how to inverse a sparse matrix in Petsc?

[Ben Tay]

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 at gmail.com> 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.
>
>