On Thu, Oct 8, 2009 at 14:20, Thomas Witkowski <thomas.witkowski at tu-dresden.de> wrote: >> This representation is purely symbolic. ?There is a permutation of the >> dofs that would look like this, but it's not the ordering that you >> actually want to use >> > > okay, but with a different ordering I'll lose symmetry of the system.
No, it is a symmetric permutation. For indefinite problems, I implement the preconditioning matrix using MatShell where I provide MatGetSubMatrix (which either returns the cached submatrix or assembles it on demand, the former is sometimes more efficient and more convenient for profiling). You should have a reasonable idea about how to precondition the subsystems before you go this route. Jed
