<div dir="ltr"><div class="gmail_quote">On Fri, Feb 3, 2012 at 15:34, Thomas Witkowski <span dir="ltr"><<a href="mailto:thomas.witkowski@tu-dresden.de">thomas.witkowski@tu-dresden.de</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div id=":gd">I did it, but gmres with boomeramg diverges. The system has three unknowns per mesh node. Each block operator is either a Laplace or the mass matrix. So each block by-itself is solvable with amg. Thus it follows that the overall system is solvable? In my case the system is not symmetric and indefinite. The boundary conditions are Neuman everywhere, but the global matrix has an empty null space. As the local blocks (in the case of the discrete Laplace) have constant null space I set -pc_hypre_boomeramg_relax_<u></u>type_coarse Jacobi for boomeramg not to make direct solves on coarse grid. Is there any theoretical reason that AMG cannot work in this case or is it a question of just the right settings for the solver?</div>
</blockquote></div><br><div>How did you order dofs?</div><div><br></div><div>How are the blocks coupled?</div><div><br></div><div>AMG is more delicate and generally less robust for systems.</div></div>