From help-request at octave dot org Thu Nov  4 02:58:26 2004
Subject: Re: Inverse Matrix Function appears a bit wonky
From: David Bateman <David dot Bateman at motorola dot com>
To: "Robert A. Macy" <macy at california dot com>
Cc: "Dmitri A. Sergatskov" <dmitri at unm dot edu>, help@octave.org
Date: Thu, 4 Nov 2004 09:54:14 +0100

According to Robert A. Macy <macy at california dot com> (on 11/04/04):
> That's it!  simply asking to return the value of rcond
> allows me to catch a "bad" matrix.  

If you are after speed don't just ask for the condition number everytime.
If you have a good expectation that the matrix isn't singular, not 
calculating the condition number is a speed gain.

Regards
David


> 
> Thank you.  
> 
>            - Robert -
> 
> On Wed, 03 Nov 2004 14:08:34 -0700
>  "Dmitri A. Sergatskov" <dmitri at unm dot edu> wrote:
> > Robert A. Macy wrote:
> > > I tried a simple test of the inverse function using a
> > "made
> > > up" matrix...
> > > 
> > > testmatrix =
> > >  1 2 3 4
> > >  2 2 3 3
> > >  1 2 1 2
> > >  3 2 2 1
> > 
> > Your matrix is no good. Set testmatrix(4,4)=-1 and try
> > again :)
> > 
> > > What do I do to make certain octave doesn't do these
> > > erroneous inverse calculations the next time I go to
> > use
> > > the inverse function?
> > > 
> > 
> > Instead of b=inv(a), you can try [b,r]=inv(a), and check
> > that r is not too small.
> > 
> > >             - Robert -
> > 
> > Cheers,
> > 
> > Dmitri.
> 
> 
> 
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-- 
David Bateman                                David dot Bateman at motorola dot com
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