[Octave-bug-tracker] [bug #59488] eigs error in dsaupd for zero matrix

[Rik]

Follow-up Comment #12, bug #59488 (project octave):

Thanks for testing.  4.4.1 and 5.2.0 are the same thing, just with a nicer
decoding of the message.  I checked the code in libinterp and liboctave and it
doesn't appear that Octave changes the vector in any way.  This means that the
error message from 4.4.1 ("error -9 in dsaupd") is correct, but attributing it
to an empty starting vector is incorrect in this case.

There is a dependence on the version of the external ARPACK library.  In my
case, I am using 3.5.0+real-2 and it just works.

It seems like ARPACK may be getting more brittle in the types of input
matrices it can accept.  That is annoying, but means we at Octave probably
have to program defensively around it.  I would suggest the nnz() test in
__eigs__.cc.

I would probably put this change on the development branch rather than the
about-to-be-released stable branch.  It is a corner case, and there is an easy
workaround.  If Octave throws this error then the programmer already knows
that all eigenvalues are zero and there is no need for any computation.

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