<div dir="ltr"><div class="gmail_quote">On Fri, Feb 3, 2012 at 10:48, 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">
Shouldn't be, but it seems that is is close to singular in computer
arithmetic. I would like to understand we it's so. The matrix is a
2x2 block matrix with no coupling between the main blocks. I know
that this does not make much sense but its for tests only and I
would like to add some couplings later. Both blocks are nonsingular
and easy solvable with direct solvers. But when adding both
together, the condition number rise to something around 10^23. Is it
only a question of scaling both matrices to the same order?</blockquote></div><br><div>If it's *very* poorly scaled, then yes, it could be. You can try to correct it with -ksp_diagonal_scale -ksp_diagonal_scale_fix.</div>
<div><br></div><div>It seems more likely to me that it's a null space issue. How many near-zero eigenvalues are there? Perhaps you effectively have an all-Neumann boundary condition (e.g. incompressible flow with all Dirichlet velocity boundary conditions leaves the pressure undetermined up to a constant).</div>
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