PETSc version 3.17.4
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# MatPtAP

Creates the matrix product C = P^T * A * P

### Synopsis

    #include "petscmat.h"
    PetscErrorCode MatPtAP(Mat A,Mat P,MatReuse scall,PetscReal fill,Mat *C)

Neighbor-wise Collective on Mat

### Input Parameters

| A | - the matrix
| P | - the projection matrix
| scall | - either MAT_INITIAL_MATRIX or MAT_REUSE_MATRIX
| fill | - expected fill as ratio of nnz(C)/(nnz(A) + nnz(P)), use PETSC_DEFAULT if you do not have a good estimate if the result is a dense matrix this is irrelevant

### Output Parameters

| C | - the product matrix

### Notes

C will be created and must be destroyed by the user with MatDestroy().

For matrix types without special implementation the function fallbacks to MatMatMult() followed by MatTransposeMatMult().

### See Also

MatMatMult(), MatRARt()

### Level

intermediate

### Location

src/mat/interface/matrix.c

### Examples

src/ksp/ksp/tutorials/ex76.c.html
src/tao/pde_constrained/tutorials/elliptic.c.html
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