[petsc-users] Nullspace for a coupled system of equations

[Matthew Knepley]

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.

Note that you should probably use FieldSplit multiplicative here since you
have a block triangular system to start with.

   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
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