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