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Am 28.08.2012 19:57, schrieb Jed Brown:
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cite="mid:CAM9tzSm3GYhoVaVDD6Mb2BTgn5PMjki1b1cgTqRm6xAgCg9Lcw@mail.gmail.com"
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<div class="gmail_quote">On Tue, Aug 28, 2012 at 12:25 PM, 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>
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<div class="im"><br>
<blockquote class="gmail_quote" style="margin:0 0 0
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GMRES uses left preconditioning by default which
forces the null space into the Krylov space at each
iteration. FGMRES uses right preconditioning and doesn't
do anything to the null space because it is naturally in
the Krylov space (since the result with fgmres contains
the constants this is why I suspect the matrix actually
doesn't have the null space you think it has). You can try
running -ksp_type gmres -ksp_pc_side right my guess
is that you will not get near zero for the dot.<br>
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Any idea how to debug this problem? Lets concentrate on the
gmres/no precon case first.</blockquote>
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<br>
<div>Left preconditioning and anything using -pc_type none works
correctly. It is only the case of right preconditioning that is
incorrect (and needs to be fixed in PETSc).</div>
</blockquote>
Is the null space projected out at every iteration step? If so, can
you give me a hint were I can find the corresponding line in source
code? Thanks.<br>
<br>
Thomas<br>
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