[SciPy-dev] Ticket #709 (was: Re: Next scipy release 0.7)
Tue Sep 30 15:23:49 CDT 2008
On Tue, 30 Sep 2008 19:43:10 +0000 (UTC)
Pauli Virtanen <email@example.com> wrote:
> Tue, 30 Sep 2008 21:21:38 +0200, Nils Wagner wrote:
>> On Tue, 30 Sep 2008 18:36:12 +0000 (UTC)
>> Pauli Virtanen <firstname.lastname@example.org> wrote:
>>> Tue, 30 Sep 2008 19:56:58 +0200, Nils Wagner wrote:
>>>> should be resolved, too.
>>> Short summary about this one: it's about a special
>>>matrix pair for which
>>> LAPACK's generalized eigenvalue problem solver DGGEV
>>> output, losing one eigenvalue from a complex eigenpair.
>>> problem should nonetheless be well-defined, AFAIK, this
>>>looks like a bug
>>> in LAPACK.)
>> In that case the LAPACK developers should be informed as
>> possible. Did you check a FORTRAN implementation of the
>>test case ?
> I didn't contact LAPACK devs yet. I'll try to see if I
>can reduce this to
> a simple Fortran testcase.
> But to my understanding, LAPACK's DGEEV has also some
>other known issues,
> see eg.
>> Can someone run the test (2dof.py) in Matlab, Octave,
> I got similar suspect result from Octave's QZ when I
>tried it (Octave
> dropped one of the eigenvalues, too).
> Matlab AFAIK does not use Lapack's DGGEV, so the
>testcase probably will
> not fail in the same way with it.
>>> The question here is what we should/can do:
>>> 1. Raise LinAlgError if we detect this condition.
>>> 2. Try to repair the returned data by filling in the
>>>other eigenvalue of
>>> the pair, as we know all complex eigenvalues come in
>> Well, this holds for real matrices but what could be
>>done in case of
>> complex matrices ?
>For complex matrices the eigenvectors are returned by
>LAPACK as complex
> data directly (and don't need to be separately
>constructed from real
> data, as in the real case), so they are simply passed
>throught by Scipy.
> If the results are wrong, we probably can't do anything
>to fix or detect
> Pauli Virtanen
Please find attached an u n t e s t e d FORTRAN
implementation of 2dof.py.
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