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

Jason Grout jason-sage at creativetrax.com
Tue Apr 26 12:22:46 EDT 2011


On 4/26/11 11:12 AM, Jason Grout wrote:
> On 4/26/11 11:07 AM, Jason Grout wrote:
>> And indeed, I get a 0 row as the last row of the V**H matrix
>
> I just double-checked things one last time and saw that I actually
> hadn't changed the first argument of zgesdd to "A" in the program that I
> actually ran.  So with this change, I get a nonzero last row of the V**H
> matrix from the C call to zgesdd.  So everything is consistent between
> the C call to zgesdd and the numpy svd call.
>
> So now my remaining question is: if the Lapack docs only say that V**H
> is the full n-by-n matrix if M>=N, why is numpy returning it even if M<N?

One more post talking to myself...

I notice that the zgesvd routine docs guarantee that the V returned is 
unitary, regardless of the size of A.  So this might be another argument 
for calling zgesvd instead of zgesdd.

Thanks,

Jason




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