# TSSetIFunction
Set the function to compute F(t,U,U_t) where F() = 0 is the DAE to be solved. 
## Synopsis
```
#include "petscts.h"  
PetscErrorCode TSSetIFunction(TS ts, Vec r, TSIFunction f, void *ctx)
```
Logically Collective on ts


## Input Parameters

- ***ts  -*** the `TS` context obtained from `TSCreate()`
- ***r   -*** vector to hold the residual (or NULL to have it created internally)
- ***f   -*** the function evaluation routine
- ***ctx -*** user-defined context for private data for the function evaluation routine (may be NULL)



## Calling sequence of f
```none
PetscErrorCode f(TS ts,PetscReal t,Vec u,Vec u_t,Vec F,ctx);
```


- ***t   -*** time at step/stage being solved
- ***u   -*** state vector
- ***u_t -*** time derivative of state vector
- ***F   -*** function vector
- ***ctx -*** [optional] user-defined context for matrix evaluation routine





## Note
The user MUST call either this routine or `TSSetRHSFunction()` to define the ODE.  When solving DAEs you must use this function.


## See Also
 [](chapter_ts), `TS`, `TSSetRHSJacobian()`, `TSSetRHSFunction()`, `TSSetIJacobian()`

## Level
beginner

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/interface/ts.c.html#TSSetIFunction">src/ts/interface/ts.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex10.c.html">src/ts/tutorials/ex10.c.html</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex14.c.html">src/ts/tutorials/ex14.c.html</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex15.c.html">src/ts/tutorials/ex15.c.html</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex16.c.html">src/ts/tutorials/ex16.c.html</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex17.c.html">src/ts/tutorials/ex17.c.html</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex19.c.html">src/ts/tutorials/ex19.c.html</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex20.c.html">src/ts/tutorials/ex20.c.html</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex20adj.c.html">src/ts/tutorials/ex20adj.c.html</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex20fwd.c.html">src/ts/tutorials/ex20fwd.c.html</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex20opt_ic.c.html">src/ts/tutorials/ex20opt_ic.c.html</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex20opt_p.c.html">src/ts/tutorials/ex20opt_p.c.html</A><BR>


---
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[Index of all TS routines](index.md)  
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[Index of all manual pages](/docs/manualpages/singleindex.md)  
