MatCreateComposite

Creates a matrix as the sum of zero or more matrices

#include "petscmat.h" 
PetscErrorCode PETSCMAT_DLLEXPORT MatCreateComposite(MPI_Comm comm,PetscInt nmat,const Mat *mats,Mat *mat)

Collective on MPI_Comm

A -the matrix

Alternative construction

      MatCreate(comm,&mat);

      MatSetSizes(mat,m,n,M,N);

      MatSetType(mat,MATCOMPOSITE);

      MatCompositeAddMat(mat,mats[0]);

      ....

      MatCompositeAddMat(mat,mats[nmat-1]);