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

[Matthew Knepley]

On Fri, Aug 17, 2012 at 2:06 PM, Thomas Witkowski <
thomas.witkowski at tu-dresden.de> wrote:

>  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> 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.

   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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