<html><head><meta http-equiv="Content-Type" content="text/html charset=iso-8859-1"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On Mar 20, 2013, at 6:14 PM, John Mousel <<a href="mailto:john.mousel@gmail.com">john.mousel@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div dir="ltr">Can you comment on a GASM type approach to find a solution for the null
space? I notice that the null vectors that successfully make the true
residual drop are only complicated in a very thin band around the
interface. This band is easy to identify using a level set. Other than
that, the null space vector has a low frequency variation. My thought
was to break the matrix into two sub-matrices, and somehow apply GAMG as
a preconditioner on the far matrix, and ILU on the interface-adjacent
matrix. Is this dumb or a complete misunderstanding of GASM?</div><div class="gmail_extra"><br></div></blockquote><div><br></div><div>The null space in GAMG is not a projection, it does not have to be exact. Do you try using the 6 RBM or giving GAMG coordinates?</div><div><br></div><div>Also, you might try not smoothing the (-pc_gamg_nsmooths 0). Unsymetric matrices can work better this way.</div><div><br></div><br><blockquote type="cite"><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Mar 20, 2013 at 5:08 PM, Jed Brown <span dir="ltr"><<a href="mailto:jedbrown@mcs.anl.gov" target="_blank">jedbrown@mcs.anl.gov</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="im"><br><div class="gmail_quote">On Wed, Mar 20, 2013 at 5:04 PM, John Mousel <span dir="ltr"><<a href="mailto:john.mousel@gmail.com" target="_blank">john.mousel@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">I've wanted to scrap this approach for a long time, but
moving away from these GFM-type treatments is not a choice that I've
been allowed to follow through on for various reasons which are out of
my control.</blockquote></div><br></div>Unless there are some clever tricks to characterize the null space or to keep preconditioners compatible with the null space, the folks making the decisions might have to reconsider. It doesn't matter how sexy a method looks if it requires a solve and that solve cannot be done efficiently.</div>
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