inconsistent condition numbers from cond, det, inv functions

[Rolf Fabian]

octave-3.0.0.exe:> version, computer
ans = 3.0.0
i686-pc-msdosmsvc

# a singular matrix
octave-3.0.0.exe:> x=[1,2,3;2,3,4;3,4,5]
x =
   1   2   3
   2   3   4
   3   4   5

octave-3.0.0.exe:> [dum,rc1]=det(x)
dum = 0
rc1 = 2.3130e-018

octave-3.0.0.exe:> [dum,rc2]=inv(x); rc2
rc2 = 2.3130e-018

octave-3.0.0.exe:> rc3 = 1./cond(x)
rc3 = 4.1351e-017

The help parts of 'det' and 'inv' do not explain
whether a different definition of condition
number than 'cond' is used during evaluation.

Thus all results of 'rc' are expected to be
identical, but result from 1/cond(x) differs
unfortunately. Needs to be fixed.

No idea what MatLab returns for the
three cases. Probably somebody has the
time to check it.

Rolf Fabian



-----
Rolf Fabian
<r dot fabian at jacobs-university dot de>

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