how to inverse a sparse matrix in Petsc?

Lisandro Dalcin dalcinl at
Wed Feb 6 11:10:01 CST 2008

On 2/6/08, Ben Tay <zonexo at> 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.

Lisandro Dalcín
Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
PTLC - Güemes 3450, (3000) Santa Fe, Argentina
Tel/Fax: +54-(0)342-451.1594

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