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[Octave-bug-tracker] [bug #55256] Wrong result raise a non-diagonalizabl
From: |
Rik |
Subject: |
[Octave-bug-tracker] [bug #55256] Wrong result raise a non-diagonalizable matrix by a fractional exponent |
Date: |
Thu, 20 Dec 2018 20:13:08 -0500 (EST) |
User-agent: |
Mozilla/5.0 (X11; Linux i586; rv:31.0) Gecko/20100101 Firefox/31.0 |
Follow-up Comment #1, bug #55256 (project octave):
Confirmed.
If you calculate the condition number with respect to eig, you can see that
this is going to be a problem.
A=[1 1 1; 0 1 1; 0 0 1];
condeig (A)
warning: matrix singular to machine precision, rcond = 2.46519e-32
warning: called from
condeig at line 94 column 8
ans =
6.3691e+15
2.0282e+31
2.0282e+31
Also, for grins, in this case one could use sqrtm rather than X^0.5 and that
functions succeeds.
sqrtm (A)
ans =
1.00000 0.50000 0.37500
0.00000 1.00000 0.50000
0.00000 0.00000 1.00000
So, it looks like at a minimum Octave should warn when the matrix is
ill-conditioned and the results are unlikely to be accurate.
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