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# PCSORSetSymmetric
Sets the SOR preconditioner to use symmetric (SSOR), backward, or forward relaxation.  The local variants perform SOR on each processor.  By default forward relaxation is used. 
## Synopsis
```
#include "petscpc.h" 
PetscErrorCode PCSORSetSymmetric(PC pc, MatSORType flag)
```
Logically Collective


## Input Parameters

- ***pc -*** the preconditioner context
- ***flag -*** one of the following


```none
    SOR_FORWARD_SWEEP
    SOR_BACKWARD_SWEEP
    SOR_SYMMETRIC_SWEEP
    SOR_LOCAL_FORWARD_SWEEP
    SOR_LOCAL_BACKWARD_SWEEP
    SOR_LOCAL_SYMMETRIC_SWEEP
```



## Options Database Keys

- ***-pc_sor_symmetric -*** Activates symmetric version
- ***-pc_sor_backward -*** Activates backward version
- ***-pc_sor_local_forward -*** Activates local forward version
- ***-pc_sor_local_symmetric -*** Activates local symmetric version
- ***-pc_sor_local_backward -*** Activates local backward version



## Note
To use the Eisenstat trick with SSOR, employ the PCEISENSTAT preconditioner,
which can be chosen with the option

- ***-pc_type eisenstat -*** Activates Eisenstat trick





## See Also
 `PCSOR`, `PCEisenstatSetOmega()`, `PCSORSetIterations()`, `PCSORSetOmega()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/sor/sor.c.html#PCSORSetSymmetric">src/ksp/pc/impls/sor/sor.c</A>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/sor/sor.c.html#PCSORSetSymmetric_SOR">PCSORSetSymmetric_SOR in src/ksp/pc/impls/sor/sor.c</A><BR>


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


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