Solving a singular matrix with BoomerAMG

[Jed Brown]

Thomas Witkowski wrote:
> I want so solve a system with a singular matrix (just the laplace
> discretized with the fem using pure Neumann boundary conditions) with
> BoomerAMG. Is there any way to do it directly in BoomerAMG/PETSc, or
> must I change the matrix to make it nonsingular?

The user's manual has a section on solving singular systems, you just
need to tell KSP about the null space (either with KSPSetNullSpace or
-ksp_constant_null_space).  Small known null spaces are rarely an issue
with iterative methods.  With multigrid, there is some risk that the
coarse operator is also singular; this can cause trouble since it is
usually solved with a direct solver, but should not happen in your case.

Jed

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