[Octave-bug-tracker] [bug #49523] eigs (R2013b and later) uses NaNs, not zeros, for unconverged Ritz values

[Marco Caliari]

Follow-up Comment #9, bug #49523 (project octave):

In the nonsymmetric case, the nonconverged Ritz values are set to NaN+NaNi in
Matlab. You can try


A = toeplitz ([-2, 1, zeros(1, 8)]) + magic (10) * 1e-10;
A = kron (A, eye (10)) + kron (eye (10), A);
opts.v0 = (1:100)';
opts.maxit = 1;
eigs (A, 8, 'sm', opts)


The converged values are complex with imaginary part zero. If you omit opts,
you see that all the converged eigenvalues are double. If we follow Matlab and
comment #5, in case of nonconverged Ritz values we should set them and the
corresponding eigenvectors to NaN+NaNi. An alternative would be: if the
converged Ritz values are double, set the nonconverged and the corresponding
eigenvectors to (double) NaN. This would save some memory.

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