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# MatResidual
Default routine to calculate the residual r = b - Ax 
## Synopsis
```
#include "petscmat.h" 
PetscErrorCode MatResidual(Mat mat, Vec b, Vec x, Vec r)
```
Collective


## Input Parameters

- ***mat -*** the matrix
- ***b   -*** the right-hand-side
- ***x   -*** the approximate solution



## Output Parameter

- ***r -*** location to store the residual





## See Also
 [](ch_matrices), `Mat`, `MatMult()`, `MatMultAdd()`, `PCMGSetResidual()`

## Level
developer

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/interface/matrix.c.html#MatResidual">src/mat/interface/matrix.c</A>


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


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