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Am 17.08.2012 16:24, schrieb Matthew Knepley:
<blockquote
cite="mid:CAMYG4G=XMe-SmHWFTwWu-XNa0KZv9yq_fhJGHgq7ZkW+MYqNiA@mail.gmail.com"
type="cite">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>
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
I 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>
<div><br>
</div>
<div>If I understand you correctly, your null space vector is (0
I). I use the same null space for SNES ex62.</div>
</div>
</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?<br>
<br>
Thomas<br>
<br>
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