[petsc-users] Nullspaces for schur complement PCs

[Matthew Knepley]

On Mon, Nov 10, 2014 at 10:28 PM, Jed Brown <jed at jedbrown.org> wrote:

> Matthew Knepley <knepley at gmail.com> writes:
> >> at which point the nullspace is not attached, no?
> >>
> >
> > Crap, that is right. I think we should propagate the nullspace from
> pmat[1].
>
> Uh, there is no reason pmat[1] can be expected to have the same null
> space.  pmat[1] is the zero matrix in some important cases, and the null
> space of the Schur complement (or its SIMPLE-type approximation) depends
> on boundary conditions in a different block (thus problematic for
> segregated assembly).  I've been unsuccessful so far in my search for an
> elegant solution, but passing the null space along in this way is
> encouraging overt lying and can be expected to break other algorithms as
> well as any consistency tests we might add in the future.
>

I know it can break, however here is my thinking. The user is completely in
control
here, and this is the only channel we have to pass the information. So I
would make
them responsible for breakage. We have excellent diagnostics here, so we
can tell
who has a null space right away. We have a lot of interface that allow a
user to break
PETSc in order to allow them to customize it sufficiently.

  Thanks,

     Matt

-- 
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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