[Pkg-scicomp-devel] Bug#569436: Bug#569436: lapack: zgesvd seems to give incorrect results

[Kumar Appaiah]

Dear Sylvestre,

I've zeroed down the bug to the last blas upload. Note that for the
intel program, the expected Left singular vectors are:

 Left singular vectors (stored columnwise)
 ( -0.86,  0.00) (  0.40,  0.00) (  0.32,  0.00)
 ( -0.35,  0.13) ( -0.24, -0.21) ( -0.63,  0.60)
 (  0.15,  0.32) (  0.61,  0.61) ( -0.36,  0.10)

Here is with the older Blas:

[$ /tmp] sudo dpkg -i libblas3gf_1.2-2_amd64.deb 
(Reading database ... 22762 files and directories currently
installed.)
Preparing to replace libblas3gf 1.2-2 (using
libblas3gf_1.2-2_amd64.deb) ...
Unpacking replacement libblas3gf ...
Setting up libblas3gf (1.2-2) ...
[$ /tmp] ./a.out 
 ZGESVD Example Program Results

 Singular values
  17.63  11.61   6.78

 Left singular vectors (stored columnwise)
 ( -0.86, -0.00) (  0.40, -0.00) (  0.32,  0.00)
 ( -0.35,  0.13) ( -0.24, -0.21) ( -0.63,  0.60)
 (  0.15,  0.32) (  0.61,  0.61) ( -0.36,  0.10)

 Right singular vectors (stored rowwise)
 ( -0.22,  0.51) ( -0.37, -0.32) ( -0.53,  0.11) (  0.15,  0.38)
 (  0.31,  0.31) (  0.09, -0.57) (  0.18, -0.39) (  0.38, -0.39)
 (  0.53,  0.24) (  0.49,  0.28) ( -0.47, -0.25) ( -0.15,  0.19)


With the newer Blas:

[# /tmp] sudo dpkg -i libblas3gf_1.2-3_amd64.deb 
(Reading database ... 22762 files and directories currently
installed.)
Preparing to replace libblas3gf 1.2-2 (using
libblas3gf_1.2-3_amd64.deb) ...
Unpacking replacement libblas3gf ...
Setting up libblas3gf (1.2-3) ...
[# /tmp] ./a.out 
 ZGESVD Example Program Results

 Singular values
  17.63  11.61   6.78

 Left singular vectors (stored columnwise)
 ( -0.86, -0.00) (  0.40, -0.00) (  0.32,  0.00)
 ( -0.35, -0.21) ( -0.24, -0.21) ( -0.63,  0.60)
 (  0.15,  0.61) (  0.61,  0.61) ( -0.36,  0.10)

 Right singular vectors (stored rowwise)
 ( -0.22,  0.51) ( -0.37, -0.32) ( -0.53,  0.11) (  0.15,  0.38)
 (  0.31,  0.31) (  0.09, -0.57) (  0.18, -0.39) (  0.38, -0.39)
 (  0.53,  0.24) (  0.49,  0.28) ( -0.47, -0.25) ( -0.15,  0.19)

Note that the second line of the Left Singular vectors doesn't match.

I couldn't progress further on my probe, but hope this helps when you
come back to this bug.

Thanks!

Kumar
-- 
We do not stray out of all words into the ever silent;
We do not raise our hands to the void for things beyond hope.
- Rabindranath Tagore (The Gardener, 1915)
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