Petsc And Slepc, singular system

[Barry Smith]

    See the manual page for MatNullSpaceCreate() and KSPSetNullSpace()

    Barry

On Jul 21, 2009, at 4:24 PM, Matthew Knepley wrote:

> 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