On Sat, Jun 25, 2011 at 8:57 PM, NAN ZHAO <zhaonanavril at gmail.com> wrote:
> 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. > Use -pc_type lu -ksp_type preonly. You will get zero or it will tell you that the system is singular. Matt > Thanks > > > On Sat, Jun 25, 2011 at 2:58 PM, Matthew Knepley <knepley at gmail.com>wrote: > >> On Sat, Jun 25, 2011 at 2:49 PM, NAN ZHAO <zhaonanavril at gmail.com> wrote: >> >>> Dear all, >>> >>> 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? >>> >> >> 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? >> >> Matt >> >> >>> Thanks >>> >> -- >> What most experimenters take for granted before they begin their >> experiments is infinitely more interesting than any results to which their >> experiments lead. >> -- Norbert Wiener >> > > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20110625/85f13a01/attachment.htm>
