[Octave-bug-tracker] [bug #46683] Need better configure test for bad ARPACK library

[Marco Caliari]

Follow-up Comment #17, bug #46683 (project octave):

Hi all,

I started with the job by searching all previous bugs related to eigs in order
to understand whether they were caused by ARPACK. To be clear, I refer to the
problem described here

https://forge.scilab.org/index.php/p/arpack-ng/issues/1315/

fixed in ARPACK 3.2.0 using dlahqr from LAPACK 2.0

https://github.com/opencollab/arpack-ng/pull/2

changed in ARPACK 3.3.0 using my hint on the previous web page. Right now, I
have a rough fortran code which fails with ARPACK 3.1.5 and succeeds with both
ARPACK 3.2.0 and 3.3.0. Good. But while searching the previous eigs bugs, I
found some incompatible behaviours with ml. This can due to different arpack
libraries behind or different eigs implementations or, probably, both.
Moreover, I have to say that I cannot reproduce Octave's eigs behaviour with
simple fortran files and ARPACK library. The main problem here is that I do
not fully understand how eigs uses ARPACK. My proposal is to make a group that
wants to understand and writes a document, like David's Sparse matrix
implementation in Octave. I think this is a nice project for students in
mathematics or computer science, too.


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