**petsc-master 2020-07-05**

# PetscSFComposeInverse

Compose a new PetscSF by putting the inverse of the second SF under the first one
### Synopsis

#include "petscsf.h"
PetscErrorCode PetscSFComposeInverse(PetscSF sfA,PetscSF sfB,PetscSF *sfBA)

### Input Parameters

### Output Parameters

**sfBA ** -The composite SF.

### Notes

Currently, the two SFs must be defined on congruent communicators and they must be true star
forests, i.e. the same leaf is not connected with different roots. Even more, all roots of the
second SF must have a degree of 1, i.e., no roots have more than one leaf connected.
sfA's leaf space and sfB's leaf space might be partially overlapped. The composition builds
a graph with sfA's roots and sfB's roots only when there is a path between them. Unconnected
roots are not in sfBA. Doing a Bcast on the new SF is equivalent to doing a Bcast on sfA, then
a Reduce on sfB, on connected roots.

### See Also

PetscSF, PetscSFCompose(), PetscSFGetGraph(), PetscSFSetGraph(), PetscSFCreateInverseSF()

### Level

developer

### Location

src/vec/is/sf/interface/sf.c

Index of all PetscSF routines

Table of Contents for all manual pages

Index of all manual pages