# TSComputeIFunction
Evaluates the DAE residual written in implicit form F(t,U,Udot)=0 
## Synopsis
```
#include "petscts.h"  
PetscErrorCode TSComputeIFunction(TS ts, PetscReal t, Vec U, Vec Udot, Vec Y, PetscBool imex)
```
Collective on ts


## Input Parameters

- ***ts -*** the `TS` context
- ***t -*** current time
- ***U -*** state vector
- ***Udot -*** time derivative of state vector
- ***imex -*** flag indicates if the method is `TSIMEX` so that the RHSFunction should be kept separate



## Output Parameter

- ***Y -*** right hand side





## Note
Most users should not need to explicitly call this routine, as it
is used internally within the nonlinear solvers.

If the user did did not write their equations in implicit form, this
function recasts them in implicit form.


## See Also
 [](chapter_ts), `TS`, `TSSetIFunction()`, `TSComputeRHSFunction()`

## Level
developer

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


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/ts/interface/ts.c)


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