Schur system + MatShell

[tribur at vision.ee.ethz.ch]

Dear Matt,

> This does not make sense to me. You decide how PETSc partitions things (if
> you want), And, I really do not understand what you want in parallel.
> If you mean
> that you solve the local Schur complements independently, then use a local
> matrix for each one. The important thing is to work out the linear 
> algebra prior
> to coding. Then wrapping it with PETSc Mat/Vec is easy.

The linear algebra is completely clear. Again: I have the local Schur 
systems given (and NOT the solution of the local Schur systems), and I 
would like to solve the global Schur complement system in parallel. The 
global Schur complement system is theoretically constructed by putting 
and adding elements of the local systems in certain locations of a 
global matrix. Wrapping this with PETSc Mat/Vec, without the 
time-intensive assembling, is not easy for me as a PETSc-beginner. But 
I'm curious of the solution you propose...