[petsc-users] Null spaces with fieldsplit preconditioning

[Lawrence Mitchell]

Hello,

I have a mixed FEM system, for which the null space is constant
functions in one of the sub spaces.  That is, I have W = V * Q and the
null space is a constant function in Q (and zero in V).  If I assemble
the operator, I get a block structure:

X = [A B,
     C D]

When I solve this monolithically, I project the null space out by
attaching it to X.  However, if I use Schur complement preconditioning,
then S = D - C Ainv B has a null space of the constant functions as
well.  Do I therefore need to attach null spaces to both X and S in this
case, or will attaching it to X be enough?  If I do need to hang
something on S, is there an easy way to do it?

Cheers,

Lawrence