OK, thanks, I understand now.
FormJacobian() has two matrices as arguments, the Jacobian Amat and the preconditioner Pmat, which may be the same. My FormJacobian() routine sets the values of the Jacobian in Pmat. So, to calculate the effective nullspace of the submatrix, I want to solve Pmat u = b. To do this within FormJacobian(), can I assemble Pmat, call SNESGetKSP(snes,&ksp), and then KSPSolve(ksp,b,u)? Is it safe to assume that the KSP matrix is Pmat?
