[petsc-users] Nullspaces for schur complement PCs

[Jed Brown]

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.
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