[petsc-dev] Lagging the numerical factorization for one of the fields of fieldsplit preconditioner

[Jed Brown]

On Wed, Mar 16, 2011 at 00:13, Barry Smith <bsmith at mcs.anl.gov> wrote:

> You cannot just check the row and column separately. You need to check them
> as a pair.  If row < m and column < n mark A as changed.


But you don't know what "A" is yet so all you can mark is some particular
pair (row,col) as changed. And you can't store cheaply in the form
(row_range, col_range) because you don't know when to extend an existing
range or when to create a new range. Maybe there is something clever that
works in a lot of common cases, but I don't think there is a cheap automatic
way to do this. I think you would have to declare index sets for submatrices
up-front.
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