caliarim

Which eigenvalues are you trying to compute? Which mode?

I'm away from my computer. Is it possible that M0 is not SPD? Can you compute chol (M0) in octave?

Ok, I will give a look when back from my holidays.

Which version of arpack are you using?

In the documentation of dnaup2 it is written that info = -8 (lapack error) should never happen. So, yes, either there is a problem with lapack libraries (and this affects...

@manavbhatia: Am I right that you are using lapack

So, 3.0.0? Anyway, I can reproduce the problem with lapack 3.5.0. The problem is in dlahqr.f. It was fixed in 3.6.0 http://www.netlib.org/lapack/errata_from_350_to_360.html#_strong_span_class_green_bug125_span_strong_xlahqr_v3_5_does_not_converge_on_some_matrices_while_xlahqr_v2_converges

Did you use a plain fortran implementation when you discovered the bug? If yes, you can download dlahqr.f from lapack 3.8.0 and add it to the compilation. I did ```...

ATLAS provides only few lapack routines. And, unfortunately, I'm not sure ATLAS is still developed: the last email from its main author is from August 2017. I recently switched to...

> I feel a simple utility is missing to run/test arpack on user matrix [matrix market format] just to see/check eigen val/vec I think we should distinguish run and see...