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# MatMFFDSetBase
Sets the vector `U` at which matrix vector products of the Jacobian are computed for the `MATMFFD` matrix
## Synopsis
```
#include "petscmat.h"
PetscErrorCode MatMFFDSetBase(Mat J, Vec U, Vec F)
```
Logically Collective
## Input Parameters
- ***J -*** the `MATMFFD` matrix
- ***U -*** the vector
- ***F -*** (optional) vector that contains F(u) if it has been already computed
## Notes
This is rarely used directly
If `F` is provided then it is not recomputed. Otherwise the function is evaluated at the base
point during the first `MatMult()` after each call to `MatMFFDSetBase()`.
## See Also
[](ch_matrices), `Mat`, `MATMFFD`, `MatMult()`, `MatMFFDSetBase()`
## Level
advanced
## Location
src/mat/impls/mffd/mffd.c
## Examples
src/snes/tutorials/ex22.c
## Implementations
MatMFFDSetBase_MFFD in src/mat/impls/mffd/mffd.c
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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/mat/impls/mffd/mffd.c)
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