PETSc version 3.15.5
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# MatSchurComplementGetPmat

Obtain a preconditioning matrix for the Schur complement by assembling Sp = A11 - A10 inv(diag(A00)) A01

### Synopsis

    #include "petscksp.h"
    PetscErrorCode  MatSchurComplementGetPmat(Mat S,MatReuse preuse,Mat *Sp)

Collective on S

### Input Parameters

| S | - matrix obtained with MatCreateSchurComplement() (or equivalent) and implementing 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 S = A11 - A10 inv(diag(A00)) A01

### Note

Since the real Schur complement is usually dense, providing a good approximation to newpmat usually requires application-specific information. The default for assembled matrices is to use the inverse of the diagonal of the (0,0) block A00 in place of A00^{-1}. This rarely produce a scalable algorithm. Optionally, A00 can be lumped before forming inv(diag(A00)).

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 Notes

The API that includes MatGetSchurComplement(), MatCreateSchurComplement(), MatSchurComplementGetPmat() should be refactored to remove redundancy and be clearer and simplier.

### See Also

MatCreateSubMatrix(), PCFIELDSPLIT, MatGetSchurComplement(), MatCreateSchurComplement(), MatSchurComplementSetAinvType()

### Level

advanced

### Location

src/ksp/ksp/utils/schurm/schurm.c
Index of all KSP routines
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Index of all manual pages