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On Fri, Aug 17, 2012 at 3:10 AM, Thomas Witkowski <span dir="ltr"><<a
moz-do-not-send="true"
href="mailto:thomas.witkowski@tu-dresden.de" target="_blank">thomas.witkowski@tu-dresden.de</a>></span>
wrote:<br>
<blockquote
cite="mid:CAMYG4Gm7HYujyYqAkPGuqC4Jv+F0BEU8-y8KXGsJ4Dn5P1=p2Q@mail.gmail.com"
type="cite">
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<blockquote class="gmail_quote" style="margin:0 0 0
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want to solve some (weakly) coupled system of
equations of the following form:<br>
<br>
A B u<br>
. = .....<br>
0 C v<br>
<br>
<br>
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?</blockquote>
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<div>If I understand you correctly, your null space
vector is (0 I). I use the same null space for SNES
ex62.</div>
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</blockquote>
(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?</div>
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<div>Maybe you could as your question again. I am not
understanding what you want.</div>
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</blockquote>
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. <br>
<br>
Thomas<br>
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