[petsc-users] How can get the inverse matrix of P(transpose)AP

[Cong Li]

Hi

I met a problem when trying to get a inverse matrix.

The problem is like this:
I have a matrix P (m x k matrix, and k<<m) and a matrix A (m x m matrix,
large SPD matrix). Now I want to get the inverse matrix of
P(transpose)*A*p.
Since P(transpose)*A*p is only a k x k matrix, and PETSc only supports
sequential direct solver, I want to store P(transpose)*A*p in MATSEQDENSE
type.
And the questions are
1. given P and A are PETSc parallel matrix, for instance MATMPIAIJ, how can
I get sequential dense matrix P(transpose)*A*p?
2. If I need to multiply P(transpose)*A*p with other parallel dense matrix,
how I can do it?

Thanks a lot.

Cong
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