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[Octave-bug-tracker] [bug #49523] eigs (R2013b and later) uses NaNs, not

From: Rik
Subject: [Octave-bug-tracker] [bug #49523] eigs (R2013b and later) uses NaNs, not zeros, for unconverged Ritz values
Date: Fri, 1 Dec 2017 00:36:15 -0500 (EST)
User-agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:55.0) Gecko/20100101 Firefox/55.0

Follow-up Comment #52, bug #49523 (project octave):

Maybe Carlo has an idea on how to relax this.  I found that by increasing the
maximum number of iterations, opts.maxit, ARPACK would converge and find all
the eigenvalues.  That is not what is needed here.

I restored maxit to 1, and then started changing the initialization vector v0.
 For some vectors there was rapid convergence and I got the three eigenvalues
listed in comment #50 (although they were all real, no imaginary parts).

Maybe there is a specific initial vector which is always far enough from a
solution that the answer from comment #46 is obtained.

Otherwise, we could work around it by coding the BIST test like this

if (isreal (d))
  assert (d(4), NaN);
  assert (d(4), NaN+1i*NaN);


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