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

From: Marco Caliari
Subject: [Octave-bug-tracker] [bug #49523] eigs (R2013b and later) uses NaNs, not zeros, for unconverged Ritz values
Date: Fri, 1 Dec 2017 02:22:57 -0500 (EST)
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:57.0) Gecko/20100101 Firefox/57.0

Follow-up Comment #53, bug #49523 (project octave):

I agree with Rik's proposal in comment #52: even if we can find a magic
combination of v0, tol, maxit working with the existing combinations of
blas/lapack/arpack/whatever, it is not sure it will work for any future
combination. Here the important thing is to get either NaN or Nan+1i*NaN (and
not NaN+1i*0).

To comment #37: -14 in XYeupd is "did not find any eigenvalues to sufficient
accuracy". It is just non-convergence with the given parameters and it could
be expected in our cases since the maximum number of iterations was strongly
decreased. We could add a string to the error message like: "try to increase
the tolerance, the maximum number of iterations or the number of requested
eigenvalues". The third hint seems strange, but arpack uses internally Krylov
spaces (spaces in which to look for eigenpairs) of default dimension opts.p =
2 * number_of_requested_eigenvalues. 


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