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# MatSchurComplementGetPmat
Obtain a preconditioning matrix for the Schur complement by assembling Sp = A11 - A10 inv(DIAGFORM(A00)) A01 
## Synopsis
```
#include "petscksp.h" 
PetscErrorCode MatSchurComplementGetPmat(Mat S, MatReuse preuse, Mat *Sp)
```
Collective


## Input Parameters

- ***S      -*** matrix obtained with MatCreateSchurComplement() (or equivalent) that implements the action of A11 - A10 ksp(A00,Ap00) A01
- ***preuse -*** `MAT_INITIAL_MATRIX` for a new Sp, or `MAT_REUSE_MATRIX` to reuse an existing Sp, or `MAT_IGNORE_MATRIX` to put nothing in Sp



## Output Parameter

- ***Sp     -*** approximate Schur complement suitable for preconditioning the exact Schur complement S = A11 - A10 inv(A00) A01





## Notes
The approximation of Sp depends on the the argument passed to to `MatSchurComplementSetAinvType()`
`MAT_SCHUR_COMPLEMENT_AINV_DIAG`, `MAT_SCHUR_COMPLEMENT_AINV_LUMP`, `MAT_SCHUR_COMPLEMENT_AINV_BLOCK_DIAG`, or `MAT_SCHUR_COMPLEMENT_AINV_FULL`
-mat_schur_complement_ainv_type <diag,lump,blockdiag,full>

Sometimes users would like to provide problem-specific data in the Schur complement, usually only
for special row and column index sets.  In that case, the user should call `PetscObjectComposeFunction()` to set
"MatSchurComplementGetPmat_C" to their function.  If their function needs to fall back to the default implementation,
it should call `MatSchurComplementGetPmat_Basic()`.


## Developer Note
The API that includes `MatGetSchurComplement()`, `MatCreateSchurComplement()`, `MatSchurComplementGetPmat()` should be refactored to
remove redundancy and be clearer and simpler.

This routine should be called `MatSchurComplementCreatePmat()`


## See Also
 [](ch_ksp), `MatCreateSubMatrix()`, `PCFIELDSPLIT`, `MatGetSchurComplement()`, `MatCreateSchurComplement()`, `MatSchurComplementSetAinvType()`

## Level
advanced

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/utils/schurm/schurm.c.html#MatSchurComplementGetPmat">src/ksp/ksp/utils/schurm/schurm.c</A>


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


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