question on MatMatMult

Randall Mackie rlmackie862 at
Mon Mar 17 18:03:18 CDT 2008

Matthew Knepley wrote:
> 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.

I know how to form the Laplacian (that's easy) but I do not know how
to form the biharmonic directly, on a non-uniform grid. If anyone
knows how, other than direct multiplication, and can point me in
the right direction, that would be most appreciated.


>   Matt
>>  Thanks, Randy

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