Re: Octave hangs forever trying to invert singular matrix

[Przemek Klosowski]

On 12/22/2009 01:19 PM, Carlo de Falco wrote:
> On 22 Dec 2009, at 19:17, Carlo de Falco wrote:
>
> > Hi,
> >
> > I have a 480x480 sparse matrix of rank 479
> > trying to solve
> >
> > A\ones(480,1)
> >
> > makes matlab throw a warning and return garbage
> >
> > > > A\ones(480,1)
> > Warning: Matrix is close to singular or badly scaled.
> >         Results may be inaccurate. RCOND = 3.815118e-20.
> >
> > ans =
> >
> >   1.0e+17 *
> >
> >   -0.2637
> >   -0.2637
> >   -0.2637
> >   -0.2637
> >   -0.2637
> >   -0.2637
> >   -0.2637
On my stock Fedora 12:

system("rpm -q kernel octave")
kernel-2.6.31.6-166.fc12.i686
octave-3.2.3-2.fc12.1.i686

Using your matrix (which by the way shows rank 478 not 479 as you reported:

A\ones(480,1)
warning: warning 1, at line 613 in file 
../Supernodal/t_cholmod_super_numeric.c
warning: matrix not positive definite
ans =

  -2.6969e+17
  -2.6969e+17
  -2.6969e+17
  -2.6969e+17
  -2.6969e+17
  -2.6969e+17