[Libmesh-devel] sparse matrix multiply

[Wout R. <wou...@gm...>]

Dear developers,

I was thinking about the following.
When solving an elastostatics problem with penalty boundary conditions
Ku=f
you get u out by setting diag(K) to P and f to P*u, where P the penalty.
This gives u but there doesn't seem to be an easy way to get out the
reaction forces (which would be Ku but without the penalty).

I propose to copy the unpenalized stiffness matrix (can already be
done in System) and add a matrix-vector multiply (petsc probably has
that) to sparsematrix.
It is obviously inefficient storage-wise so it should be available
only when called explicitly.
It could be done by looping over all elements connected to a DOF for
which the reaction force needs to be known and multiply the element
stiffness, but that seems nastier since it would require recoding of
the assembly function.

Let me know there are any thoughts/ways of doing this.
W