question on MatMatMult

Matthew Knepley knepley at
Mon Mar 17 17:40:12 CDT 2008

On Mon, Mar 17, 2008 at 5:28 PM, Randall Mackie <rlmackie862 at> wrote:
> If I want to compute H = L^T L where L is a sparse matrix which is an approximation to the
>  laplacian, and hence H is the biharmonic (also sparse), and if I have L as an MPI matrix
>  in PETSc, will MatMatMult work for this (assuming I create the transpose of L first).
>  In other words, does MatMatMult look at the non-zero structure only that would result,
>  or does it think the result is a dense matrix?

It builds the structure dynamically, which explains the "fill"
argument. This is not a great
thing to do unless you have no idea how to form it directly.


>  Thanks, Randy
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which
their experiments lead.
-- Norbert Wiener

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