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On 10.01.2012 04:57, Jed Brown wrote:
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cite="mid:CAM9tzS=sMY6vR=OzrGJcQ1oJHYhsXzqsvXrqJbtfk8=RThH5YQ@mail.gmail.com"
type="cite">
<div class="gmail_quote">On Mon, Jan 9, 2012 at 22:47, Mohamad M.
Nasr-Azadani <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:mmnasr@gmail.com">mmnasr@gmail.com</a>></span>
wrote:<br>
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<div>It might be a naive question, but I am wondering how
KSPSetNullSpace() works when it is passed to a linear system
ksp context?</div>
<div>Say, for instance, we have the simple case of Poisson
equation solved in a square domain and Neumann boundary
condition applied to all boundaries. </div>
<div>Does it take the integral of the solution and set it to
zero as an extra constraint? </div>
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<br>
<div>It just projects out whatever you provide as a null space, so
the Krylov method effectively runs in the remaining subspace.</div>
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In examples listed on this page:<br>
<br>
<a class="moz-txt-link-freetext" href="http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetNullSpace.html">http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetNullSpace.html</a><br>
<br>
You set nullspace without any particular information:<br>
<br>
ierr = MatNullSpaceCreate(PETSC_COMM_WORLD, PETSC_TRUE, 0,
PETSC_NULL, &nullsp);<br>
ierr = KSPSetNullSpace(ksp, nullsp);<br>
<br>
What does it project and how works in this case?<br>
<br>
Regards,<br>
Alexander<br>
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