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Re: how to inverse a sparse matrix in Petsc?

Ya, Lisandro, it's my mistake. It is indeed SPD. Thank you for your reminder!

Lisandro Dalcin wrote:
On 2/6/08, Ben Tay <zonexo@xxxxxxxxx> wrote:
Because my grid's are
non-uniform in both x,y directions. Shouldn't that result in a
non-symmetric matrix? But I think it's still PD, positive definite.
Correct me if I'm wrong.

I believe you are wrong, unless you are using a non-standart spatial
discretization method. Is your Poisson equation using some additional
terms than the usual Laplace operator? For standard finite elements
and finite diferences, your matrix should be symmetric. Of course,
symmetry can be lost if you use the common trick of zeroing-out rows
for boundary conditions (using MatZeroRows and related). But even in
that case, I believe you can still use CG.