On Fri, Feb 3, 2012 at 10:48, Thomas Witkowski < thomas.witkowski at tu-dresden.de> wrote:
> 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? 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. 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). -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120203/a1b43e3f/attachment-0001.htm>
