[petsc-users] What do with singular blocks in block matrix preconditioning?

[Jed Brown]

On Fri, Feb 17, 2012 at 06:54, Thomas Witkowski <
thomas.witkowski at tu-dresden.de> wrote:

> Maybe some related question: Most textbooks write that the compatibility
> condition to solve a system with constant null space is that the right hand
> side has zero mean value. Today I read part of the Multigrid-book written
> by Trottenberg, and there the condition is written in a different form (eq
> 5.6.22 on page 185): the integral of the right hand side must be equal on
> the whole domain and on the boundary. Does any of you have an explanation
> for this condition? Is there a book/paper that considers the compatibility
> condition in more details?


It's common to project the null space out of the right hand side (yielding
a "consistent" right hand side).
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