[petsc-users] Assembly of local Mat's to a parallel Mat

[Hui Zhang]

Assembly of local Mat's to a parallel Mat formally defined by A = \sum_i Ri^T Ai Ri.  

My Ai's are originally defined in sub-communicators.  In Petsc, I was told before there are no counterpart of Ri that can promote the mat Ai to sup-communicator where A stays. (VecScatter can only acts on Vec but not Mat)

So, what is the easiest way to achieve this task?  Note that Ai's are already defined in sub-communicators.