On Sat, Jun 25, 2011 at 8:57 PM, NAN ZHAO <span dir="ltr"><<a href="mailto:zhaonanavril@gmail.com">zhaonanavril@gmail.com</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 think I may rephrase my problem like that, I have a huge matrix with n by n, at row m, I put a 1 in the location (m,m), other element in the row is zero. Then I construct a vector with size n, put zero in the m location. All other location I just put random number. I should get a answer vector with 0 in the mth element, But I some times got unreasonable number in this location. I am wondering if there is some option in petsc to avoid this case.<br>
</blockquote><div><br></div><div>Use -pc_type lu -ksp_type preonly. You will get zero or it will tell you that the system is singular.</div><div><br></div><div> Matt</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
Thanks<div><div></div><div class="h5"><br><br><div class="gmail_quote">On Sat, Jun 25, 2011 at 2:58 PM, Matthew Knepley <span dir="ltr"><<a href="mailto:knepley@gmail.com" target="_blank">knepley@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div>On Sat, Jun 25, 2011 at 2:49 PM, NAN ZHAO <span dir="ltr"><<a href="mailto:zhaonanavril@gmail.com" target="_blank">zhaonanavril@gmail.com</a>></span> wrote:<br></div><div class="gmail_quote"><div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Dear all,<div><br></div><div>I have a question about how to use the ksp in a smart way to solve a linear system. I had a simple test to generate a random matrix and vector, but I put only one nonzero value in a row (let's say 1), and in the respect location of the RHS vector I put a zero. The size of the matrix is kind of big, I saw petsc some time give unreasonable value at that loaction (it should be zero or some really small number). I want to know if there is a way to avoid it?</div>
</blockquote><div><br></div></div><div>The problem sounds degenerate. A full rank A with 1 nonzero/row and b = 0 would not have a solution other than 0. What will this tell you?</div><div><br></div><div> Matt</div><div>
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<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>Thanks</div></blockquote></div>-- <br><font color="#888888">What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br>
-- Norbert Wiener<br>
</font></blockquote></div><br>
</div></div></blockquote></div><br><br clear="all"><br>-- <br>What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br>
-- Norbert Wiener<br>