# PCMGResidualDefault
Default routine to calculate the residual. 
## Synopsis
```
#include "petscksp.h" 
PetscErrorCode PCMGResidualDefault(Mat mat, Vec b, Vec x, Vec r)
```
Collective on mat


## Input Parameters

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



## Output Parameter

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





## See Also
 `PCMG`, `PCMGSetResidual()`, `PCMGSetMatResidual()`

## Level
developer

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/mg/mgfunc.c.html#PCMGResidualDefault">src/ksp/pc/impls/mg/mgfunc.c</A>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/ksp/pc/impls/mg/mgfunc.c)


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