# 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 PETSCKSP_DLLEXPORT PCSORSetSymmetric(PC pc,MatSORType flag)

Collective on PC

### Input Parameters

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

        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

### Notes

To use the Eisenstat trick with SSOR, employ the PCEISENSTAT preconditioner, which can be chosen with the option
-pc_type eisenstat -Activates Eisenstat trick

### Keywords

PC, SOR, SSOR, set, relaxation, sweep, forward, backward, symmetric

### See Also

PCEisenstatSetOmega(), PCSORSetIterations(), PCSORSetOmega()

Level:intermediate
Location:src/ksp/pc/impls/sor/sor.c
Index of all PC routines
Table of Contents for all manual pages
Index of all manual pages

### Examples

src/ksp/examples/tutorials/ex21f.F.html