[petsc-users] Nullspaces for schur complement PCs

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:
>    Perhaps we need a generic way of attaching null spaces to parts of
>    matrices and to parts of matrices in different basis (which is what
>    a Schur complement is). So (and this is likely overly simplistic)
>    one could supply a null space, an index set, and a flag indicating
>    either "part of the matrix" or the "Schur complement of the matrix
>    with respect to an index set".  These provided null spaces could
>    then be used by the preconditioners (like Fieldsplit) that
>    manipulate parts of the matrix or Schur complement.

Yes, this is conceptually right, but it looks to me like it gets hairy
for recursive splitting.
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