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MathGroup Archive 2005

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Re: Re: SingularValueDecomposition bug: fifty examples

  • To: mathgroup at smc.vnet.net
  • Subject: [mg61227] Re: [mg61183] Re: SingularValueDecomposition bug: fifty examples
  • From: Bruce Miller <brucem at wolfram.com>
  • Date: Thu, 13 Oct 2005 01:39:35 -0400 (EDT)
  • References: <200510060745.DAA08051@smc.vnet.net> <200510120542.BAA09202@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

General suggestion: if one of you thinks he has an example of
a bug in Mathematica, please send it to support at wolfram.com.
ONE example per email, with an explanation of what appears to be
wrong with that example.
Include the Mathematica input, the output, and the VERSION NUMBER.

Mr. Sidles's  tarball is being looked at, but it is one
of many things in that Tech Support engineer's queue.

Bruce

On Oct 12, 2005, at 12:42 AM, John Sidles wrote:

> Dear MathGroup folks
>
> Per my exploration of errors in SingularValueDecomposition[],
> the directory:
>
>     http://faculty.washington.edu/sidles/mma_svd_failure_example/
>
> contains an updated tarball whose contents are summarized in
> the PDF file "readme_v4.0.pdf".
>
> The new findings include:
>
>   (1) SingularValueDecomposition[] is shown to fail on both G4 and G5
>       computers.  However, the patterns of failure are slightly
>       different for the two architectures.  Reason unknown.
>
>   (2) Formerly, the only examples of SVD failure in Mathematica
>       were of Hermitian matrices.  Now, the tarball includes
>       examples of failing non-Hermitian matrices.  These are generated
>       by randomly permuting the rows and columns of the failing
>       Hermitian matrices.
>
> One plausible explanation might be that Mathematica's implementation
> does not set LAPACK's machine precision parameters (which determine
> convergence) quite right for the new 64-bit archtecture of the G4
> and G5.  This would be easy to fix.
>
> This concludes my posting on this topic until December, which
> will be the earliest time that I can further investigate this
> topic.  Hopefully, by then Wolfram Research will have fixed
> the problem!
>
> Sincerely ... John Sidles


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