Petsc And Slepc, singular system

[Matthew Knepley]

On Tue, Jul 21, 2009 at 4:09 PM, Umut Tabak <u.tabak at tudelft.nl> wrote:

> On Tue, Jul 21, 2009 at 03:56:11PM -0500, Matthew Knepley wrote:
> > If your system is badly scaled, roundoff errors could result in a pivot
> > larger than our tolerance. It is also possible that your preconditioner
> > resulted in a badly scaled system.
> >
> >    Matt
> >
> Hi,
>    Thanks for the fast reply.
>    since the system is singular condition number will be bad any way,
>    but I was wondering if there were already ways to overcome this
>    problem. And are there ways to prevent the preconditioner to give a
>    badly scaled system, what I mean is that the system is singular so
>    will preconditioning improve that? I definitely read about these.
>

Well, the default is ILU which can do horrendously things. Try running with
Jacobi as a test. It should fail like you want. In my opinion, blackbox PCs
almost never work, and what you really need is something tailored to your
problem (which is linear algebra heresy).

  Matt


>
>    Thanks and best,
>    Umut
>
> --
> Quote:      You can not teach a man anything, you can only help him find it
> within himself.
> Source:     Galileo Galilei
>



-- 
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
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