PETSc version 3.17.0
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# KSPInitialResidual

Computes the residual. Either b - A*C*u = b - A*x with right preconditioning or C*(b - A*x) with left preconditioning; the latter residual is often called the "preconditioned residual".

### Synopsis

    #include "petscksp.h"
    PetscErrorCode  KSPInitialResidual(KSP ksp,Vec vsoln,Vec vt1,Vec vt2,Vec vres,Vec vb)

Collective on ksp

### Input Parameters

| vsoln | - solution to use in computing residual
| vt1, vt2 | - temporary work vectors
| vb | - right-hand-side vector

### Output Parameters

| vres | - calculated residual

### Notes

This routine assumes that an iterative method, designed for

        A x = b

will be used with a preconditioner, C, such that the actual problem is either

        AC u = b (right preconditioning) or

        CA x = Cb (left preconditioning).

This means that the calculated residual will be scaled and/or preconditioned; the true residual

        b-Ax

is returned in the vt2 temporary.

### See Also

KSPMonitor()

### Level

developer

### Location

src/ksp/ksp/interface/itres.c
Index of all KSP routines
Table of Contents for all manual pages
Index of all manual pages