bugGNU Octave - Bugs: bug #32121, logm returns complex numbers


bug #32121: logm returns complex numbers

Submitter:  Guillaume <gyom>
Submitted:  Thu 13 Jan 2011 07:57:42 PM UTC
Category:  Libraries Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Incorrect Result
Status:  Fixed Assigned to:  None
Originator Name:  Guillaume Open/Closed:  * Closed
Release:  * dev Operating System:  * GNU/Linux
Fixed Release:  None Planned Release:  None
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Sun 04 Sep 2011 11:53:40 PM UTC, comment #4: 
Rik <rik5>
Group administrator
Wed 02 Feb 2011 12:48:44 PM UTC, comment #3: 

Agreed. As the eigenvalues are computed at the beginning of the function (and tested for non-positiveness), it should be fairly straightforward to return real(s) if (all (diag (s) >= 0)) [the two 's' variables are different here].

Guillaume <gyom>
Tue 01 Feb 2011 08:14:41 PM UTC, comment #2: 

Actually no threshold is needed in this specific case. A real matrix without non-positive real eigenvalues always has a real logarithm, and as far as I can tell the employed algorithm does find the real solution, so any imaginary parts should be due to numerical imprecision and can be thrown away.

Gunnar Farneb├Ąck <gf>
Fri 14 Jan 2011 06:33:17 AM UTC, comment #1: 

What threshold should be used to determine if the imaginary parts that are produced are insignificant?

John W. Eaton <jwe>
Group administrator
Thu 13 Jan 2011 07:57:42 PM UTC, original submission:  

logm sometimes returns complex numbers with Octave dev, while it doesn't with Octave 3.2.4 or MATLAB.

For example, with the attached file:

>> load R.mat
>> lR = logm(R)

Octave dev returns:
lR =
  -2.6321e-16 - 1.3010e-18i  -1.5583e-03 + 3.4694e-18i  -3.6555e-02 - 5.2042e-18i
   1.5583e-03 + 6.9389e-18i   1.2423e-16 - 4.1633e-17i   1.9404e-01 + 2.0817e-17i
   3.6555e-02 - 5.2042e-18i  -1.9404e-01 + 2.0817e-17i  -6.9390e-16 + 1.3878e-17i

while Octave 3.2.4 returns:
lR =
  -1.4738e-16  -1.5583e-03  -3.6555e-02
   1.5583e-03   4.1562e-16   1.9404e-01
   3.6555e-02  -1.9404e-01  -6.0125e-16

and MATLAB returns:
lR =
   -0.0000   -0.0016   -0.0366
    0.0016    0.0000    0.1940
    0.0366   -0.1940   -0.0000

Guillaume <gyom>


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Attach Files:

Attached Files
file #22403:  R.mat added by gyom (384B - application/octet-stream)


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    Follow 4 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2011-09-04 rik5 StatusNone Fixed
        Open/ClosedOpen Closed
    2011-02-02 gyom Carbon-Copy- Added caliari
    2011-01-13 gyom Attached File- Added R.mat, #22403

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