Matthew Knepley <[email protected]> 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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