I'm about to write a preconditioner for a 2x2 block matrix coming from the FEM discretization (2D, equidistance mesh) of a 4th order PDE. The (0,0) block is the discrete laplace with Neumann boundary conditions. I want to precondition this block by using some amg iterations. Just for tests, the preconditioner should solve in each step the neumann problem with some rhs using a gmres solver with hypre/boomeramg preconditioner up to a given tolerance. But in most iterations it ends up with DIVERGED_DTOL. What could be the reason for? Is AMG is linear preconditioner? The constant null space of the matrix is set via KSPSetNullSpace.
Thomas
