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DSYEVR returns non-orthogonal vectors #151

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miroi opened this issue May 15, 2017 · 8 comments

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@miroi
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commented May 15, 2017

Greetings,

thanks for moving lapack resources to github.

i) I wish to remind the old reported bug, http://icl.cs.utk.edu/lapack-forum/viewtopic.php?f=2&t=1820#p11388 , showing up with the newest lapack-3.7.0, as I am reporting here, https://github.com/miroi/lapack-dsyevr-test .

Based on the lapack bug tracker ( http://www.netlib.org/lapack/bug_list.html - bug 126), you are taking care of this nasty bug, right ?

ii) Freel free to take this specific vectors orthogonality test ( https://github.com/miroi/lapack-dsyevr-test ) and add it to lapack tests.

Best, Miro

@langou

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commented May 16, 2017

Hi Miro, Thanks for adding the BUG 126 as a GitHub issue. This is great. Osni ( @oamarques ) is working on #100. Thanks for sharing your test code with us. Yes there is a plan to work on a new test suite for LAPACK and any nasty matrix is welcome. Osni has many such matrices and the goal would be to grow a collection of these, along with an infrastructure for testing. In any case, thanks for adding the BUG 126 as a GitHub issue. This is great. Cheers, Julien.

@oamarques

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commented Jun 7, 2017

This bug is related to the splitting criterion used in DSTEMR, starting at line 606 in the corresponding source file (as in LAPACK 3.7.0). Based on the input variable TRYRAC, the subroutine DLARRR returns INFO=1 (this seems to contradict the expected behavior of DLARRR), which subsequently triggers a harmful splitting of the (particular) tridiagonal matrix passed as input. If we ignore what DLARRR does and set IINFO=0 just before "Set the splitting criterion" the returned vectors are orthogonal, when using a new version of DLARRF, see issue 100. However, this is not a proper fix to this bug: we need to figure out what the correct behavior of DLARRR should be.)

@miroi

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commented Mar 24, 2018

Hi, any news concerning solving this bug?

@uihsnv

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commented Apr 5, 2018

@oamarques Does using DSTEGR instead of the underlying DSTEMR mitigate this issue?

@langou

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commented Apr 5, 2018

@uihsnv: good question. STEGR is not maintained any longer. There are issues with it. We are only working on and maintaining STEMR. But, yes, it would be a good idea to see what STEGR does on this problem.

@uihsnv

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commented Apr 5, 2018

@langou Oh! I had no idea!! I'm using a couple of instances of STEGR in my code. Should I switch to using STEMR?

Also, I've made some modifications to the test that @miroi shared, which if he accepts, you can see the differences.

@langou

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commented Apr 5, 2018

I'm using a couple of instances of STEGR in my code. Should I switch to using STEMR?

I think so. I hope @oamarques will follow up soon on this with us.

@langou

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commented Apr 5, 2018

Also, I've made some modifications to the test that @miroi shared, which if he accepts, you can see the differences.

It would be great to share the modified test with @oamarques

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