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# PCEISENSTAT
An implementation of SSOR (symmetric successive over relaxation, symmetric Gauss-Seidel) preconditioning that incorporates Eisenstat's trick to reduce the amount of computation needed. 
## Options Database Keys

- ***-pc_eisenstat_omega <omega> -*** Sets omega
- ***-pc_eisenstat_no_diagonal_scaling -*** Activates `PCEisenstatSetNoDiagonalScaling()`





## Notes
Only implemented for the `MATAIJ` matrix format.

Not a true parallel SOR, in parallel this implementation corresponds to block Jacobi with SOR on each block.


## Developer Note
Since this algorithm runs the Krylov method on a transformed linear system the implementation provides `PCPreSolve()` and `PCPostSolve()`
routines that `KSP` uses to set up the transformed linear system.


## See Also
 `PCCreate()`, `PCSetType()`, `PCType`, `PC`, `PCEisenstatGetOmega()`,
`PCEisenstatSetNoDiagonalScaling()`, `PCEisenstatSetOmega()`, `PCSOR`

## Level
beginner

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/eisens/eisen.c.html#PCEISENSTAT">src/ksp/pc/impls/eisens/eisen.c</A>


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


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