)
            sage: E = EllipticCurve(K,[7,0])
            sage: E2=EllipticCurve(K,[5,0])
            sage: phi=E.isogeny(E(0))
            sage: phi(E2(0))
            (0 : 1 : 0)
            sage: E2(20,90)
            (20 : 90 : 1)
            sage: phi(E2(20,90))
            Traceback (most recent call last):
            ...
            TypeError: (20 : 90 : 1) fails to convert into the map's domain Elliptic Curve defined by y^2 = x^3 + 7*x over Number Field in th with defining polynomial x^2 + 3, but a `pushforward` method is not properly implemented

        """
        if P.is_zero():
            return self.__E2(0)

        (xP, yP) = P.xy()
        # if there is a pre isomorphism, apply it
        if self.__pre_isomorphism is not None:
            temp_xP = self.__prei_x_coord_ratl_map(xP)
            temp_yP = self.__prei_y_coord_ratl_map(xP, yP)
            (xP, yP) = (temp_xP, temp_yP)

        if "velu" == self.__algorithm:
            outP = self.__compute_via_velu_numeric(xP, yP)
        elif "kohel" == self.__algorithm:
            outP = self.__compute_via_kohel_numeric(xP,yP)

        # the intermediate functions return the point at infinity
        # if the input point is in the kernel
        if (outP == self.__intermediate_codomain(0)):
            return self.__E2(0)

        # if there is a post isomorphism, apply it
        if (self.__post_isomorphism is not None):
            tempX = self.__posti_x_coord_ratl_map(outP[0])
            tempY = self.__posti_y_coord_ratl_map(outP[0], outP[1])
            outP = (tempX, tempY)

        return self.__E2(outP)

    def __getitem__(self, i):
        r"""
        Return one of the rational map components.

        .. NOTE::

           Both components are returned as elements of the function
           field `F(x,y)` in two variables over the base field `F`,
           though the first only involves `x`.  To obtain the
           `x`-coordinate function as a rational function in `F(x)`,
           use :meth:`x_rational_map`.

        EXAMPLES::

            sage: E = EllipticCurve(QQ, [0,2,0,1,-1])
            sage: phi = EllipticCurveIsogeny(E, [1])
            sage: phi[0]
            x
            sage: phi[1]
            y

            sage: E = EllipticCurve(GF(17), [0,0,0,3,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: phi[0]
            (x^2 + 3)/x
            sage: phi[1]
            (x^2*y - 3*y)/x^2
        """
        return self.rational_maps()[i]

    def __iter__(self):
        r"""
        Return an iterator through the rational map components.

        EXAMPLES::

            sage: E = EllipticCurve(QQ, [0,2,0,1,-1])
            sage: phi = EllipticCurveIsogeny(E, [1])
            sage: for c in phi: print(c)
            x
            y

            sage: E = EllipticCurve(GF(17), [0,0,0,3,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: for c in phi: print(c)
            (x^2 + 3)/x
            (x^2*y - 3*y)/x^2
        """
        return iter(self.rational_maps())

    def __hash__(self):
        r"""
        Function that implements the hash ability of Isogeny objects.

        This hashes the underlying kernel polynomial so that equal
        isogeny objects have the same hash value.  Also, this hashes
        the base field, and domain and codomain curves as well, so
        that isogenies with the same kernel polynomial (over different
        base fields / curves) hash to different values.

        EXAMPLES::

            sage: E = EllipticCurve(QQ, [0,0,0,1,0])
            sage: phi_v = EllipticCurveIsogeny(E, E((0,0)))
            sage: phi_k = EllipticCurveIsogeny(E, [0,1])
            sage: phi_k.__hash__() == phi_v.__hash__()
            True
            sage: E_F17 = EllipticCurve(GF(17), [0,0,0,1,1])
            sage: phi_p = EllipticCurveIsogeny(E_F17, E_F17([0,1]))
            sage: phi_p.__hash__() == phi_v.__hash__()
            False

            sage: E = EllipticCurve('49a3')
            sage: R.<X> = QQ[]
            sage: EllipticCurveIsogeny(E,X^3-13*X^2-58*X+503,check=False)
            Isogeny of degree 7 from Elliptic Curve defined by y^2 + x*y = x^3 - x^2 - 107*x + 552 over Rational Field to Elliptic Curve defined by y^2 + x*y = x^3 - x^2 - 5252*x - 178837 over Rational Field

        """

        if self.__this_hash is not None:
            return self.__this_hash

        ker_poly_list = self.__kernel_polynomial_list

        if ker_poly_list is None:
            ker_poly_list = self.__init_kernel_polynomial()

        this_hash = 0

        for a in ker_poly_list:
            this_hash ^= hash(a)

        this_hash ^= hash(self.__E1)
        this_hash ^= hash(self.__E2)
        this_hash ^= hash(self.__base_field)

        self.__this_hash = this_hash

        return self.__this_hash

    def _richcmp_(self, other, op):
        r"""
        Function that implements comparisons between isogeny objects.

        This function works by comparing the underlying kernel
        objects.

        EXAMPLES::

            sage: E = EllipticCurve(QQ, [0,0,0,1,0])
            sage: phi_v = EllipticCurveIsogeny(E, E((0,0)))
            sage: phi_k = EllipticCurveIsogeny(E, [0,1])
            sage: phi_k == phi_v
            True
            sage: E_F17 = EllipticCurve(GF(17), [0,0,0,1,0])
            sage: phi_p = EllipticCurveIsogeny(E_F17, [0,1])
            sage: phi_p == phi_v
            False
            sage: E = EllipticCurve('11a1')
            sage: phi = E.isogeny(E(5,5))
            sage: phi == phi
            True
            sage: phi == -phi
            False
            sage: psi = E.isogeny(phi.kernel_polynomial())
            sage: phi == psi
            True
            sage: phi.dual() == psi.dual()
            True
        """
        if (self.__kernel_polynomial is None):
            self.__init_kernel_polynomial()

        # We cannot just compare kernel polynomials, as was done until
        # Trac #11327, as then phi and -phi compare equal, and
        # similarly with phi and any composition of phi with an
        # automorphism of its codomain, or any post-isomorphism.
        # Comparing domains, codomains and rational maps seems much
        # safer.
        lx = self.domain()
        rx = other.domain()
        if lx != rx:
            return richcmp_not_equal(lx, rx, op)
        lx = self.codomain()
        rx = other.codomain()
        if lx != rx:
            return richcmp_not_equal(lx, rx, op)
        return richcmp(self.rational_maps(), other.rational_maps(), op)

    def __neg__(self):
        r"""
        Function to implement unary negation (-) operator on
        isogenies. Return a copy of this isogeny that has been
        negated.

        EXAMPLES:

        The following examples inherently exercise this function::

            sage: E = EllipticCurve(j=GF(17)(0))
            sage: phi = EllipticCurveIsogeny(E,  E((-1,0)) )
            sage: negphi = -phi
            sage: phi(E((0,1))) + negphi(E((0,1))) == 0
            True

            sage: E = EllipticCurve(j=GF(19)(1728))
            sage: R.<x> = GF(19)[]
            sage: phi = EllipticCurveIsogeny(E, x)
            sage: negphi = -phi
            sage: phi(E((3,7))) + negphi(E((3,12))) == phi(2*E((3,7)))
            True
            sage: negphi(E((18,6)))
            (17 : 0 : 1)

            sage: R.<x> = QQ[]
            sage: E = EllipticCurve('17a1')
            sage: R.<x> = QQ[]
            sage: f = x - 11/4
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: negphi = -phi
            sage: phi.rational_maps()[0] == negphi.rational_maps()[0]
            True
            sage: P = E((7,13))
            sage: phi(P) + negphi(P) == 0
            True

        """
        # save off the kernel lists
        kernel_list = self.__kernel_list
        self.__kernel_list = None

        output = copy(self)

        # reset the kernel lists
        output.__kernel_list = copy(kernel_list)
        self.__kernel_list = kernel_list

        output.switch_sign()
        return output

    #
    # Sage Special Functions
    #

    def _repr_(self):
        r"""
        Special sage specific function that implement the
        functionality to display the isogeny self as a string.

        EXAMPLES::

            sage: E = EllipticCurve(GF(31), [1,0,1,1,0])
            sage: phi = EllipticCurveIsogeny(E, E((0,0)) )
            sage: phi._repr_()
            'Isogeny of degree 17 from Elliptic Curve defined by y^2 + x*y + y = x^3 + x over Finite Field of size 31 to Elliptic Curve defined by y^2 + x*y + y = x^3 + 14*x + 9 over Finite Field of size 31'

            sage: E = EllipticCurve(QQ, [1,0,0,1,9])
            sage: phi = EllipticCurveIsogeny(E, [2,1])
            sage: phi._repr_()
            'Isogeny of degree 2 from Elliptic Curve defined by y^2 + x*y = x^3 + x + 9 over Rational Field to Elliptic Curve defined by y^2 + x*y = x^3 - 59*x + 165 over Rational Field'

        """
        return 'Isogeny of degree %r from %r to %r' % (
                self.__degree, self.__E1, self.__E2)

    def _latex_(self):
        r"""
        Special sage specific function that implements functionality
        to display an isogeny object as a latex string.

        This function returns a latex string representing the isogeny
        self as the `x` and `y` coordinate rational functions.

        EXAMPLES::

            sage: E = EllipticCurve(QQ, [0,0,0,1,-1])
            sage: phi = EllipticCurveIsogeny(E, E(0))
            sage: phi._latex_()
            '\\left( x , y \\right)'

            sage: E = EllipticCurve(GF(17), [0,0,0,1,-1])
            sage: R.<X> = GF(17)[]
            sage: phi = EllipticCurveIsogeny(E, X+11)
            sage: phi._latex_()
            '\\left( \\frac{x^{2} + 11 x + 7}{x + 11} , \\frac{x^{2} y + 5 x y + 12 y}{x^{2} + 5 x + 2} \\right)'


        """
        ratl_maps = self.rational_maps()
        return '\\left( %s , %s \\right)' % (ratl_maps[0]._latex_(), ratl_maps[1]._latex_())


    ###########################
    # Private Common Functions
    ###########################

    # delete the hash value
    def __clear_cached_values(self):
        r"""
        A private function to clear the hash if the codomain has been
        modified by a pre or post isomorphism.

        EXAMPLES::

            sage: F = GF(7)
            sage: E = EllipticCurve(j=F(0))
            sage: phi = EllipticCurveIsogeny(E, [E((0,-1)), E((0,1))])
            sage: old_hash = hash(phi)
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: phi.set_post_isomorphism(WeierstrassIsomorphism(phi.codomain(), (-1,2,-3,4)))
            sage: hash(phi) == old_hash
            False

            sage: R.<x> = QQ[]
            sage: E = EllipticCurve(QQ, [0,0,0,1,0])
            sage: phi = EllipticCurveIsogeny(E, x)
            sage: old_ratl_maps = phi.rational_maps()
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: phi.set_post_isomorphism(WeierstrassIsomorphism(phi.codomain(), (-1,0,0,0)))
            sage: old_ratl_maps == phi.rational_maps()
            False
            sage: old_ratl_maps[1] == -phi.rational_maps()[1]
            True

            sage: F = GF(127); R.<x> = F[]
            sage: E = EllipticCurve(j=F(1728))
            sage: f = x^5 + 43*x^4 + 97*x^3 + 81*x^2 + 42*x + 82
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: old_hash = hash(phi)
            sage: old_ratl_maps = phi.rational_maps()
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: phi.set_post_isomorphism(WeierstrassIsomorphism(phi.codomain(), (-13,13,-13,13)))
            sage: old_hash == hash(phi)
            False
            sage: old_ratl_maps == phi.rational_maps()
            False
            sage: phi._EllipticCurveIsogeny__clear_cached_values()

        """
        self.__this_hash = None
        self.__rational_maps_initialized = False
        self.__X_coord_rational_map = None
        self.__Y_coord_rational_map = None
        self.__dual = None

    # performs the inheritance house keeping
    def __perform_inheritance_housekeeping(self):
        r"""
        Internal helper function, sets values on the super classes of
        this class.

        EXAMPLES:

        The following examples will implicitly exercise this
        function::

            sage: E = EllipticCurve(GF(43), [2,3,5,7,11])
            sage: R.<x> = GF(43)[]; f = x + 42
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: phi._EllipticCurveIsogeny__perform_inheritance_housekeeping()
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: E2 = phi.codomain()
            sage: post_isom = WeierstrassIsomorphism(E2, (41, 37, 31, 29))
            sage: phi.set_post_isomorphism(post_isom)
            sage: E1pr = WeierstrassIsomorphism(E, (-1, 2, -3, 4)).codomain().codomain()
            sage: pre_isom = E1pr.isomorphism_to(E)
            sage: phi.set_pre_isomorphism(pre_isom)

        """
        # one of the superclasses uses these fields
        self._domain = self.__E1
        self._codomain = self.__E2

        # sets up the parent
        parent = homset.Hom(self.__E1, self.__E2)
        Morphism.__init__(self, parent)

    def __init_algebraic_structs(self, E):
        r"""
        An internal function for EllipticCurveIsogeny objects that
        sets up the member variables necessary for algebra.

        EXAMPLES::

            sage: E = EllipticCurve(j=GF(17)(0))
            sage: phi = EllipticCurveIsogeny(E,  E((-1,0)))

        The constructor calls this function itself, so the fields it
        sets are already defined::

            sage: phi._EllipticCurveIsogeny__E1
            Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 17
            sage: phi._EllipticCurveIsogeny__base_field
            Finite Field of size 17
            sage: phi._EllipticCurveIsogeny__poly_ring
            Univariate Polynomial Ring in x over Finite Field of size 17
            sage: phi._EllipticCurveIsogeny__mpoly_ring
            Multivariate Polynomial Ring in x, y over Finite Field of size 17
            sage: phi._EllipticCurveIsogeny__intermediate_domain
            Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 17

        Now, calling the initialization function does nothing more::

            sage: phi._EllipticCurveIsogeny__init_algebraic_structs(E)
            sage: phi._EllipticCurveIsogeny__E1
            Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 17
            sage: phi._EllipticCurveIsogeny__base_field
            Finite Field of size 17
            sage: phi._EllipticCurveIsogeny__poly_ring
            Univariate Polynomial Ring in x over Finite Field of size 17
            sage: phi._EllipticCurveIsogeny__mpoly_ring
            Multivariate Polynomial Ring in x, y over Finite Field of size 17
            sage: phi._EllipticCurveIsogeny__intermediate_domain
            Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 17

            sage: E = EllipticCurve(QQ, [0,0,0,1,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: phi._EllipticCurveIsogeny__init_algebraic_structs(E)
            sage: phi._EllipticCurveIsogeny__E1
            Elliptic Curve defined by y^2 = x^3 + x over Rational Field
            sage: phi._EllipticCurveIsogeny__base_field
            Rational Field
            sage: phi._EllipticCurveIsogeny__poly_ring
            Univariate Polynomial Ring in x over Rational Field
            sage: phi._EllipticCurveIsogeny__mpoly_ring
            Multivariate Polynomial Ring in x, y over Rational Field
            sage: phi._EllipticCurveIsogeny__intermediate_domain
            Elliptic Curve defined by y^2 = x^3 + x over Rational Field

            sage: F = GF(19); R.<x> = F[]
            sage: E = EllipticCurve(j=GF(19)(0))
            sage: phi = EllipticCurveIsogeny(E, x)
            sage: phi._EllipticCurveIsogeny__init_algebraic_structs(E)
            sage: phi._EllipticCurveIsogeny__E1
            Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 19
            sage: phi._EllipticCurveIsogeny__base_field
            Finite Field of size 19
            sage: phi._EllipticCurveIsogeny__poly_ring
            Univariate Polynomial Ring in x over Finite Field of size 19
            sage: phi._EllipticCurveIsogeny__mpoly_ring
            Multivariate Polynomial Ring in x, y over Finite Field of size 19
            sage: phi._EllipticCurveIsogeny__intermediate_domain
            Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 19
        """
        self.__E1 = E
        self.__base_field = E.base_ring()
        self.__poly_ring = PolynomialRing(self.__base_field, ['x'])
        self.__mpoly_ring = PolynomialRing(self.__base_field, ['x','y'])
        from sage.rings.all import FractionField
        self.__xfield = FractionField(self.__poly_ring)
        self.__xyfield = FractionField(self.__mpoly_ring)
        self.__intermediate_domain = E

    def __compute_E2(self):
        r"""
        Private function that computes and sets the isogeny codomain.

        EXAMPLES:

        These examples inherently exercise this function::

            sage: E = EllipticCurve(j=GF(7)(1728))
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: phi.codomain()
            Elliptic Curve defined by y^2 = x^3 + 3*x over Finite Field of size 7
            sage: phi._EllipticCurveIsogeny__compute_E2()

            sage: R.<x> = GF(7)[]
            sage: phi = EllipticCurveIsogeny(E, x)
            sage: phi.codomain()
            Elliptic Curve defined by y^2 = x^3 + 3*x over Finite Field of size 7
            sage: phi._EllipticCurveIsogeny__compute_E2()

        """

        if ("velu" == self.__algorithm):
            E2 = self.__compute_E2_via_velu()
        elif ("kohel" == self.__algorithm):
            E2 = self.__compute_E2_via_kohel()

        self.__E2 = E2
        self.__intermediate_codomain = E2

    # initializes the rational maps fields
    def __initialize_rational_maps(self, precomputed_maps=None):
        r"""
        Private function that computes and initializes the rational
        maps.

        INPUT:

        - ``precomputed_maps`` (default None) -- tuple (X,Y) of
          rational functions in x,y

        EXAMPLES:

        The following examples inherently exercise this function::

            sage: E = EllipticCurve(j=GF(7)(1728))
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: phi._EllipticCurveIsogeny__initialize_rational_maps()
            sage: phi.rational_maps()
            ((x^2 + 1)/x, (x^2*y - y)/x^2)

            sage: R.<x> = GF(7)[]
            sage: phi = EllipticCurveIsogeny(E, x)
            sage: phi = EllipticCurveIsogeny(E, x)
            sage: phi.rational_maps()
            ((x^2 + 1)/x, (x^2*y - y)/x^2)
            sage: phi._EllipticCurveIsogeny__initialize_rational_maps()

            sage: E = EllipticCurve([1,2,3,4,5])
            sage: Eshort = E.short_weierstrass_model()
            sage: phi = E.isogeny(E(0), Eshort)
            sage: phiX, phiY = phi.rational_maps()
            sage: phiX(1,2), phiY(1,2)
            (63, 864)
        """
        if self.__rational_maps_initialized:
            return

        if precomputed_maps is None:
            if ("velu"==self.__algorithm):
                (X_map, Y_map) = self.__initialize_rational_maps_via_velu()

            if ("kohel"==self.__algorithm):
                (X_map, Y_map) = self.__initialize_rational_maps_via_kohel()
        else:
            X_map, Y_map = precomputed_maps
            # cannot coerce directly in xfield for some reason
            X_map = self.__poly_ring(X_map.numerator())/self.__poly_ring(X_map.denominator())

        if self.__prei_x_coord_ratl_map is not None:
            prei_X_map = self.__prei_x_coord_ratl_map
            prei_Y_map = self.__prei_y_coord_ratl_map
            X_map = X_map(prei_X_map)
            Y_map = Y_map([prei_X_map, prei_Y_map])

        if self.__posti_x_coord_ratl_map is not None:
            # Do not reverse the order here!
            Y_map = self.__posti_y_coord_ratl_map([X_map, Y_map])
            X_map = self.__posti_x_coord_ratl_map(X_map)

        self.__X_coord_rational_map = self.__xfield(X_map)
        self.__Y_coord_rational_map = self.__xyfield(Y_map)
        self.__rational_maps_initialized = True


    def __init_kernel_polynomial(self):
        r"""
        Private function that initializes the kernel polynomial (if
        the algorithm does not take it as a parameter).

        EXAMPLES:

        The following examples inherently exercise this function::

            sage: E = EllipticCurve(j=GF(7)(1728))
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: phi.kernel_polynomial()
            x
            sage: phi._EllipticCurveIsogeny__init_kernel_polynomial()
            [0, 1]
        """

        if (self.__kernel_polynomial_list is not None):
            return self.__kernel_polynomial_list

        if ("velu" == self.__algorithm):
            ker_poly_list = self.__init_kernel_polynomial_velu()
        else:
            raise ValueError("The kernel polynomial should already be defined!")

        return ker_poly_list

    def __set_pre_isomorphism(self, domain, isomorphism):
        r"""
        Private function to set the pre isomorphism and domain (and
        keep track of the domain of the isogeny).

        EXAMPLES::

            sage: E = EllipticCurve(GF(43), [2,3,5,7,11])
            sage: R.<x> = GF(43)[]; f = x + 42
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: phi._EllipticCurveIsogeny__perform_inheritance_housekeeping()
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: E1pr = WeierstrassIsomorphism(E, (-1, 2, -3, 4)).codomain().codomain()
            sage: pre_isom = E1pr.isomorphism_to(E)
            sage: phi.set_pre_isomorphism(pre_isom)
            sage: phi._EllipticCurveIsogeny__set_pre_isomorphism(E, WeierstrassIsomorphism(E, (-1, 3, -3, 4)))
            sage: E == phi.domain()
            True

        """

        self.__E1 = domain

        # set the isomorphism
        self.__pre_isomorphism = isomorphism

        # calculate the isomorphism as a rational map.

        u, r, s, t = [self.__base_field(c) for c in isomorphism.tuple()]
        uinv = 1/u
        uinv2 = uinv**2
        uinv3 = uinv*uinv2

        x = self.__poly_ring.gen()
        y = self.__xyfield.gen(1) # not mpoly_ring.gen(1) else we end
                                  # up in K(x)[y] and trouble ensues

        self.__prei_x_coord_ratl_map = (x - r) * uinv2
        self.__prei_y_coord_ratl_map = (y - s*(x-r) - t) * uinv3

        if (self.__kernel_polynomial is not None):
            ker_poly = self.__kernel_polynomial
            ker_poly = ker_poly(self.__prei_x_coord_ratl_map)
            self.__kernel_polynomial = ker_poly.monic()

        self.__perform_inheritance_housekeeping()


    def __set_post_isomorphism(self, codomain, isomorphism):
        r"""
        Private function to set the post isomorphism and codomain (and
        keep track of the codomain of the isogeny).

        EXAMPLES:

        The following examples inherently exercise this function::

            sage: E = EllipticCurve(j=GF(7)(1728))
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: E2 = phi.codomain()
            sage: isom = WeierstrassIsomorphism(E2, (-1,2,-3,4))
            sage: phi.set_post_isomorphism(isom)
            sage: phi._EllipticCurveIsogeny__set_post_isomorphism(E2, WeierstrassIsomorphism(phi.codomain(), (1,-2,3,-4)))
            sage: E2 == phi.codomain()
            True

        """

        # set the codomains
        self.__E2 = codomain

        # set the isomorphism
        self.__post_isomorphism = isomorphism

        # calculate the isomorphism as a rational map.

        u, r, s, t = [self.__base_field(c) for c in isomorphism.tuple()]
        uinv = 1/u
        uinv2 = uinv**2
        uinv3 = uinv*uinv2

        x = self.__poly_ring.gen()
        y = self.__xyfield.gen(1)

        self.__posti_x_coord_ratl_map = (x - r) * uinv2
        self.__posti_y_coord_ratl_map = (y - s*(x-r) - t) * uinv3

        self.__perform_inheritance_housekeeping()

    def __setup_post_isomorphism(self, codomain, model):
        r"""
        Private function to set up the post isomorphism given the
        codomain.

        EXAMPLES:

        The following examples inherently exercise this function::

            sage: E = EllipticCurve(j=GF(7)(1728))
            sage: E2 = EllipticCurve(GF(7), [0,0,0,5,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)), E2); phi
            Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + x over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 5*x over Finite Field of size 7
            sage: E3 = EllipticCurve(GF(7), [0,0,0,6,0])
            sage: phi._EllipticCurveIsogeny__setup_post_isomorphism(E3, None)
            sage: phi
            Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + x over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 6*x over Finite Field of size 7

            sage: R.<x> = QQ[]
            sage: E = EllipticCurve(j=1728)
            sage: f = x^3 - x
            sage: phi = EllipticCurveIsogeny(E, f, model='minimal'); phi
            Isogeny of degree 4 from Elliptic Curve defined by y^2 = x^3 - x over Rational Field to Elliptic Curve defined by y^2 = x^3 - x over Rational Field

            sage: phi = EllipticCurveIsogeny(E, f, model=None)
            sage: phi._EllipticCurveIsogeny__setup_post_isomorphism(None, 'minimal')
            sage: phi
            Isogeny of degree 4 from Elliptic Curve defined by y^2 = x^3 - x over Rational Field to Elliptic Curve defined by y^2 = x^3 - x over Rational Field

        """
        # TODO: add checks to make sure that codomain and model
        # parameters are consistent with the algorithm used.

        post_isom = None
        newE2 = None

        oldE2 = self.__E2

        if (model is not None):

            if (codomain is not None):
                raise ValueError("Cannot specify a codomain and model flag simultaneously.")

            if ('minimal' == model):

                if (not is_NumberField(oldE2.base_field())):
                    raise ValueError("specifying minimal for model flag only valid with curves over number fields.")

                newE2 = oldE2.global_minimal_model(semi_global=True)
                post_isom = oldE2.isomorphism_to(newE2)

            else:
                raise ValueError("Unknown value of model flag.")

        elif (codomain is not None):
            if (not is_EllipticCurve(codomain)):
                raise ValueError("Codomain parameter must be an elliptic curve.")

            if (not oldE2.is_isomorphic(codomain)):
                raise ValueError("Codomain parameter must be isomorphic to computed codomain isogeny")

            newE2 = codomain
            post_isom = oldE2.isomorphism_to(newE2)

        if (post_isom is not None):
            self.__set_post_isomorphism(newE2, post_isom)

        return

    ###########################
    # Velu's Formula Functions
    ###########################

    #
    # Setup function for Velu's formula
    #

    def __init_from_kernel_list(self, kernel_gens):
        r"""
        Private function that initializes the isogeny from a list of
        points which generate the kernel (For Velu's formulas.)

        EXAMPLES:

        The following example inherently exercises this function::

            sage: E = EllipticCurve(GF(7), [0,0,0,-1,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0))); phi
            Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 6*x over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 4*x over Finite Field of size 7
            sage: phi._EllipticCurveIsogeny__init_from_kernel_list([E(0), E((0,0))])

        The following example demonstrates the necessity of avoiding any calls
        to P.order(), since such calls involve factoring the group order which
        could take a long time. ::

            sage: p = 12 * next_prime(2^180) * next_prime(2^194) - 1
            sage: F = FiniteField(p, proof=False)
            sage: E = EllipticCurve([F(1), F(0)])
            sage: P = E(0).division_points(3)[1]
            sage: EllipticCurveIsogeny(E, P)
            Isogeny of degree 3 from Elliptic Curve defined by y^2 = x^3 + x over Finite Field of size 461742260113997803268895001173557974278278194575766957660028841364655249961609425998827452443620996655395008156411 to Elliptic Curve defined by y^2 = x^3 + 80816485163488178037199320944019099858815874115367810482828676054000067654558381377552245721755005198633191074893*x + 301497584865165444049833326660609767433467459033532853758006118022998267706948164646650354324860226263546558337993 over Finite Field of size 461742260113997803268895001173557974278278194575766957660028841364655249961609425998827452443620996655395008156411

        """
        if self.__check :
            for P in kernel_gens:
                if not P.has_finite_order():
                    raise ValueError("The points in the kernel must be of finite order.")

        # Compute a list of points in the subgroup generated by the
        # points in kernel_gens.  This is very naive: when finite
        # subgroups are implemented better, this could be simplified,
        # but it won't speed things up too much.

        kernel_set = Set([self.__E1(0)])
        from sage.misc.all import flatten

        def all_multiples(itr, terminal):
            mult_list = [terminal]
            R = terminal + itr
            while R != terminal:
                mult_list.append(R)
                R = R + itr
            return mult_list
        for P in kernel_gens:
            kernel_set += Set(flatten([all_multiples(P,Q)
                                       for Q in kernel_set]))
        self.__kernel_list = kernel_set.list()
        self.__kernel_2tor = {}
        self.__kernel_non2tor = {}
        self.__degree = Integer(len(kernel_set))
        self.__sort_kernel_list()

    #
    # Precompute the values in Velu's Formula.
    #
    def __sort_kernel_list(self):
        r"""
        Private function that sorts the list of points in the kernel
        (For Velu's formulas). Sorts out the 2 torsion points, and
        puts them in a dictionary.

        EXAMPLES:

        The following example inherently exercises this function::

            sage: E = EllipticCurve(GF(7), [0,0,0,-1,0])
            sage: P = E((4,2))
            sage: phi = EllipticCurveIsogeny(E, P); phi
            Isogeny of degree 4 from Elliptic Curve defined by y^2 = x^3 + 6*x over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 2*x over Finite Field of size 7
            sage: phi._EllipticCurveIsogeny__kernel_2tor = {}
            sage: phi._EllipticCurveIsogeny__kernel_non2tor = {}
            sage: phi._EllipticCurveIsogeny__sort_kernel_list()

        """

        a1,a2,a3,a4,a6 = self.__E1.ainvs()

        v = 0
        w = 0

        for Q in self.__kernel_list:

            if Q.is_zero():
                continue

            (xQ,yQ) = Q.xy()

            gxQ = 3*xQ**2 + 2*a2*xQ + a4 - a1*yQ
            gyQ = -2*yQ - a1*xQ - a3

            uQ = gyQ**2

            # sort torsion points:
            if (2*yQ == -a1*xQ - a3): # Q is 2-torsion
                vQ = gxQ
                self.__kernel_2tor[xQ] = (xQ,yQ,gxQ,gyQ,vQ,uQ)
                v = v + vQ
                w = w + (uQ + xQ*vQ)
            elif xQ not in self.__kernel_non2tor: # Q is not a 2-torsion
                vQ = 2*gxQ - a1*gyQ
                self.__kernel_non2tor[xQ] = (xQ,yQ,gxQ,gyQ,vQ,uQ)
                v = v + vQ
                w = w + (uQ + xQ*vQ)

        self.__v = v
        self.__w = w

    #
    # Velu's formula computing the codomain curve
    #
    def __compute_E2_via_velu(self):
        r"""
        Private function that computes the codomain via Velu's
        formulas.

        EXAMPLES:

        The following example inherently exercises this function::

            sage: E = EllipticCurve(GF(7), [0,0,0,-1,0])
            sage: P = E((4,2))
            sage: phi = EllipticCurveIsogeny(E, P)
            sage: phi.codomain()
            Elliptic Curve defined by y^2 = x^3 + 2*x over Finite Field of size 7
            sage: phi._EllipticCurveIsogeny__compute_E2_via_velu()
            Elliptic Curve defined by y^2 = x^3 + 2*x over Finite Field of size 7

        """
        v = self.__v
        w = self.__w

        return compute_codomain_formula(self.__E1, v,w)


    def __velu_sum_helper(self, Qvalues, a1, a3, x, y):
        r"""
        Private function for Velu's formulas, helper function to help
        perform the summation.

        EXAMPLES:

        The following example inherently exercises this function::

            sage: E = EllipticCurve(GF(7), [0,0,0,-1,0])
            sage: P = E((4,2))
            sage: phi = EllipticCurveIsogeny(E, P)
            sage: Q = E((0,0)); phi(Q)
            (0 : 0 : 1)
            sage: phi.rational_maps()
            ((x^4 - 2*x^3 + x^2 - 3*x)/(x^3 - 2*x^2 + 3*x - 2), (x^5*y - 2*x^3*y - x^2*y - 2*x*y + 2*y)/(x^5 + 3*x^3 + 3*x^2 + x - 1))

            sage: F = GF(7)
            sage: E = EllipticCurve(F, [0,0,0,1,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)) )
            sage: Qvals = phi._EllipticCurveIsogeny__kernel_2tor[0]
            sage: phi._EllipticCurveIsogeny__velu_sum_helper(Qvals, 0, 0, F(5), F(5))
            (3, 3)
            sage: R.<x,y> = GF(7)[]
            sage: phi._EllipticCurveIsogeny__velu_sum_helper(Qvals, 0, 0, x, y)
            (1/x, y/x^2)

        """
        xQ = Qvalues[0]
        yQ = Qvalues[1]
        gxQ = Qvalues[2]
        gyQ = Qvalues[3]
        vQ = Qvalues[4]
        uQ = Qvalues[5]

        t1 = x - xQ
        inv_t1 = t1**-1
        inv_t1_2 = inv_t1**2
        inv_t1_3 = inv_t1_2*inv_t1

        tX = vQ*inv_t1 + uQ*(inv_t1_2)

        tY0 = uQ*(2*y + a1*x + a3)
        tY1 = vQ*(a1*t1 + y - yQ)
        tY2 = a1*uQ - gxQ*gyQ

        # Without this explicit coercion, tY ends up in K(x)[y]
        # instead of K(x,y), and trouble ensues!
        from sage.rings.all import FractionField
        F = FractionField(y.parent())
        tY =  ( tY0*F(inv_t1_3) + (tY1 + tY2)*F(inv_t1_2) )

        return (tX, tY)


    def __compute_via_velu_numeric(self, xP, yP):
        r"""
        Private function that sorts the list of points in the kernel
        (for Velu's formulas). Sorts out the 2 torsion points, and
        puts them in a dictionary.

        EXAMPLES:

        The following example inherently exercises this function::

            sage: F = GF(7)
            sage: E = EllipticCurve(F, [0,0,0,-1,0])
            sage: P = E((4,2))
            sage: phi = EllipticCurveIsogeny(E, P)
            sage: Q = E((0,0)); phi(Q)
            (0 : 0 : 1)
            sage: Q = E((-1,0)); phi(Q)
            (0 : 0 : 1)
            sage: phi._EllipticCurveIsogeny__compute_via_velu_numeric(F(0), F(0))
            (0, 0)
            sage: phi._EllipticCurveIsogeny__compute_via_velu_numeric(F(-1), F(0))
            (0, 0)

        """
        # first check if the point is in the kernel
        if xP in self.__kernel_2tor or xP in self.__kernel_non2tor:
            return self.__intermediate_codomain(0)

        outP = self.__compute_via_velu(xP,yP)

        return outP


    def __compute_via_velu(self, xP, yP):
        r"""
        Private function for Velu's formulas, to perform the summation.

        EXAMPLES:

        The following example inherently exercises this function::

            sage: F = GF(7)
            sage: E = EllipticCurve(F, [0,0,0,-1,0])
            sage: P = E((4,2))
            sage: phi = EllipticCurveIsogeny(E, P)
            sage: Q = E((0,0)); phi(Q)
            (0 : 0 : 1)
            sage: phi.rational_maps()
            ((x^4 - 2*x^3 + x^2 - 3*x)/(x^3 - 2*x^2 + 3*x - 2), (x^5*y - 2*x^3*y - x^2*y - 2*x*y + 2*y)/(x^5 + 3*x^3 + 3*x^2 + x - 1))
            sage: phi._EllipticCurveIsogeny__compute_via_velu(F(0), F(0))
            (0, 0)
            sage: R.<x,y> = GF(7)[]
            sage: phi._EllipticCurveIsogeny__compute_via_velu(x, y)
            ((x^4 - 2*x^3 + x^2 - 3*x)/(x^3 - 2*x^2 + 3*x - 2),
             (x^5*y - 2*x^3*y - x^2*y - 2*x*y + 2*y)/(x^5 + 3*x^3 + 3*x^2 + x - 1))
        """
        ker_2tor = self.__kernel_2tor
        ker_non2tor = self.__kernel_non2tor

        X = 0
        Y = 0

        a1 = self.__E1.a1()
        a3 = self.__E1.a3()

        # next iterate over the 2torsion points of the kernel
        for Qvalues in ker_2tor.values():
            (tX, tY) = self.__velu_sum_helper(Qvalues, a1, a3, xP, yP)
            X = X + tX
            Y = Y + tY

        for Qvalues in ker_non2tor.values():
            (tX, tY) = self.__velu_sum_helper(Qvalues, a1, a3, xP, yP)
            X = X + tX
            Y = Y + tY

        X = xP + X
        Y = yP - Y

        return (X,Y)


    def __initialize_rational_maps_via_velu(self):
        r"""
        Private function for Velu's formulas, helper function to
        initialize the rational maps.

        EXAMPLES:

        The following example inherently exercises this function::

            sage: E = EllipticCurve(GF(7), [0,0,0,-1,0])
            sage: P = E((4,2))
            sage: phi = EllipticCurveIsogeny(E, P)
            sage: phi.rational_maps()
            ((x^4 - 2*x^3 + x^2 - 3*x)/(x^3 - 2*x^2 + 3*x - 2), (x^5*y - 2*x^3*y - x^2*y - 2*x*y + 2*y)/(x^5 + 3*x^3 + 3*x^2 + x - 1))
            sage: phi._EllipticCurveIsogeny__initialize_rational_maps_via_velu()
            ((x^4 + 5*x^3 + x^2 + 4*x)/(x^3 + 5*x^2 + 3*x + 5), (x^5*y - 2*x^3*y - x^2*y - 2*x*y + 2*y)/(x^5 + 3*x^3 + 3*x^2 + x - 1))
        """
        x = self.__poly_ring.gen()
        y = self.__mpoly_ring.gen(1)

        return self.__compute_via_velu(x,y)


    def __init_kernel_polynomial_velu(self):
        r"""
        Private function for Velu's formulas, helper function to
        initialize the rational maps.

        EXAMPLES:

        The following example inherently exercises this function::

            sage: E = EllipticCurve(GF(7), [0,0,0,-1,0])
            sage: P = E((4,2))
            sage: phi = EllipticCurveIsogeny(E, P)
            sage: phi.kernel_polynomial()
            x^2 + 2*x + 4
            sage: phi._EllipticCurveIsogeny__init_kernel_polynomial_velu()
            [4, 2, 1]
        """
        poly_ring = self.__poly_ring
        x = poly_ring.gen()

        invX = 0

        if (self.__pre_isomorphism is not None):
            pre_isom = self.__pre_isomorphism
            u = pre_isom.u
            r = pre_isom.r
            invX = (u**2)*x + r
        else:
            invX = x

        psi = poly_ring(1)

        for Qvalues in self.__kernel_2tor.values():
            xQ = invX(x=Qvalues[0])
            psi = psi*(x - xQ)

        for Qvalues in self.__kernel_non2tor.values():
            xQ = invX(x=Qvalues[0])
            psi = psi*(x - xQ)

        ker_poly_list = psi.list()

        self.__kernel_polynomial_list = ker_poly_list
        self.__kernel_polynomial = psi

        return ker_poly_list



    ###################################
    # Kohel's Variant of Velu's Formula
    ###################################

    def __init_from_kernel_polynomial(self, kernel_polynomial):
        r"""
        Private function that initializes the isogeny from a kernel
        polynomial.

        EXAMPLES:

        These examples inherently exercise this private function::

            sage: R.<x> = GF(7)[]
            sage: E = EllipticCurve(GF(7), [0,0,0,-1,0])
            sage: phi = EllipticCurveIsogeny(E, x);phi
            Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 6*x over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 4*x over Finite Field of size 7

            sage: phi._EllipticCurveIsogeny__init_from_kernel_polynomial(x)

            sage: E = EllipticCurve(GF(7), [0,-1,0,0,1])
            sage: phi = EllipticCurveIsogeny(E, x+6, degree=3); phi
            Isogeny of degree 3 from Elliptic Curve defined by y^2 = x^3 + 6*x^2 + 1 over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 6*x^2 + 4*x + 2 over Finite Field of size 7

            sage: phi._EllipticCurveIsogeny__init_from_kernel_polynomial(x+6)

        """
        poly_ring = self.__poly_ring
        E = self.__E1

        # Convert to a univariate polynomial, even if it had a
        # bivariate parent, or was given as a list:
        self.__kernel_polynomial = psi = poly_ring(kernel_polynomial)

        if psi.leading_coefficient() != 1:
            raise ValueError("The kernel polynomial must be monic.")

        self.__kernel_polynomial_list = psi.list()

        #
        # Determine if kernel polynomial is entirely a two torsion
        #
        psi_G = two_torsion_part(E, psi).monic()

        if (0 != psi_G.degree()): # even degree case

            psi_quo = psi//psi_G

            if (0 != psi_quo.degree()):
                raise NotImplementedError("For basic Kohel's algorithm, if the kernel degree is even then the kernel must be contained in the two torsion.")

            (phi, omega, v, w, n, d) = self.__init_even_kernel_polynomial(E, psi_G)

        else: # odd degree case

            (phi, omega, v, w, n, d) = self.__init_odd_kernel_polynomial(E, psi)


        #
        # Set up the necessary instance variables
        #

        self.__kernel_polynomial = psi
        self.__inner_kernel_polynomial = psi

        self.__degree = Integer(d)  # degree of the isogeny

        # As a rational map, the isogeny maps (x,y) to (X,Y), where
        # X=phi(x)/psi(x)^2 and Y=omega(x,y)/psi(x)^3.  Both phi and
        # psi are univariate polynomials in x, while omega is a
        # bivariate polynomial in x, y.  The names are compatible so
        # that univariate polynomials automatically coerce into the
        # bivariate polynomial ring.

        self.__psi = psi
        self.__phi = phi
        self.__omega = omega

        self.__v = v
        self.__w = w

    def __init_even_kernel_polynomial(self, E, psi_G):
        r"""
        Return the isogeny parameters for the 2-part of an isogeny.

        INPUT:

        - ``E`` -- an elliptic curve

        - ``psi_G`` -- a univariate polynomial over the base field of
          ``E`` of degree 1 or 3 dividing its 2-division polynomial

        OUTPUT:

        (phi, omega, v, w, n, d) where:

        - ``phi`` is a univariate polynomial, the numerator of the
          `X`-coordinate of the isogeny;

        - ``omega`` is a bivariate polynomial, the numerator of the
          `Y`-coordinate of the isogeny;

        - ``v``, ``w`` are the Velu parameters of the isogeny;

        - ``n`` is the degree of ``psi``;

        - ``d`` is the degree of the isogeny.

        EXAMPLES:

        These examples inherently exercise this private function::

            sage: R.<x> = GF(7)[]
            sage: E = EllipticCurve(GF(7), [-1,0])
            sage: phi = EllipticCurveIsogeny(E, x); phi
            Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 6*x over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 4*x over Finite Field of size 7

            sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import two_torsion_part
            sage: psig = two_torsion_part(E,x)
            sage: phi._EllipticCurveIsogeny__init_even_kernel_polynomial(E,psig)
            (x^3 + 6*x, x^3*y + x*y, 6, 0, 1, 2)

            sage: F = GF(2^4, 'alpha'); R.<x> = F[]
            sage: E = EllipticCurve(F, [1,1,0,1,0])
            sage: phi = EllipticCurveIsogeny(E, x); phi
            Isogeny of degree 2 from Elliptic Curve defined by y^2 + x*y = x^3 + x^2 + x over Finite Field in alpha of size 2^4 to Elliptic Curve defined by y^2 + x*y = x^3 + x^2 + 1 over Finite Field in alpha of size 2^4

            sage: psig = two_torsion_part(E,x)
            sage: phi._EllipticCurveIsogeny__init_even_kernel_polynomial(E,psig)
            (x^3 + x, x^3*y + x^2 + x*y, 1, 0, 1, 2)

            sage: E = EllipticCurve(GF(7), [0,-1,0,0,1])
            sage: R.<x> = GF(7)[]
            sage: f = x^3 + 6*x^2 + 1
            sage: phi = EllipticCurveIsogeny(E, f); phi
            Isogeny of degree 4 from Elliptic Curve defined by y^2 = x^3 + 6*x^2 + 1 over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 6*x^2 + 2*x + 5 over Finite Field of size 7
            sage: psig = two_torsion_part(E,f)
            sage: psig = two_torsion_part(E,f)
            sage: phi._EllipticCurveIsogeny__init_even_kernel_polynomial(E,psig)
            (x^7 + 5*x^6 + 2*x^5 + 6*x^4 + 3*x^3 + 5*x^2 + 6*x + 3, x^9*y - 3*x^8*y + 2*x^7*y - 3*x^3*y + 2*x^2*y + x*y - y, 1, 6, 3, 4)
        """
        #check if the polynomial really divides the two_torsion_polynomial
        if  self.__check and E.division_polynomial(2, x=self.__poly_ring.gen()) % psi_G  != 0 :
            raise ValueError("The polynomial {} does not define a finite subgroup of {}.".format(psi_G,E))

        n = psi_G.degree() # 1 or 3
        d = n+1            # 2 or 4

        base_field = self.__base_field
        char = base_field.characteristic()

        a1,a2,a3,a4,a6 = E.ainvs()
        b2,b4,_,_ = E.b_invariants()
        x = self.__poly_ring.gen()
        y = self.__mpoly_ring.gen(1)

        if (1 == n):
            x0 = -psi_G.constant_coefficient()

            # determine y0
            if (2 == char):
                y0 = (x0**3 + a2*x0**2 + a4*x0 + a6).sqrt()
            else:
                y0 = -(a1*x0 + a3)/2

            (v,w) = compute_vw_kohel_even_deg1(x0,y0,a1,a2,a4)

            phi = (x*psi_G + v)*psi_G
            omega = (y*psi_G**2 - v*(a1*psi_G + (y - y0)))*psi_G

        elif (3 == n):
            s = psi_G.list()
            s1 = -s[n-1]
            s2 = s[n-2]
            s3 = -s[n-3]

            psi_G_pr = psi_G.derivative()
            psi_G_prpr = psi_G_pr.derivative()

            phi = (psi_G_pr**2) + (-2*psi_G_prpr + (4*x - s1))*psi_G
            phi_pr = phi.derivative(x)

            psi_2 = 2*y + a1*x + a3

            omega = (psi_2*(phi_pr*psi_G - phi*psi_G_pr) - (a1*phi + a3*psi_G)*psi_G)/2

            phi = phi*psi_G
            omega = omega*psi_G

            (v,w) = compute_vw_kohel_even_deg3(b2,b4,s1,s2,s3)

        else:
            raise ValueError("input polynomial must be of degree 1 or 3, not %d" % n)

        return (phi, omega, v, w, n, d)


    def __init_odd_kernel_polynomial(self, E, psi):
        r"""
        Return the isogeny parameters for a cyclic isogeny of odd degree.

        INPUT:

        - ``E`` -- an elliptic curve

        - ``psi`` -- a univariate polynomial over the base field of
          ``E``, assumed to be a kernel polynomial

        OUTPUT:

        (phi, omega, v, w, n, d) where:

        - ``phi`` is a univariate polynomial, the numerator of the
          `X`-coordinate of the isogeny;

        - ``omega`` is a bivariate polynomial, the numerator of the
          `Y`-coordinate of the isogeny;

        - ``v``, ``w`` are the Velu parameters of the isogeny;

        - ``n`` is the degree of ``psi``;

        - ``d`` is the degree of the isogeny.

        EXAMPLES:

        These examples inherently exercise this private function::

            sage: R.<x> = GF(7)[]
            sage: E = EllipticCurve(GF(7), [0,-1,0,0,1])
            sage: phi = EllipticCurveIsogeny(E, x+6, degree=3); phi
            Isogeny of degree 3 from Elliptic Curve defined by y^2 = x^3 + 6*x^2 + 1 over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 6*x^2 + 4*x + 2 over Finite Field of size 7

            sage: R.<x> = GF(7)[]
            sage: phi._EllipticCurveIsogeny__init_odd_kernel_polynomial(E, x+6)
            (x^3 + 5*x^2 + 3*x + 2, x^3*y - 3*x^2*y + x*y, 2, 6, 1, 3)

            sage: F = GF(2^4, 'alpha'); R.<x> = F[]
            sage: alpha = F.gen()
            sage: E = EllipticCurve(F, [1,1,F.gen(),F.gen()^2+1,1])
            sage: f = x + alpha^2 + 1
            sage: phi = EllipticCurveIsogeny(E, f); phi
            Isogeny of degree 3 from Elliptic Curve defined by y^2 + x*y + alpha*y = x^3 + x^2 + (alpha^2+1)*x + 1 over Finite Field in alpha of size 2^4 to Elliptic Curve defined by y^2 + x*y + alpha*y = x^3 + x^2 + alpha*x + alpha^3 over Finite Field in alpha of size 2^4

            sage: R.<x> = F[]
            sage: f = x + alpha^2 + 1
            sage: phi._EllipticCurveIsogeny__init_odd_kernel_polynomial(E, f)
            (x^3 + (alpha^2 + 1)*x + alpha^3 + alpha^2 + alpha, x^3*y + (alpha^2 + 1)*x^2*y + (alpha^2 + alpha + 1)*x^2 + (alpha^2 + 1)*x*y + (alpha^2 + alpha)*x + (alpha)*y + (alpha), alpha^2 + alpha + 1, alpha^3 + alpha^2 + alpha, 1, 3)

            sage: E = EllipticCurve(j=-262537412640768000)
            sage: f = (E.isogenies_prime_degree()[0]).kernel_polynomial()
            sage: f.degree()
            81
            sage: E.isogeny(kernel=f, check=False)
            Isogeny of degree 163 from Elliptic Curve defined by y^2 + y = x^3 - 2174420*x + 1234136692 over Rational Field to Elliptic Curve defined by y^2 + y = x^3 - 57772164980*x - 5344733777551611 over Rational Field
        """
        n = psi.degree()
        d = 2*n + 1

        # check if the polynomial really divides the torsion polynomial :
        if self.__check:
            from .isogeny_small_degree import is_kernel_polynomial
            if not is_kernel_polynomial(E, d, psi):
                raise ValueError("The polynomial {} does not define a finite subgroup of {}.".format(psi,E))

        b2, b4, b6, _ = E.b_invariants()

        psi_coeffs = psi.list()

        s1 = 0
        s2 = 0
        s3 = 0

        if 1 <= n:
            s1 = -psi_coeffs[n-1]

        if 2 <= n:
            s2 = psi_coeffs[n-2]

        if 3 <= n:
            s3 = -psi_coeffs[n-3]

        # initializing these allows us to calculate E2.
        (v,w) = compute_vw_kohel_odd(b2,b4,b6,s1,s2,s3,n)

        # initialize the polynomial temporary variables

        psi_pr = psi.derivative()
        psi_prpr = psi_pr.derivative()

        x = self.__poly_ring.gen()

        phi = (4*x**3 + b2*x**2 + 2*b4*x + b6)*(psi_pr**2 - psi_prpr*psi) - \
                (6*x**2 + b2*x + b4)*psi_pr*psi + (d*x - 2*s1)*psi**2

        phi_pr = phi.derivative(x)

        if (2 != self.__base_field.characteristic()):
            omega = self.__compute_omega_fast(E, psi, psi_pr, phi, phi_pr)
        else:
            omega = self.__compute_omega_general(E, psi, psi_pr, phi, phi_pr)

        return (phi, omega, v, w, n, d)


    #
    # This is the fast omega computation that works when characteristic is not 2
    #
    def __compute_omega_fast(self, E, psi, psi_pr, phi, phi_pr):
        r"""
        Return omega from phi, psi and their derivatives, used when
        the characteristic field is not 2.

        INPUT:

        - ``E`` -- an elliptic curve.

        - ``psi, psi_pr, phi, phi_pr`` -- univariate polynomials over
          the base field of ``E``, where ``psi`` is the kernel
          polynomial and ``phi`` the numerator of the `X`-coordinate
          of the isogeny, together with their derivatives.

        OUTPUT:

        - ``omega`` -- a bivariate polynomial giving the numerator of
          the `Y`-coordinate of the isogeny.

        EXAMPLES:

        These examples inherently exercise this private function::

            sage: R.<x> = GF(7)[]
            sage: E = EllipticCurve(GF(7), [0,-1,0,0,1])
            sage: phi = EllipticCurveIsogeny(E, x+6, degree=3); phi
            Isogeny of degree 3 from Elliptic Curve defined by y^2 = x^3 + 6*x^2 + 1 over Finite Field of size 7 to Elliptic Curve defined by y^2 = x^3 + 6*x^2 + 4*x + 2 over Finite Field of size 7

            sage: R.<x,y> = GF(7)[]
            sage: psi = phi._EllipticCurveIsogeny__psi
            sage: psi_pr = psi.derivative()
            sage: fi = phi._EllipticCurveIsogeny__phi
            sage: fi_pr = fi.derivative()
            sage: phi._EllipticCurveIsogeny__compute_omega_fast(E, psi, psi_pr, fi, fi_pr)
            x^3*y - 3*x^2*y + x*y

        """

        a1 = E.a1()
        a3 = E.a3()

        x, y = self.__mpoly_ring.gens()

        psi_2 = 2*y + a1*x + a3

        # note, the formula below is correct
        # the formula in Kohel's thesis has some typos
        # notably the first plus sign should be a minus
        # as it is here below.

        return phi_pr*psi*psi_2/2 - phi*psi_pr*psi_2 - (a1*phi + a3*psi**2)*psi/2

    def __compute_omega_general(self, E, psi, psi_pr, phi, phi_pr):
        r"""
        Return omega from phi, psi and their derivatives, in any
        characteristic.

        INPUT:

        - ``E`` -- an elliptic curve.

        - ``psi, psi_pr, phi, phi_pr`` -- univariate polynomials over
          the base field of ``E``, where ``psi`` is the kernel
          polynomial and ``phi`` the numerator of the `X`-coordinate
          of the isogeny, together with their derivatives.

        OUTPUT:

        - ``omega`` -- a bivariate polynomial giving the numerator of
          the `Y`-coordinate of the isogeny.

        EXAMPLES:

        These examples inherently exercise this private function::

            sage: F = GF(2^4, 'alpha'); R.<x> = F[]
            sage: alpha = F.gen()
            sage: E = EllipticCurve(F, [1,1,F.gen(),F.gen()^2+1,1])
            sage: f = x + alpha^2 + 1
            sage: phi = EllipticCurveIsogeny(E, f); phi
            Isogeny of degree 3 from Elliptic Curve defined by y^2 + x*y + alpha*y = x^3 + x^2 + (alpha^2+1)*x + 1 over Finite Field in alpha of size 2^4 to Elliptic Curve defined by y^2 + x*y + alpha*y = x^3 + x^2 + alpha*x + alpha^3 over Finite Field in alpha of size 2^4

            sage: R.<x,y> = F[]
            sage: psi = phi._EllipticCurveIsogeny__psi
            sage: psi_pr = psi.derivative()
            sage: fi = phi._EllipticCurveIsogeny__phi
            sage: fi_pr = fi.derivative()
            sage: phi._EllipticCurveIsogeny__compute_omega_general(E, psi, psi_pr, fi, fi_pr)
            x^3*y + (alpha^2 + 1)*x^2*y + (alpha^2 + alpha + 1)*x^2 + (alpha^2 + 1)*x*y + (alpha^2 + alpha)*x + (alpha)*y + (alpha)

        A bug fixed in :trac:`7907`::

            sage: F = GF(128,'a')
            sage: a = F.gen()
            sage: E = EllipticCurve([1,0,0,0,(a**6+a**4+a**2+a)])
            sage: x = polygen(F)
            sage: ker =  (x^6 + (a^6 + a^5 + a^4 + a^3 + a^2 + a)*x^5 + (a^6 + a^5 + a^2 + 1)*x^4 + (a^6 + a^5 + a^4 + a^3 + a^2 + 1)*x^3 + (a^6 + a^3 + a)*x^2 + (a^4 + a^3 + 1)*x + a^5 + a^4 + a)
            sage: E.isogeny(ker)
            Isogeny of degree 13 from Elliptic Curve defined by y^2 + x*y = x^3 + (a^6+a^4+a^2+a) over Finite Field in a of size 2^7 to Elliptic Curve defined by y^2 + x*y = x^3 + (a^6+a^5+a^4+a^3+a^2+a)*x + (a^5+a^3) over Finite Field in a of size 2^7


        """
        a1, a2, a3, a4, a6 = E.ainvs()
        b2, b4, _, _ = E.b_invariants()

        n = psi.degree()
        d = 2 * n + 1

        x, y = self.__mpoly_ring.gens()

        psi_2 = 2 * y + a1 * x + a3

        psi_coeffs = psi.list()

        if (0 < n):
            s1 = -psi_coeffs[n - 1]
        else:
            s1 = 0

        psi_prpr = 0
        cur_x_pow = 1

        # Note: we now get the "derivatives" of psi
        # these are not actually the derivatives
        # furthermore, the formulas in Kohel's
        # thesis are wrong, the correct formulas
        # are coded below

        from sage.arith.all import binomial

        for j in range(n - 1):
            psi_prpr += binomial(j+2, 2) * psi_coeffs[(j+2)] * cur_x_pow
            cur_x_pow = x * cur_x_pow

        psi_prprpr = 0
        cur_x_pow = 1

        for j in range(n - 2):
            psi_prprpr += (3 * binomial(j+3,3)) * psi_coeffs[(j+3)] * cur_x_pow
            cur_x_pow = x * cur_x_pow

        omega = phi_pr*psi*y - phi*psi_pr*psi_2 + \
                ((a1*x + a3)*(psi_2**2)*(psi_prpr*psi_pr-psi_prprpr*psi) + \
                (a1*psi_2**2 - 3*(a1*x + a3)*(6*x**2 + b2*x + b4))*psi_prpr*psi + \
                (a1*x**3 + 3*a3*x**2 + (2*a2*a3 - a1*a4)*x + (a3*a4 - 2*a1*a6))*psi_pr**2 + \
                (-(3*a1*x**2 + 6*a3*x + (-a1*a4 + 2*a2*a3)) + \
                (a1*x + a3)*(d*x - 2*s1) )*psi_pr*psi + (a1*s1 + a3*n)*psi**2)*psi

        return omega

    def __compute_via_kohel_numeric(self, xP, yP):
        r"""
        Private function that computes the image of a point under this
        isogeny, using Kohel's formulas.

        EXAMPLES:

        These examples inherently exercise this private function::

            sage: R.<x> = GF(7)[]
            sage: E = EllipticCurve(GF(7), [0,-1,0,0,1])
            sage: phi = EllipticCurveIsogeny(E, x+6, degree=3)
            sage: P = E((0,1)); phi(P)
            (2 : 0 : 1)
            sage: P = E((1,1)); phi(P)
            (0 : 1 : 0)
            sage: phi._EllipticCurveIsogeny__compute_via_kohel_numeric(0, 1)
            (2, 0)
            sage: phi._EllipticCurveIsogeny__compute_via_kohel_numeric(1, 1)
            (0 : 1 : 0)

        """
        # first check if this point is in the kernel:

        if 0 == self.__inner_kernel_polynomial(x=xP):
            return self.__intermediate_codomain(0)

        (xP_out, yP_out) = self.__compute_via_kohel(xP, yP)

        # xP_out and yP_out do not always get evaluated to field
        # elements but rather constant polynomials, so we do some
        # explicit casting

        return (self.__base_field(xP_out), self.__base_field(yP_out))

    def __compute_via_kohel(self, xP, yP):
        r"""
        Private function that applies Kohel's formulas.

        EXAMPLES:

        These examples inherently exercise this private function::

            sage: R.<x> = GF(7)[]
            sage: E = EllipticCurve(GF(7), [0,-1,0,0,1])
            sage: phi = EllipticCurveIsogeny(E, x+6, degree=3)
            sage: P = E((0,1)); phi(P)
            (2 : 0 : 1)
            sage: phi.rational_maps()
            ((x^3 - 2*x^2 + 3*x + 2)/(x^2 - 2*x + 1), (x^3*y - 3*x^2*y + x*y)/(x^3 - 3*x^2 + 3*x - 1))
            sage: phi._EllipticCurveIsogeny__compute_via_kohel(0,1)
            (2, 0)
            sage: R.<x,y> = GF(7)[]
            sage: phi._EllipticCurveIsogeny__compute_via_kohel(x,y)
            ((x^3 - 2*x^2 + 3*x + 2)/(x^2 - 2*x + 1), (x^3*y - 3*x^2*y + x*y)/(x^3 - 3*x^2 + 3*x - 1))

        """
        a = self.__phi(xP)
        b = self.__omega(xP, yP)
        c = self.__psi(xP)
        cc = self.__mpoly_ring(c)

        return (a/c**2, b/cc**3)

    def __initialize_rational_maps_via_kohel(self):
        r"""
        Private function that computes and initializes the rational
        maps of this isogeny.

        EXAMPLES:

        These examples inherently exercise this private function::

            sage: R.<x> = GF(7)[]
            sage: E = EllipticCurve(GF(7), [0,-1,0,0,1])
            sage: phi = EllipticCurveIsogeny(E, x+6, degree=3)
            sage: phi.rational_maps()
            ((x^3 - 2*x^2 + 3*x + 2)/(x^2 - 2*x + 1), (x^3*y - 3*x^2*y + x*y)/(x^3 - 3*x^2 + 3*x - 1))
            sage: phi._EllipticCurveIsogeny__initialize_rational_maps_via_kohel()
            ((x^3 + 5*x^2 + 3*x + 2)/(x^2 + 5*x + 1), (x^3*y - 3*x^2*y + x*y)/(x^3 - 3*x^2 + 3*x - 1))


        """
        x = self.__poly_ring.gen()
        y = self.__mpoly_ring.gen(1)
        return self.__compute_via_kohel(x,y)

    #
    # Kohel's formula computing the codomain curve
    #
    def __compute_E2_via_kohel(self):
        r"""
        Private function that computes and initializes the codomain of
        the isogeny (via Kohel's.)

        EXAMPLES:

        These examples inherently exercise this private function::

            sage: R.<x> = GF(7)[]
            sage: E = EllipticCurve(GF(7), [0,-1,0,0,1])
            sage: phi = EllipticCurveIsogeny(E, x+6, degree=3)
            sage: phi.codomain()
            Elliptic Curve defined by y^2 = x^3 + 6*x^2 + 4*x + 2 over Finite Field of size 7
            sage: phi._EllipticCurveIsogeny__compute_E2_via_kohel()
            Elliptic Curve defined by y^2 = x^3 + 6*x^2 + 4*x + 2 over Finite Field of size 7

        """

        v = self.__v
        w = self.__w

        return compute_codomain_formula(self.__E1, v,w)

    #
    # public isogeny methods
    #

    def degree(self):
        r"""
        Return the degree of this isogeny.

        EXAMPLES::

            sage: E = EllipticCurve(QQ, [0,0,0,1,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: phi.degree()
            2
            sage: phi = EllipticCurveIsogeny(E, [0,1,0,1])
            sage: phi.degree()
            4

            sage: E = EllipticCurve(GF(31), [1,0,0,1,2])
            sage: phi = EllipticCurveIsogeny(E, [17, 1])
            sage: phi.degree()
            3

        """
        return self.__degree

    def rational_maps(self):
        r"""
        Return the pair of rational maps defining this isogeny.

        .. NOTE::

           Both components are returned as elements of the function
           field `F(x,y)` in two variables over the base field `F`,
           though the first only involves `x`.  To obtain the
           `x`-coordinate function as a rational function in `F(x)`,
           use :meth:`x_rational_map`.

        EXAMPLES::

            sage: E = EllipticCurve(QQ, [0,2,0,1,-1])
            sage: phi = EllipticCurveIsogeny(E, [1])
            sage: phi.rational_maps()
            (x, y)

            sage: E = EllipticCurve(GF(17), [0,0,0,3,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: phi.rational_maps()
            ((x^2 + 3)/x, (x^2*y - 3*y)/x^2)
        """
        if (not self.__rational_maps_initialized):
            self.__initialize_rational_maps()
        return (self.__xyfield(self.__X_coord_rational_map),
                self.__Y_coord_rational_map)

    def x_rational_map(self):
        r"""
        Return the rational map giving the `x`-coordinate of this isogeny.

        .. NOTE::

           This function returns the `x`-coordinate component of the
           isogeny as a rational function in `F(x)`, where `F` is the
           base field.  To obtain both coordinate functions as
           elements of the function field `F(x,y)` in two variables,
           use :meth:`rational_maps`.

        EXAMPLES::

            sage: E = EllipticCurve(QQ, [0,2,0,1,-1])
            sage: phi = EllipticCurveIsogeny(E, [1])
            sage: phi.x_rational_map()
            x

            sage: E = EllipticCurve(GF(17), [0,0,0,3,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: phi.x_rational_map()
            (x^2 + 3)/x
        """
        if (not self.__rational_maps_initialized):
            self.__initialize_rational_maps()
        return self.__X_coord_rational_map

    def is_separable(self):
        r"""
        Return whether or not this isogeny is separable.

        .. NOTE::

           This function always returns ``True`` as currently this
           class only implements separable isogenies.

        EXAMPLES::

            sage: E = EllipticCurve(GF(17), [0,0,0,3,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: phi.is_separable()
            True

            sage: E = EllipticCurve('11a1')
            sage: phi = EllipticCurveIsogeny(E, E.torsion_points())
            sage: phi.is_separable()
            True
        """
        return self.__separable

    def kernel_polynomial(self):
        r"""
        Return the kernel polynomial of this isogeny.

        EXAMPLES::

            sage: E = EllipticCurve(QQ, [0,0,0,2,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: phi.kernel_polynomial()
            x

            sage: E = EllipticCurve('11a1')
            sage: phi = EllipticCurveIsogeny(E, E.torsion_points())
            sage: phi.kernel_polynomial()
            x^2 - 21*x + 80

            sage: E = EllipticCurve(GF(17), [1,-1,1,-1,1])
            sage: phi = EllipticCurveIsogeny(E, [1])
            sage: phi.kernel_polynomial()
            1

            sage: E = EllipticCurve(GF(31), [0,0,0,3,0])
            sage: phi = EllipticCurveIsogeny(E, [0,3,0,1])
            sage: phi.kernel_polynomial()
            x^3 + 3*x
        """
        if self.__kernel_polynomial is None:
            self.__init_kernel_polynomial()

        return self.__kernel_polynomial


    def set_pre_isomorphism(self, preWI):
        r"""
        Modify this isogeny by precomposing with a Weierstrass isomorphism.

        EXAMPLES::

            sage: E = EllipticCurve(GF(31), [1,1,0,1,-1])
            sage: R.<x> = GF(31)[]
            sage: f = x^3 + 9*x^2 + x + 30
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: Epr = E.short_weierstrass_model()
            sage: isom = Epr.isomorphism_to(E)
            sage: phi.set_pre_isomorphism(isom)
            sage: phi.rational_maps()
            ((-6*x^4 - 3*x^3 + 12*x^2 + 10*x - 1)/(x^3 + x - 12), (3*x^7 + x^6*y - 14*x^6 - 3*x^5 + 5*x^4*y + 7*x^4 + 8*x^3*y - 8*x^3 - 5*x^2*y + 5*x^2 - 14*x*y + 14*x - 6*y - 6)/(x^6 + 2*x^4 + 7*x^3 + x^2 + 7*x - 11))
            sage: phi(Epr((0,22)))
            (13 : 21 : 1)
            sage: phi(Epr((3,7)))
            (14 : 17 : 1)

            sage: E = EllipticCurve(GF(29), [0,0,0,1,0])
            sage: R.<x> = GF(29)[]
            sage: f = x^2 + 5
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: phi
            Isogeny of degree 5 from Elliptic Curve defined by y^2 = x^3 + x over Finite Field of size 29 to Elliptic Curve defined by y^2 = x^3 + 20*x over Finite Field of size 29
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: inv_isom = WeierstrassIsomorphism(E, (1,-2,5,10))
            sage: Epr = inv_isom.codomain().codomain()
            sage: isom = Epr.isomorphism_to(E)
            sage: phi.set_pre_isomorphism(isom); phi
            Isogeny of degree 5 from Elliptic Curve defined by y^2 + 10*x*y + 20*y = x^3 + 27*x^2 + 6 over Finite Field of size 29 to Elliptic Curve defined by y^2 = x^3 + 20*x over Finite Field of size 29
            sage: phi(Epr((12,1)))
            (26 : 0 : 1)
            sage: phi(Epr((2,9)))
            (0 : 0 : 1)
            sage: phi(Epr((21,12)))
            (3 : 0 : 1)
            sage: phi.rational_maps()[0]
            (x^5 - 10*x^4 - 6*x^3 - 7*x^2 - x + 3)/(x^4 - 8*x^3 + 5*x^2 - 14*x - 6)

            sage: E = EllipticCurve('11a1')
            sage: R.<x> = QQ[]
            sage: f = x^2 - 21*x + 80
            sage: phi = EllipticCurveIsogeny(E, f); phi
            Isogeny of degree 5 from Elliptic Curve defined by y^2 + y = x^3 - x^2 - 10*x - 20 over Rational Field to Elliptic Curve defined by y^2 + y = x^3 - x^2 - 7820*x - 263580 over Rational Field
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: Epr = E.short_weierstrass_model()
            sage: isom = Epr.isomorphism_to(E)
            sage: phi.set_pre_isomorphism(isom)
            sage: phi
            Isogeny of degree 5 from Elliptic Curve defined by y^2 = x^3 - 13392*x - 1080432 over Rational Field to Elliptic Curve defined by y^2 + y = x^3 - x^2 - 7820*x - 263580 over Rational Field
            sage: phi(Epr((168,1188)))
            (0 : 1 : 0)
        """
        WIdom = preWI.domain().codomain()
        WIcod = preWI.codomain().codomain()

        if not isinstance(preWI, WeierstrassIsomorphism):
            raise ValueError("Invalid parameter: isomorphism must be of type Weierstrass isomorphism.")

        if (self.__E1 != WIcod):
            raise ValueError("Invalid parameter: isomorphism must have codomain curve equal to this isogenies' domain.")

        if (self.__pre_isomorphism is None):
            isom = preWI
            domain = WIdom
        else:
            isom = self.__pre_isomorphism*preWI
            domain = WIdom

        self.__clear_cached_values()

        self.__set_pre_isomorphism(domain, isom)

        return


    def set_post_isomorphism(self, postWI):
        r"""
        Modify this isogeny by postcomposing with a Weierstrass isomorphism.

        EXAMPLES::

            sage: E = EllipticCurve(j=GF(31)(0))
            sage: R.<x> = GF(31)[]
            sage: phi = EllipticCurveIsogeny(E, x+18)
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: phi.set_post_isomorphism(WeierstrassIsomorphism(phi.codomain(), (6,8,10,12)))
            sage: phi
            Isogeny of degree 3 from Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 31 to Elliptic Curve defined by y^2 + 24*x*y + 7*y = x^3 + 22*x^2 + 16*x + 20 over Finite Field of size 31

            sage: E = EllipticCurve(j=GF(47)(0))
            sage: f = E.torsion_polynomial(3)/3
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: E2 = phi.codomain()
            sage: post_isom = E2.isomorphism_to(E)
            sage: phi.set_post_isomorphism(post_isom)
            sage: phi.rational_maps() == E.multiplication_by_m(3)
            False
            sage: phi.switch_sign()
            sage: phi.rational_maps() == E.multiplication_by_m(3)
            True

        Example over a number field::

            sage: R.<x> = QQ[]
            sage: K.<a> = NumberField(x^2 + 2)
            sage: E = EllipticCurve(j=K(1728))
            sage: ker_list = E.torsion_points()
            sage: phi = EllipticCurveIsogeny(E, ker_list)
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: post_isom = WeierstrassIsomorphism(phi.codomain(), (a,2,3,5))
            sage: phi
            Isogeny of degree 4 from Elliptic Curve defined by y^2 = x^3 + x over Number Field in a with defining polynomial x^2 + 2 to Elliptic Curve defined by y^2 = x^3 + (-44)*x + 112 over Number Field in a with defining polynomial x^2 + 2

        """
        WIdom = postWI.domain().codomain()
        WIcod = postWI.codomain().codomain()

        if not isinstance(postWI, WeierstrassIsomorphism):
            raise ValueError("Invalid parameter: isomorphism must be of type Weierstrass isomorphism.")

        if (self.__E2 != WIdom):
            raise ValueError("Invalid parameter: isomorphism must have domain curve equal to this isogenies' codomain.")

        if (self.__post_isomorphism is None):
            isom = postWI
            codomain = WIcod
        else:
            isom = postWI*self.__post_isomorphism
            codomain = WIcod

        self.__clear_cached_values()

        self.__set_post_isomorphism(codomain, isom)

        return


    def get_pre_isomorphism(self):
        r"""
        Return the pre-isomorphism of this isogeny, or ``None``.

        EXAMPLES::

            sage: E = EllipticCurve(GF(31), [1,1,0,1,-1])
            sage: R.<x> = GF(31)[]
            sage: f = x^3 + 9*x^2 + x + 30
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: phi.get_post_isomorphism()
            sage: Epr = E.short_weierstrass_model()
            sage: isom = Epr.isomorphism_to(E)
            sage: phi.set_pre_isomorphism(isom)
            sage: isom == phi.get_pre_isomorphism()
            True

            sage: E = EllipticCurve(GF(83), [1,0,1,1,0])
            sage: R.<x> = GF(83)[]; f = x+24
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: E2 = phi.codomain()
            sage: phi2 = EllipticCurveIsogeny(E, None, E2, 2)
            sage: phi2.get_pre_isomorphism()
            Generic morphism:
              From: Abelian group of points on Elliptic Curve defined by y^2 + x*y + y = x^3 + x over Finite Field of size 83
              To:   Abelian group of points on Elliptic Curve defined by y^2 = x^3 + 62*x + 74 over Finite Field of size 83
              Via:  (u,r,s,t) = (1, 76, 41, 3)
        """
        return self.__pre_isomorphism

    def get_post_isomorphism(self):
        r"""
        Return the post-isomorphism of this isogeny, or ``None``.

        EXAMPLES::

            sage: E = EllipticCurve(j=GF(31)(0))
            sage: R.<x> = GF(31)[]
            sage: phi = EllipticCurveIsogeny(E, x+18)
            sage: phi.get_post_isomorphism()
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: isom = WeierstrassIsomorphism(phi.codomain(), (6,8,10,12))
            sage: phi.set_post_isomorphism(isom)
            sage: isom == phi.get_post_isomorphism()
            True

            sage: E = EllipticCurve(GF(83), [1,0,1,1,0])
            sage: R.<x> = GF(83)[]; f = x+24
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: E2 = phi.codomain()
            sage: phi2 = EllipticCurveIsogeny(E, None, E2, 2)
            sage: phi2.get_post_isomorphism()
            Generic morphism:
            From: Abelian group of points on Elliptic Curve defined by y^2 = x^3 + 65*x + 69 over Finite Field of size 83
            To:   Abelian group of points on Elliptic Curve defined by y^2 + x*y + y = x^3 + 4*x + 16 over Finite Field of size 83
            Via:  (u,r,s,t) = (1, 7, 42, 42)
        """
        return self.__post_isomorphism


    def switch_sign(self):
        r"""
        Compose this isogeny with `[-1]` (negation).

        EXAMPLES::

            sage: E = EllipticCurve(GF(23), [0,0,0,1,0])
            sage: f = E.torsion_polynomial(3)/3
            sage: phi = EllipticCurveIsogeny(E, f, E)
            sage: phi.rational_maps() == E.multiplication_by_m(3)
            False
            sage: phi.switch_sign()
            sage: phi.rational_maps() == E.multiplication_by_m(3)
            True

            sage: E = EllipticCurve(GF(17), [-2, 3, -5, 7, -11])
            sage: R.<x> = GF(17)[]
            sage: f = x+6
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: phi
            Isogeny of degree 2 from Elliptic Curve defined by y^2 + 15*x*y + 12*y = x^3 + 3*x^2 + 7*x + 6 over Finite Field of size 17 to Elliptic Curve defined by y^2 + 15*x*y + 12*y = x^3 + 3*x^2 + 4*x + 8 over Finite Field of size 17
            sage: phi.rational_maps()
            ((x^2 + 6*x + 4)/(x + 6), (x^2*y - 5*x*y + 8*x - 2*y)/(x^2 - 5*x + 2))
            sage: phi.switch_sign()
            sage: phi
            Isogeny of degree 2 from Elliptic Curve defined by y^2 + 15*x*y + 12*y = x^3 + 3*x^2 + 7*x + 6 over Finite Field of size 17 to Elliptic Curve defined by y^2 + 15*x*y + 12*y = x^3 + 3*x^2 + 4*x + 8 over Finite Field of size 17
            sage: phi.rational_maps()
            ((x^2 + 6*x + 4)/(x + 6),
             (2*x^3 - x^2*y - 5*x^2 + 5*x*y - 4*x + 2*y + 7)/(x^2 - 5*x + 2))

            sage: E = EllipticCurve('11a1')
            sage: R.<x> = QQ[]
            sage: f = x^2 - 21*x + 80
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: (xmap1, ymap1) = phi.rational_maps()
            sage: phi.switch_sign()
            sage: (xmap2, ymap2) = phi.rational_maps()
            sage: xmap1 == xmap2
            True
            sage: ymap1 == -ymap2 - E.a1()*xmap2 - E.a3()
            True

            sage: K.<a> = NumberField(x^2 + 1)
            sage: E = EllipticCurve(K, [0,0,0,1,0])
            sage: R.<x> = K[]
            sage: phi = EllipticCurveIsogeny(E, x-a)
            sage: phi.rational_maps()
            ((x^2 + (-a)*x - 2)/(x + (-a)), (x^2*y + (-2*a)*x*y + y)/(x^2 + (-2*a)*x - 1))
            sage: phi.switch_sign()
            sage: phi.rational_maps()
            ((x^2 + (-a)*x - 2)/(x + (-a)), (-x^2*y + (2*a)*x*y - y)/(x^2 + (-2*a)*x - 1))

        """
        self.set_post_isomorphism(WeierstrassIsomorphism(self.__E2, (-1,0,-self.__E2.a1(),-self.__E2.a3())))

    def is_normalized(self, via_formal=True, check_by_pullback=True):
        r"""
        Return whether this isogeny is normalized.

        .. NOTE::

           An isogeny `\varphi\colon E\to E_2` between two given
           Weierstrass equations is said to be normalized if the
           constant `c` is `1` in `\varphi*(\omega_2) = c\cdot\omega`,
           where `\omega` and `omega_2` are the invariant
           differentials on `E` and `E_2` corresponding to the given
           equation.

        INPUT:

        - ``via_formal`` - (default: ``True``) If ``True`` it simply
          checks if the leading term of the formal series is
          1. Otherwise it uses a deprecated algorithm involving the
          second optional argument.

        - ``check_by_pullback`` -  (default:``True``) Deprecated.

        EXAMPLES::

            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: E = EllipticCurve(GF(7), [0,0,0,1,0])
            sage: R.<x> = GF(7)[]
            sage: phi = EllipticCurveIsogeny(E, x)
            sage: phi.is_normalized()
            True
            sage: isom = WeierstrassIsomorphism(phi.codomain(), (3, 0, 0, 0))
            sage: phi.set_post_isomorphism(isom)
            sage: phi.is_normalized()
            False
            sage: isom = WeierstrassIsomorphism(phi.codomain(), (5, 0, 0, 0))
            sage: phi.set_post_isomorphism(isom)
            sage: phi.is_normalized()
            True
            sage: isom = WeierstrassIsomorphism(phi.codomain(), (1, 1, 1, 1))
            sage: phi.set_post_isomorphism(isom)
            sage: phi.is_normalized()
            True

            sage: F = GF(2^5, 'alpha'); alpha = F.gen()
            sage: E = EllipticCurve(F, [1,0,1,1,1])
            sage: R.<x> = F[]
            sage: phi = EllipticCurveIsogeny(E, x+1)
            sage: isom = WeierstrassIsomorphism(phi.codomain(), (alpha, 0, 0, 0))
            sage: phi.is_normalized()
            True
            sage: phi.set_post_isomorphism(isom)
            sage: phi.is_normalized()
            False
            sage: isom = WeierstrassIsomorphism(phi.codomain(), (1/alpha, 0, 0, 0))
            sage: phi.set_post_isomorphism(isom)
            sage: phi.is_normalized()
            True
            sage: isom = WeierstrassIsomorphism(phi.codomain(), (1, 1, 1, 1))
            sage: phi.set_post_isomorphism(isom)
            sage: phi.is_normalized()
            True

            sage: E = EllipticCurve('11a1')
            sage: R.<x> = QQ[]
            sage: f = x^3 - x^2 - 10*x - 79/4
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: isom = WeierstrassIsomorphism(phi.codomain(), (2, 0, 0, 0))
            sage: phi.is_normalized()
            True
            sage: phi.set_post_isomorphism(isom)
            sage: phi.is_normalized()
            False
            sage: isom = WeierstrassIsomorphism(phi.codomain(), (1/2, 0, 0, 0))
            sage: phi.set_post_isomorphism(isom)
            sage: phi.is_normalized()
            True
            sage: isom = WeierstrassIsomorphism(phi.codomain(), (1, 1, 1, 1))
            sage: phi.set_post_isomorphism(isom)
            sage: phi.is_normalized()
            True
        """
        # easy algorithm using the formal expansion.
        if via_formal:
            phi_formal = self.formal(prec=5)
            return phi_formal[1] == 1

        # this is the old algorithm. it should be deprecated.
        check_prepost_isomorphism = False

        f_normalized = True

        if (check_by_pullback):

            (Xmap, Ymap) = self.rational_maps()

            E1 = self.__E1
            E2 = self.__E2

            a1 = E1.a1()
            a3 = E1.a3()

            a1pr = E2.a1()
            a3pr = E2.a3()

            x, y = self.__mpoly_ring.gens()

            Xmap_pr = Xmap.derivative(x)

            domain_inv_diff = 1/(2*y + a1*x + a3)
            codomain_inv_diff = Xmap_pr/(2*Ymap + a1pr*Xmap + a3pr)

            inv_diff_quo = domain_inv_diff/codomain_inv_diff

            if (1 == inv_diff_quo):
                f_normalized = True
            else:
                # For some reason, in certain cases, when the isogeny
                # is pre or post composed with a translation the
                # resulting rational functions are too complicated for
                # sage to simplify down to a constant in this case, we
                # do some cheating by checking if the post-composition
                # by isogeny has a non 1 scaling factor
                if ( inv_diff_quo.numerator().is_constant() and (inv_diff_quo.denominator().is_constant) ):
                    f_normalized = False
                else:
                    check_prepost_isomorphism = True
        else:
            check_prepost_isomorphism = True

        # If we skip checking by the pullback of the invariant
        # differential OR if that was inconclusive We explicitly check
        # if there is a post isomorphism and if it has a non 1 scaling
        # factor or if it is a just a translation.  NOTE: This only
        # works because we are using algorithms for calculating the
        # isogenies that calculate a separable normalized isogeny, if
        # this changes, this check will no longer be correct.
        #
        if (check_prepost_isomorphism):
            post_isom = self.__post_isomorphism
            if (post_isom is not None):
                if (1 == self.__base_field(post_isom.u)):
                    f_post_normalized = True
                else:
                    f_post_normalized = False
            else:
                f_post_normalized = True

            pre_isom = self.__pre_isomorphism
            if (pre_isom is not None):
                if (1 == self.__base_field(pre_isom.u)):
                    f_pre_normalized = True
                else:
                    f_pre_normalized = False
            else:
                f_pre_normalized = True

            f_normalized = f_pre_normalized and f_post_normalized

        return f_normalized

    def dual(self):
        r"""
        Return the isogeny dual to this isogeny.

        .. NOTE::

           If `\varphi\colon E \to E_2` is the given isogeny and `n`
           is its degree, then the dual is by definition the unique
           isogeny `\hat\varphi\colon E_2\to E` such that the
           compositions `\hat\varphi\circ\varphi` and
           `\varphi\circ\hat\varphi` are the multiplication-by-`n`
           maps on `E` and `E_2`, respectively.

        EXAMPLES::

            sage: E = EllipticCurve('11a1')
            sage: R.<x> = QQ[]
            sage: f = x^2 - 21*x + 80
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: phi_hat = phi.dual()
            sage: phi_hat.domain() == phi.codomain()
            True
            sage: phi_hat.codomain() == phi.domain()
            True
            sage: (X, Y) = phi.rational_maps()
            sage: (Xhat, Yhat) = phi_hat.rational_maps()
            sage: Xm = Xhat.subs(x=X, y=Y)
            sage: Ym = Yhat.subs(x=X, y=Y)
            sage: (Xm, Ym) == E.multiplication_by_m(5)
            True

            sage: E = EllipticCurve(GF(37), [0,0,0,1,8])
            sage: R.<x> = GF(37)[]
            sage: f = x^3 + x^2 + 28*x + 33
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: phi_hat = phi.dual()
            sage: phi_hat.codomain() == phi.domain()
            True
            sage: phi_hat.domain() == phi.codomain()
            True
            sage: (X, Y) = phi.rational_maps()
            sage: (Xhat, Yhat) = phi_hat.rational_maps()
            sage: Xm = Xhat.subs(x=X, y=Y)
            sage: Ym = Yhat.subs(x=X, y=Y)
            sage: (Xm, Ym) == E.multiplication_by_m(7)
            True

            sage: E = EllipticCurve(GF(31), [0,0,0,1,8])
            sage: R.<x> = GF(31)[]
            sage: f = x^2 + 17*x + 29
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: phi_hat = phi.dual()
            sage: phi_hat.codomain() == phi.domain()
            True
            sage: phi_hat.domain() == phi.codomain()
            True
            sage: (X, Y) = phi.rational_maps()
            sage: (Xhat, Yhat) = phi_hat.rational_maps()
            sage: Xm = Xhat.subs(x=X, y=Y)
            sage: Ym = Yhat.subs(x=X, y=Y)
            sage: (Xm, Ym) == E.multiplication_by_m(5)
            True

        Test for :trac:`23928`::

            sage: E = EllipticCurve(j=GF(431**2)(4))
            sage: phi = E.isogeny(E.lift_x(0))
            sage: phi.dual()
            Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 427*x over Finite Field in z2 of size 431^2 to Elliptic Curve defined by y^2 = x^3 + x over Finite Field in z2 of size 431^2

        Test (for :trac:`7096`)::

            sage: E = EllipticCurve('11a1')
            sage: phi = E.isogeny(E(5,5))
            sage: phi.dual().dual() == phi
            True

            sage: k = GF(103)
            sage: E = EllipticCurve(k,[11,11])
            sage: phi = E.isogeny(E(4,4))
            sage: phi
            Isogeny of degree 5 from Elliptic Curve defined by y^2 = x^3 + 11*x + 11 over Finite Field of size 103 to Elliptic Curve defined by y^2 = x^3 + 25*x + 80 over Finite Field of size 103
            sage: from sage.schemes.elliptic_curves.weierstrass_morphism import WeierstrassIsomorphism
            sage: phi.set_post_isomorphism(WeierstrassIsomorphism(phi.codomain(),(5,0,1,2)))
            sage: phi.dual().dual() == phi
            True

            sage: E = EllipticCurve(GF(103),[1,0,0,1,-1])
            sage: phi = E.isogeny(E(60,85))
            sage: phi.dual()
            Isogeny of degree 7 from Elliptic Curve defined by y^2 + x*y = x^3 + 84*x + 34 over Finite Field of size 103 to Elliptic Curve defined by y^2 + x*y = x^3 + x + 102 over Finite Field of size 103

        Check that :trac:`17293` is fixed:

            sage: k.<s> = QuadraticField(2)
            sage: E = EllipticCurve(k, [-3*s*(4 + 5*s), 2*s*(2 + 14*s + 11*s^2)])
            sage: phi = E.isogenies_prime_degree(3)[0]
            sage: (-phi).dual() == -(phi.dual())
            True
            sage: phi._EllipticCurveIsogeny__clear_cached_values()  # forget the dual
            sage: -(phi.dual()) == (-phi).dual()
            True

        """
        if (self.__base_field.characteristic() in [2,3]):
            raise NotImplementedError("Computation of dual isogenies not yet implemented in characteristics 2 and 3")

        if (self.__dual is not None):
            return self.__dual

        # trac 7096
        (E1, E2pr, pre_isom, post_isom) = compute_intermediate_curves(self.codomain(), self.domain())

        F = self.__base_field
        d = self.__degree

        # trac 7096
        if F(d) == 0:
            raise NotImplementedError("The dual isogeny is not separable: only separable isogenies are currently implemented")

        # trac 7096
        # this should take care of the case when the isogeny is not normalized.
        u = self.formal()[1]
        isom = WeierstrassIsomorphism(E2pr, (u/F(d), 0, 0, 0))

        E2 = isom.codomain().codomain()

        pre_isom = self.__E2.isomorphism_to(E1)
        post_isom = E2.isomorphism_to(self.__E1)

        phi_hat = EllipticCurveIsogeny(E1, None, E2, d)

        phi_hat.set_pre_isomorphism(pre_isom)
        phi_hat.set_post_isomorphism(post_isom)
        phi_hat.__perform_inheritance_housekeeping()

        assert phi_hat.codomain() == self.domain()

        # trac 7096 : this adjusts a posteriori the automorphism on
        # the codomain of the dual isogeny.  we used _a_ Weierstrass
        # isomorphism to get to the original curve, but we may have to
        # change it by an automorphism.  We impose the condition that
        # the composition has the degree as a leading coefficient in
        # the formal expansion.

        phi_sc = self.formal()[1]
        phihat_sc = phi_hat.formal()[1]

        sc = phi_sc * phihat_sc/F(d)

        if sc == 0:
            raise RuntimeError("Bug in computing dual isogeny: sc = 0")

        if sc != 1:
            auts = self.__E1.automorphisms()
            aut = [a for a in auts if a.u == sc]
            if len(aut) != 1:
                raise ValueError("There is a bug in dual().")
            phi_hat.set_post_isomorphism(aut[0])

        self.__dual = phi_hat

        return phi_hat

    def formal(self,prec=20):
        r"""
        Return the formal isogeny as a power series in the variable
        `t=-x/y` on the domain curve.

        INPUT:

        - ``prec`` - (default = 20), the precision with which the
          computations in the formal group are carried out.

        EXAMPLES::

            sage: E = EllipticCurve(GF(13),[1,7])
            sage: phi = E.isogeny(E(10,4))
            sage: phi.formal()
            t + 12*t^13 + 2*t^17 + 8*t^19 + 2*t^21 + O(t^23)

            sage: E = EllipticCurve([0,1])
            sage: phi = E.isogeny(E(2,3))
            sage: phi.formal(prec=10)
            t + 54*t^5 + 255*t^7 + 2430*t^9 + 19278*t^11 + O(t^13)

            sage: E = EllipticCurve('11a2')
            sage: R.<x> = QQ[]
            sage: phi = E.isogeny(x^2 + 101*x + 12751/5)
            sage: phi.formal(prec=7)
            t - 2724/5*t^5 + 209046/5*t^7 - 4767/5*t^8 + 29200946/5*t^9 + O(t^10)
        """
        Eh = self.__E1.formal()
        f, g = self.rational_maps()
        xh = Eh.x(prec=prec)
        if xh.valuation() != -2:
            raise RuntimeError("xh has valuation %s (should be -2)" % xh.valuation())
        yh = Eh.y(prec=prec)
        if yh.valuation() != -3:
            raise RuntimeError("yh has valuation %s (should be -3)" % yh.valuation())
        fh = f(xh,yh)
        if fh.valuation() != -2:
            raise RuntimeError("fh has valuation %s (should be -2)" % fh.valuation())
        gh = g(xh,yh)
        if gh.valuation() != -3:
            raise RuntimeError("gh has valuation %s (should be -3)" % gh.valuation())
        th = -fh/gh
        if th.valuation() != 1:
            raise RuntimeError("th has valuation %s (should be +1)" % th.valuation())
        return th

    #
    # Overload Morphism methods that we want to
    #

    def is_injective(self):
        r"""
        Return ``True`` if and only if this isogeny has trivial
        kernel.

        EXAMPLES::

            sage: E = EllipticCurve('11a1')
            sage: R.<x> = QQ[]
            sage: f = x^2 + x - 29/5
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: phi.is_injective()
            False
            sage: phi = EllipticCurveIsogeny(E, R(1))
            sage: phi.is_injective()
            True

            sage: F = GF(7)
            sage: E = EllipticCurve(j=F(0))
            sage: phi = EllipticCurveIsogeny(E, [ E((0,-1)), E((0,1))])
            sage: phi.is_injective()
            False
            sage: phi = EllipticCurveIsogeny(E, E(0))
            sage: phi.is_injective()
            True
        """
        return not (1 < self.__degree)

    def is_surjective(self):
        r"""
        Return ``True`` if and only if this isogeny is surjective.

        .. NOTE::

           This function always returns ``True``, as a non-constant
           map of algebraic curves must be surjective, and this class
           does not model the constant `0` isogeny.

        EXAMPLES::

            sage: E = EllipticCurve('11a1')
            sage: R.<x> = QQ[]
            sage: f = x^2 + x - 29/5
            sage: phi = EllipticCurveIsogeny(E, f)
            sage: phi.is_surjective()
            True

            sage: E = EllipticCurve(GF(7), [0,0,0,1,0])
            sage: phi = EllipticCurveIsogeny(E,  E((0,0)))
            sage: phi.is_surjective()
            True

            sage: F = GF(2^5, 'omega')
            sage: E = EllipticCurve(j=F(0))
            sage: R.<x> = F[]
            sage: phi = EllipticCurveIsogeny(E, x)
            sage: phi.is_surjective()
            True
        """
        return True

    def is_zero(self):
        r"""
        Return whether this isogeny is zero.

        .. NOTE::

           Currently this class does not allow zero isogenies, so this
           function will always return True.

        EXAMPLES::

            sage: E = EllipticCurve(j=GF(7)(0))
            sage: phi = EllipticCurveIsogeny(E, [ E((0,1)), E((0,-1))])
            sage: phi.is_zero()
            False
        """
        return self.degree().is_zero()

    def post_compose(self, left):
        r"""
        Return the post-composition of this isogeny with ``left``.

        EXAMPLES::

            sage: E = EllipticCurve(j=GF(7)(0))
            sage: phi = EllipticCurveIsogeny(E, [ E((0,1)), E((0,-1))])
            sage: phi.post_compose(phi)
            Traceback (most recent call last):
            ...
            NotImplementedError: post-composition of isogenies not yet implemented
        """
        raise NotImplementedError("post-composition of isogenies not yet implemented")

    def pre_compose(self, right):
        r"""
        Return the pre-composition of this isogeny with ``right``.

        EXAMPLES::

            sage: E = EllipticCurve(j=GF(7)(0))
            sage: phi = EllipticCurveIsogeny(E, [ E((0,1)), E((0,-1))])
            sage: phi.pre_compose(phi)
            Traceback (most recent call last):
            ...
            NotImplementedError: pre-composition of isogenies not yet implemented
        """
        raise NotImplementedError("pre-composition of isogenies not yet implemented")

    def n(self):
        r"""
        Numerical Approximation inherited from Map (through morphism),
        nonsensical for isogenies.

        EXAMPLES::

            sage: E = EllipticCurve(j=GF(7)(0))
            sage: phi = EllipticCurveIsogeny(E, [ E((0,1)), E((0,-1))])
            sage: phi.n()
            Traceback (most recent call last):
            ...
            NotImplementedError: Numerical approximations do not make sense for Elliptic Curve Isogenies
        """
        raise NotImplementedError("Numerical approximations do not make sense for Elliptic Curve Isogenies")

def compute_isogeny_starks(E1, E2, ell):
    r"""
    Return the kernel polynomials of an isogeny of degree ``ell``
    between ``E1`` and ``E2``.

    INPUT:

    - ``E1``  - an elliptic curve in short Weierstrass form.
    - ``E2``  - an elliptic curve in short Weierstrass form.
    - ``ell`` - the degree of the isogeny from E1 to E2.

    OUTPUT:

    polynomial over the field of definition of ``E1``, ``E2``, that is
    the kernel polynomial of the isogeny from ``E1`` to ``E2``.

    .. NOTE::

       There must be a degree ``ell``, separable, normalized cyclic
       isogeny from ``E1`` to ``E2``, or an error will be raised.

    ALGORITHM:

    This function uses Starks Algorithm as presented in section 6.2 of
    [BMSS2006]_.

    .. NOTE::

       As published in [BMSS2006]_, the algorithm is incorrect, and a
       correct version (with slightly different notation) can be found
       in [Mo2009]_.  The algorithm originates in [Sta1973]_.

    EXAMPLES::

        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import compute_isogeny_starks, compute_sequence_of_maps

        sage: E = EllipticCurve(GF(97), [1,0,1,1,0])
        sage: R.<x> = GF(97)[]; f = x^5 + 27*x^4 + 61*x^3 + 58*x^2 + 28*x + 21
        sage: phi = EllipticCurveIsogeny(E, f)
        sage: E2 = phi.codomain()
        sage: (isom1, isom2, E1pr, E2pr, ker_poly) = compute_sequence_of_maps(E, E2, 11)
        sage: compute_isogeny_starks(E1pr, E2pr, 11)
        x^10 + 37*x^9 + 53*x^8 + 66*x^7 + 66*x^6 + 17*x^5 + 57*x^4 + 6*x^3 + 89*x^2 + 53*x + 8

        sage: E = EllipticCurve(GF(37), [0,0,0,1,8])
        sage: R.<x> = GF(37)[]
        sage: f = (x + 14) * (x + 30)
        sage: phi = EllipticCurveIsogeny(E, f)
        sage: E2 = phi.codomain()
        sage: compute_isogeny_starks(E, E2, 5)
        x^4 + 14*x^3 + x^2 + 34*x + 21
        sage: f**2
        x^4 + 14*x^3 + x^2 + 34*x + 21

        sage: E = EllipticCurve(QQ, [0,0,0,1,0])
        sage: R.<x> = QQ[]
        sage: f = x
        sage: phi = EllipticCurveIsogeny(E, f)
        sage: E2 = phi.codomain()
        sage: compute_isogeny_starks(E, E2, 2)
        x
    """
    K = E1.base_field()
    R = PolynomialRing(K, 'x')
    x = R.gen()

    wp1 = E1.weierstrass_p(prec=4*ell+4)  #BMSS2006 claim 2*ell is enough, but it is not M09
    wp2 = E2.weierstrass_p(prec=4*ell+4)

    # viewed them as power series in Z = z^2
    S = LaurentSeriesRing(K, 'Z')
    Z = S.gen()
    pe1 = 1/Z
    pe2 = 1/Z
    for i in range(2 * ell + 1):
        pe1 += wp1[2*i] * Z**i
        pe2 += wp2[2*i] * Z**i
    pe1 = pe1.add_bigoh(2*ell+2)
    pe2 = pe2.add_bigoh(2*ell+2)

    n = 1
    q = [R(1), R(0)]
    T = pe2

    while ( q[n].degree() < (ell-1) ):
        n += 1
        a_n = 0
        r = -T.valuation()
        while (0 <= r):
            t_r = T[-r]
            a_n = a_n + t_r * x**r
            T = T - t_r*pe1**r
            r = -T.valuation()

        q_n = a_n*q[n-1] + q[n-2]
        q.append(q_n)

        if (n == ell+1 or T == 0):
            if (T == 0 or T.valuation()<2):
                raise ValueError("The two curves are not linked by a cyclic normalized isogeny of degree %s" % ell)
            break

        T = 1/T

    qn = q[n]
    qn = (1/qn.leading_coefficient())*qn

    return qn

def split_kernel_polynomial(poly):
    r"""
    Internal helper function for ``compute_isogeny_kernel_polynomial``.

    INPUT:

    - ``poly`` -- a nonzero univariate polynomial.

    OUTPUT:

    The maximum separable divisor of ``poly``.  If the input is a full
    kernel polynomial where the roots which are `x`-coordinates of
    points of order greater than 2 have multiplicity 1, the output
    will be a polynomial with the same roots, all of multiplicity 1.

    EXAMPLES:

    The following example implicitly exercises this function::

        sage: E = EllipticCurve(GF(37), [0,0,0,1,8])
        sage: R.<x> = GF(37)[]
        sage: f = (x + 10) * (x + 12) * (x + 16)
        sage: phi = EllipticCurveIsogeny(E, f)
        sage: E2 = phi.codomain()
        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import compute_isogeny_starks
        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import split_kernel_polynomial
        sage: ker_poly = compute_isogeny_starks(E, E2, 7); ker_poly
        x^6 + 2*x^5 + 20*x^4 + 11*x^3 + 36*x^2 + 35*x + 16
        sage: ker_poly.factor()
        (x + 10)^2 * (x + 12)^2 * (x + 16)^2
        sage: poly = split_kernel_polynomial(ker_poly); poly
        x^3 + x^2 + 28*x + 33
        sage: poly.factor()
        (x + 10) * (x + 12) * (x + 16)
    """
    from sage.misc.all import prod
    return prod([p for p,e in poly.squarefree_decomposition()])

def compute_isogeny_kernel_polynomial(E1, E2, ell, algorithm="starks"):
    r"""
    Return the kernel polynomial of an isogeny of degree ``ell``
    between ``E1`` and ``E2``.

    INPUT:

    - ``E1``        - an elliptic curve in short Weierstrass form.

    - ``E2``        - an elliptic curve in short Weierstrass form.

    - ``ell``       - the degree of the isogeny from ``E1`` to ``E2``.

    - ``algorithm`` - currently only ``starks`` (default) is implemented.

    OUTPUT:

    polynomial over the field of definition of ``E1``, ``E2``, that is
    the kernel polynomial of the isogeny from ``E1`` to ``E2``.


    .. NOTE::

       If there is no degree ``ell``, cyclic, separable, normalized
       isogeny from ``E1`` to ``E2`` then an error will be raised.


    EXAMPLES::

        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import compute_isogeny_kernel_polynomial

        sage: E = EllipticCurve(GF(37), [0,0,0,1,8])
        sage: R.<x> = GF(37)[]
        sage: f = (x + 14) * (x + 30)
        sage: phi = EllipticCurveIsogeny(E, f)
        sage: E2 = phi.codomain()
        sage: compute_isogeny_kernel_polynomial(E, E2, 5)
        x^2 + 7*x + 13
        sage: f
        x^2 + 7*x + 13

        sage: R.<x> = QQ[]
        sage: K.<i> = NumberField(x^2 + 1)
        sage: E = EllipticCurve(K, [0,0,0,1,0])
        sage: E2 = EllipticCurve(K, [0,0,0,16,0])
        sage: compute_isogeny_kernel_polynomial(E, E2, 4)
        x^3 + x
    """
    return split_kernel_polynomial(compute_isogeny_starks(E1, E2, ell))

def compute_intermediate_curves(E1, E2):
    r"""
    Return intermediate curves and isomorphisms.

    .. NOTE::

       This is used so we can compute `\wp` functions from the short
       Weierstrass model more easily.

    .. WARNING::

       The base field must be of characteristic not equal to 2,3.

    INPUT:

    - ``E1`` - an elliptic curve
    - ``E2`` - an elliptic curve

    OUTPUT:

    tuple (``pre_isomorphism``, ``post_isomorphism``,
    ``intermediate_domain``, ``intermediate_codomain``):

    - ``intermediate_domain``: a short Weierstrass model isomorphic to
      ``E1``

    - ``intermediate_codomain``: a short Weierstrass model isomorphic
      to ``E2``

    - ``pre_isomorphism``: normalized isomorphism from ``E1`` to
      intermediate_domain

    - ``post_isomorphism``: normalized isomorphism from
      intermediate_codomain to ``E2``

    EXAMPLES::

        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import compute_intermediate_curves
        sage: E = EllipticCurve(GF(83), [1,0,1,1,0])
        sage: R.<x> = GF(83)[]; f = x+24
        sage: phi = EllipticCurveIsogeny(E, f)
        sage: E2 = phi.codomain()
        sage: compute_intermediate_curves(E, E2)
        (Elliptic Curve defined by y^2 = x^3 + 62*x + 74 over Finite Field of size 83,
         Elliptic Curve defined by y^2 = x^3 + 65*x + 69 over Finite Field of size 83,
         Generic morphism:
          From: Abelian group of points on Elliptic Curve defined by y^2 + x*y + y = x^3 + x over Finite Field of size 83
          To:   Abelian group of points on Elliptic Curve defined by y^2 = x^3 + 62*x + 74 over Finite Field of size 83
          Via:  (u,r,s,t) = (1, 76, 41, 3),
         Generic morphism:
          From: Abelian group of points on Elliptic Curve defined by y^2 = x^3 + 65*x + 69 over Finite Field of size 83
          To:   Abelian group of points on Elliptic Curve defined by y^2 + x*y + y = x^3 + 4*x + 16 over Finite Field of size 83
          Via:  (u,r,s,t) = (1, 7, 42, 42))

        sage: R.<x> = QQ[]
        sage: K.<i> = NumberField(x^2 + 1)
        sage: E = EllipticCurve(K, [0,0,0,1,0])
        sage: E2 = EllipticCurve(K, [0,0,0,16,0])
        sage: compute_intermediate_curves(E, E2)
        (Elliptic Curve defined by y^2 = x^3 + x over Number Field in i with defining polynomial x^2 + 1,
         Elliptic Curve defined by y^2 = x^3 + 16*x over Number Field in i with defining polynomial x^2 + 1,
         Generic endomorphism of Abelian group of points on Elliptic Curve defined by y^2 = x^3 + x over Number Field in i with defining polynomial x^2 + 1
          Via:  (u,r,s,t) = (1, 0, 0, 0),
         Generic endomorphism of Abelian group of points on Elliptic Curve defined by y^2 = x^3 + 16*x over Number Field in i with defining polynomial x^2 + 1
          Via:  (u,r,s,t) = (1, 0, 0, 0))

    """
    if (E1.base_ring().characteristic() in [2,3]):
        raise NotImplementedError("compute_intermediate_curves is only defined for characteristics not 2 or 3")

    # We cannot just use
    # E1w = E1.short_weierstrass_model()
    # E2w = E2.short_weierstrass_model()
    # as the resulting isomorphisms would not be normalised (u=1)

    c4, c6 = E1.c_invariants()
    E1w = EllipticCurve([0,0,0,-c4/48, -c6/864])
    c4, c6 = E2.c_invariants()
    E2w = EllipticCurve([0,0,0,-c4/48, -c6/864])

    # We cannot even just use pre_iso = E1.isomorphism_to(E1w) since
    # it may have u=-1; similarly for E2

    urst = [w for w in isomorphisms(E1,E1w) if w[0]==1][0]
    pre_iso = WeierstrassIsomorphism(E1,urst,E1w)
    urst = [w for w in isomorphisms(E2w,E2) if w[0]==1][0]
    post_iso = WeierstrassIsomorphism(E2w,urst,E2)
    return (E1w, E2w, pre_iso, post_iso)

def compute_sequence_of_maps(E1, E2, ell):
    r"""
    Return intermediate curves, isomorphisms and kernel polynomial.

    INPUT:

    - ``E1``, ``E2`` -- elliptic curves.

    - ``ell`` -- a prime such that there is a degree ``ell`` separable
      normalized isogeny from ``E1`` to ``E2``.

    OUTPUT:

    (pre_isom, post_isom, E1pr, E2pr, ker_poly) where:

    - ``E1pr`` is an elliptic curve in short Weierstrass form
      isomorphic to ``E1``;

    - ``E2pr`` is an elliptic curve in short Weierstrass form
      isomorphic to ``E2``;

    - ``pre_isom`` is a normalised isomorphism from ``E1`` to
      ``E1pr``;

    - ``post_isom`` is a normalised isomorphism from ``E2pr`` to
      ``E2``;

    - ``ker_poly`` is the kernel polynomial of an ``ell``-isogeny from
      ``E1pr`` to ``E2pr``.

    EXAMPLES::

        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import compute_sequence_of_maps
        sage: E = EllipticCurve('11a1')
        sage: R.<x> = QQ[]; f = x^2 - 21*x + 80
        sage: phi = EllipticCurveIsogeny(E, f)
        sage: E2 = phi.codomain()
        sage: compute_sequence_of_maps(E, E2, 5)
        (Generic morphism:
          From: Abelian group of points on Elliptic Curve defined by y^2 + y = x^3 - x^2 - 10*x - 20 over Rational Field
          To:   Abelian group of points on Elliptic Curve defined by y^2 = x^3 - 31/3*x - 2501/108 over Rational Field
          Via:  (u,r,s,t) = (1, 1/3, 0, -1/2),
         Generic morphism:
          From: Abelian group of points on Elliptic Curve defined by y^2 = x^3 - 23461/3*x - 28748141/108 over Rational Field
          To:   Abelian group of points on Elliptic Curve defined by y^2 + y = x^3 - x^2 - 7820*x - 263580 over Rational Field
          Via:  (u,r,s,t) = (1, -1/3, 0, 1/2),
         Elliptic Curve defined by y^2 = x^3 - 31/3*x - 2501/108 over Rational Field,
         Elliptic Curve defined by y^2 = x^3 - 23461/3*x - 28748141/108 over Rational Field,
         x^2 - 61/3*x + 658/9)

        sage: K.<i> = NumberField(x^2 + 1)
        sage: E = EllipticCurve(K, [0,0,0,1,0])
        sage: E2 = EllipticCurve(K, [0,0,0,16,0])
        sage: compute_sequence_of_maps(E, E2, 4)
        (Generic endomorphism of Abelian group of points on Elliptic Curve defined by y^2 = x^3 + x over Number Field in i with defining polynomial x^2 + 1
          Via:  (u,r,s,t) = (1, 0, 0, 0),
         Generic endomorphism of Abelian group of points on Elliptic Curve defined by y^2 = x^3 + 16*x over Number Field in i with defining polynomial x^2 + 1
          Via:  (u,r,s,t) = (1, 0, 0, 0),
         Elliptic Curve defined by y^2 = x^3 + x over Number Field in i with defining polynomial x^2 + 1,
         Elliptic Curve defined by y^2 = x^3 + 16*x over Number Field in i with defining polynomial x^2 + 1,
         x^3 + x)

        sage: E = EllipticCurve(GF(97), [1,0,1,1,0])
        sage: R.<x> = GF(97)[]; f = x^5 + 27*x^4 + 61*x^3 + 58*x^2 + 28*x + 21
        sage: phi = EllipticCurveIsogeny(E, f)
        sage: E2 = phi.codomain()
        sage: compute_sequence_of_maps(E, E2, 11)
        (Generic morphism:
          From: Abelian group of points on Elliptic Curve defined by y^2 + x*y + y = x^3 + x over Finite Field of size 97
          To:   Abelian group of points on Elliptic Curve defined by y^2 = x^3 + 52*x + 31 over Finite Field of size 97
          Via:  (u,r,s,t) = (1, 8, 48, 44),
         Generic morphism:
          From: Abelian group of points on Elliptic Curve defined by y^2 = x^3 + 41*x + 66 over Finite Field of size 97
          To:   Abelian group of points on Elliptic Curve defined by y^2 + x*y + y = x^3 + 87*x + 26 over Finite Field of size 97
          Via:  (u,r,s,t) = (1, 89, 49, 49),
         Elliptic Curve defined by y^2 = x^3 + 52*x + 31 over Finite Field of size 97,
         Elliptic Curve defined by y^2 = x^3 + 41*x + 66 over Finite Field of size 97,
         x^5 + 67*x^4 + 13*x^3 + 35*x^2 + 77*x + 69)
    """
    (E1pr, E2pr, pre_isom, post_isom) = compute_intermediate_curves(E1, E2)

    ker_poly = compute_isogeny_kernel_polynomial(E1pr, E2pr, ell)

    return (pre_isom, post_isom, E1pr, E2pr, ker_poly)


# Utility function for manipulating isogeny degree matrices


def fill_isogeny_matrix(M):
    """
    Return a filled isogeny matrix giving all degrees from one giving only prime degrees.

    INPUT:

    - ``M`` -- a square symmetric matrix whose off-diagonal `i`, `j`
      entry is either a prime `l` (if the `i`'th and `j`'th curves
      have an `l`-isogeny between them), otherwise is 0.

    OUTPUT:

    (matrix) a square matrix with entries `1` on the diagonal, and in
    general the `i`, `j` entry is `d>0` if `d` is the minimal degree
    of an isogeny from the `i`'th to the `j`'th curve,

    EXAMPLES::

        sage: M = Matrix([[0, 2, 3, 3, 0, 0], [2, 0, 0, 0, 3, 3], [3, 0, 0, 0, 2, 0], [3, 0, 0, 0, 0, 2], [0, 3, 2, 0, 0, 0], [0, 3, 0, 2, 0, 0]]); M
        [0 2 3 3 0 0]
        [2 0 0 0 3 3]
        [3 0 0 0 2 0]
        [3 0 0 0 0 2]
        [0 3 2 0 0 0]
        [0 3 0 2 0 0]
        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import fill_isogeny_matrix
        sage: fill_isogeny_matrix(M)
        [ 1  2  3  3  6  6]
        [ 2  1  6  6  3  3]
        [ 3  6  1  9  2 18]
        [ 3  6  9  1 18  2]
        [ 6  3  2 18  1  9]
        [ 6  3 18  2  9  1]
    """
    from sage.matrix.all import Matrix
    from sage.rings.infinity import Infinity

    n = M.nrows()
    M0 = copy(M)
    for i in range(n):
        M0[i,i]=1

    def fix(d):
        if d == 0:
            return Infinity
        return d

    def fix2(d):
        if d == Infinity:
            return 0
        return d

    def pr(M1, M2):
        return Matrix([[fix2(min([fix(M1[i,k]*M2[k,j]) for k in range(n)])) for i in range(n)] for j in range(n)])

    M1 = M0
    M2 = pr(M0,M1)
    while M1!=M2:
        M1 = M2
        M2 = pr(M0,M1)

    return M1

def unfill_isogeny_matrix(M):
    """
    Reverses the action of ``fill_isogeny_matrix``.

    INPUT:

    - ``M`` -- a square symmetric matrix of integers.

    OUTPUT:

    (matrix) a square symmetric matrix obtained from ``M`` by
    replacing non-prime entries with `0`.

    EXAMPLES::

        sage: M = Matrix([[0, 2, 3, 3, 0, 0], [2, 0, 0, 0, 3, 3], [3, 0, 0, 0, 2, 0], [3, 0, 0, 0, 0, 2], [0, 3, 2, 0, 0, 0], [0, 3, 0, 2, 0, 0]]); M
        [0 2 3 3 0 0]
        [2 0 0 0 3 3]
        [3 0 0 0 2 0]
        [3 0 0 0 0 2]
        [0 3 2 0 0 0]
        [0 3 0 2 0 0]
        sage: from sage.schemes.elliptic_curves.ell_curve_isogeny import fill_isogeny_matrix, unfill_isogeny_matrix
        sage: M1 = fill_isogeny_matrix(M); M1
        [ 1  2  3  3  6  6]
        [ 2  1  6  6  3  3]
        [ 3  6  1  9  2 18]
        [ 3  6  9  1 18  2]
        [ 6  3  2 18  1  9]
        [ 6  3 18  2  9  1]
        sage: unfill_isogeny_matrix(M1)
        [0 2 3 3 0 0]
        [2 0 0 0 3 3]
        [3 0 0 0 2 0]
        [3 0 0 0 0 2]
        [0 3 2 0 0 0]
        [0 3 0 2 0 0]
        sage: unfill_isogeny_matrix(M1) == M
        True
    """
    n = M.nrows()
    M1 = copy(M)
    zero = Integer(0)
    for i in range(n):
        M1[i, i] = zero
        for j in range(i):
            if not M1[i, j].is_prime():
                M1[i, j] = zero
                M1[j, i] = zero
    return M1
