Am 17.08.2012 16:24, schrieb Matthew Knepley: > On Fri, Aug 17, 2012 at 3:10 AM, Thomas Witkowski > <thomas.witkowski at tu-dresden.de > <mailto: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?
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