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# MatMatTransposeMult
Performs matrix-matrix multiplication C=A*B^T. 
## Synopsis
```
#include "petscmat.h" 
PetscErrorCode MatMatTransposeMult(Mat A, Mat B, MatReuse scall, PetscReal fill, Mat *C)
```
Neighbor-wise Collective


## Input Parameters

- ***A -*** the left matrix
- ***B -*** the right matrix
- ***scall -*** either `MAT_INITIAL_MATRIX` or `MAT_REUSE_MATRIX`
- ***fill -*** expected fill as ratio of nnz(C)/(nnz(A) + nnz(B)), use `PETSC_DEFAULT` if not known



## Output Parameter

- ***C -*** the product matrix





## Notes
C will be created if `MAT_INITIAL_MATRIX` and must be destroyed by the user with `MatDestroy()`.

`MAT_REUSE_MATRIX` can only be used if the matrices A and B have the same nonzero pattern as in the previous call

To determine the correct fill value, run with -info and search for the string "Fill ratio" to see the value
actually needed.

This routine is currently only implemented for pairs of `MATSEQAIJ` matrices, for the `MATSEQDENSE` class,
and for pairs of `MATMPIDENSE` matrices.

This routine is shorthand for using `MatProductCreate()` with the `MatProductType` of `MATPRODUCT_ABt`


## Options Database Keys

- ***-matmattransmult_mpidense_mpidense_via {allgatherv,cyclic} -*** Choose between algorithms for `MATMPIDENSE` matrices: the
first redundantly copies the transposed B matrix on each process and requires O(log P) communication complexity;
the second never stores more than one portion of the B matrix at a time by requires O(P) communication complexity.



## See Also
 [](ch_matrices), `Mat`, `MatProductCreate()`, `MATPRODUCT_ABt`, `MatMatMult()`, `MatTransposeMatMult()` `MatPtAP()`, `MatProductCreate()`, `MatProductAlgorithm`, `MatProductType`, `MATPRODUCT_ABt`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/interface/matrix.c.html#MatMatTransposeMult">src/mat/interface/matrix.c</A>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/mat/interface/matrix.c)


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