[petsc-users] Null space/Near null space

Jed Brown jed at jedbrown.org
Wed Jul 22 17:56:35 CDT 2015

"Young, Matthew, Adam" <may at bu.edu> writes:

> Should I be using MatSetNearNullSpace (as opposed to MatSetNullSpace) with -pc_type hypre -pc_hypre_type boomeramg since BoomerAMG is multigrid?

As far as I know, Hypre doesn't directly use either of these.
MatSetNullSpace is for an actual null space.  It is usually set so that
the Krylov method can clean any pollution caused by the preconditioner.

MatSetNearNullSpace is for things like rigid body motions which are sort
of a local null space (i.e., it would be a null space with some
different boundary conditions, like natural/floating).  It is used by
aggregation-based multigrid methods.
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