On Fri, Aug 17, 2012 at 2:27 PM, Thomas Witkowski < thomas.witkowski at tu-dresden.de> wrote:
> On Fri, Aug 17, 2012 at 3:10 AM, Thomas Witkowski < > thomas.witkowski at tu-dresden.de> wrote: > > I want to solve some (weakly) coupled system of equations of the >>> following form: >>> >>> A B u >>> . = ..... >>> 0 C v >>> >>> >>> so, C is the discrete Laplacian and A and B are some more complicated >>> operators (I make use of linear finite elements). All boundary conditions >>> are periodic, so the unknown v is determined only up to a constant. A and B >>> contain both the identity operator, so u is fixed. Now I want to solve the >>> system on the whole (there are reasons to do it in this way!) and I must >>> provide information about the nullspace to the solver. When I am right, to >>> provide the correct nullspace I must solve one equation with A. Is there >>> any way in PETSc to circumvent the problem? >> >> >> If I understand you correctly, your null space vector is (0 I). I use >> the same null space for SNES ex62. >> >> (0 I) cannot be an element of the null space, as multiplying it with the >> matrix results in a non-zero vector. Or am I totally wrong about null >> spaces of matrices? >> > > Maybe you could as your question again. I am not understanding what you > want. > > I want to solve the block triangular system as described above. My > problem is, that it has a one dimensional null space, but I'm not able to > define it. My question is: does anyone can give me an advice how to EITHER > compute the null space explicitly OR how to solve the system in such a way > that the null space is considered by the solver. The only constraint is > that I cannot split the system of equations into two independent solve for > both variables. I know that from this description its not clear why there > is this constraint, but it would take too long to describe it. > What is your evidence that it has a null space? Matt > > Thomas > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120817/b7386307/attachment.html>
