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[Octave-bug-tracker] [bug #55710] svd may sometimes return -0 for exactl

From: Tim Mitchell
Subject: [Octave-bug-tracker] [bug #55710] svd may sometimes return -0 for exactly zero singular values
Date: Thu, 14 Feb 2019 13:44:10 -0500 (EST)
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/12.0 Safari/605.1.15

Follow-up Comment #4, bug #55710 (project octave):


1) I agree that this -0 is likely to be an upstream issue with LAPACK. 
Fortunately, it's easy to fix so I imagine it won't be too long from them to
handle it on their end.

2) In the meantime, having Octave implement a temporary workaround would be
ideal.  It's debatable which is better but I think S(S==0)=0 is better than
abs(S).  I doubt there's a difference speedwise.  But if there's ever a bug
where the SVD routines return a negative singular value, with S(S==0)=0, that
negative value would still get passed to the user so someone might actually
notice something is really wrong.  With abs(S), we'd never know.  


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