PETSc version 3.16.5
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# PCGalerkinSetComputeSubmatrix

Provide a routine that will be called to compute the Galerkin submatrix

### Synopsis

    #include "petscksp.h"
    PetscErrorCode  PCGalerkinSetComputeSubmatrix(PC pc,PetscErrorCode (*computeAsub)(PC,Mat,Mat,Mat*,void*),void *ctx)

Logically Collective

### Input Parameters

| pc | - the preconditioner context
| computeAsub | - routine that computes the submatrix from the global matrix
| ctx | - context used by the routine, or NULL

### Calling sequence of computeAsub

       computeAsub(PC pc,Mat A, Mat Ap, Mat *cAP,void *ctx);

| PC | - the Galerkin PC
| A | - the matrix in the Galerkin PC
| Ap | - the computed submatrix from any previous computation, if NULL it has not previously been computed
| cAp | - the submatrix computed by this routine
| ctx | - optional user-defined function context

### Notes

Instead of providing this routine you can call PCGalerkinGetKSP() and then KSPSetOperators() to provide the submatrix, but that will not work for multiple KSPSolves with different matrices unless you call it for each solve.

This routine is called each time the outer matrix is changed. In the first call the Ap argument is NULL and the routine should create the matrix and computes its values in cAp. On each subsequent call the routine should up the Ap matrix.

### Developer Notes

If the user does not call this routine nor call PCGalerkinGetKSP() and KSPSetOperators() then PCGalerkin could could automatically compute the submatrix via calls to MatGalerkin() or MatRARt()

### See Also

PCCreate(), PCSetType(), PCType (for list of available types), PCGALERKIN,
PCGalerkinSetRestriction(), PCGalerkinSetInterpolation(), PCGalerkinGetKSP()

### Level

Intermediate

### Location

src/ksp/pc/impls/galerkin/galerkin.c

### Implementations

PCGalerkinSetComputeSubmatrix_Galerkin(PC pc,PetscErrorCode (*computeAsub) in src/ksp/pc/impls/galerkin/galerkin.c
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