# PCGAMGSetUseSAEstEig
Use the eigen estimate from smoothed aggregation for the Chebyshev smoother during the solution process 
## Synopsis
```
#include "petscpc.h" 
#include "petscksp.h" 
PetscErrorCode PCGAMGSetUseSAEstEig(PC pc, PetscBool n)
```
Collective on pc


## Input Parameters

- ***pc -*** the preconditioner context
- ***n -*** number of its



## Options Database Key

- ***-pc_gamg_use_sa_esteig <true,false> -*** use the eigen estimate



## Notes
Smoothed aggregation constructs the smoothed prolongator $P = (I - \omega D^{-1} A) T$ where $T$ is the tentative prolongator and $D$ is the diagonal of $A$.
Eigenvalue estimates (based on a few `PCCG` or `PCGMRES` iterations) are computed to choose $\omega$ so that this is a stable smoothing operation.
If Chebyshev with Jacobi (diagonal) preconditioning is used for smoothing, then the eigenvalue estimates can be reused during the solution process
This option is only used when the smoother uses Jacobi, and should be turned off if a different `PCJacobiType` is used.
It became default in PETSc 3.17.




## See Also
 `PCGAMG`, `KSPChebyshevSetEigenvalues()`, `KSPChebyshevEstEigSet()`

## Level
advanced

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

## Implementations

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


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


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