[Numpy-discussion] Zero row in SVD's unitary matrix on some Mac's

Jason Grout jason-sage at creativetrax.com
Tue Apr 26 11:33:07 EDT 2011


On 4/25/11 12:57 PM, Pauli Virtanen wrote:
> On Mon, 25 Apr 2011 10:16:13 -0700, Rob Beezer wrote:
> [clip]
>> Many more details and complete transcripts are at:
>> http://trac.sagemath.org/sage_trac/ticket/11248
>>
>> Any thoughts or advice to help us understand this would be greatly
>> appreciated.
>
> The Numpy routine is a very thin wrapper of LAPACK's ZGESDD, and probably
> cannot have any bugs of this kind, so the problem is most likely with the
> LAPACK and BLAS libraries you use. You will probably be able to reproduce
> the problem also with an equivalent Fortran/C snippet calling LAPACK
> directly.
>
> Problems like this in BLAS/LAPACK are somewhat difficult to track. You
> could try switching to another BLAS library (or, if you use ATLAS,
> compile it differently) and checking if the problem disappears.


I was just looking up the documentation for ZGESDD and noticed that the 
value we have for rwork in the numpy call [1] does not match the Lapack 
docs.  This was changed in Lapack 3.2.2 [2].  I've submitted a pull request:

https://github.com/numpy/numpy/pull/78

but I have not tested the change.

I doubt this fix will fix the problem on this thread, but it makes sense 
to make the change anyway.

Thanks,

Jason

[1] https://github.com/numpy/numpy/blob/master/numpy/linalg/linalg.py#L1300

[2] http://icl.cs.utk.edu/lapack-forum/viewtopic.php?f=2&t=1779, or 
http://www.netlib.org/lapack/lapack-3.2.2.html (bug 0046)




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