[Octave-bug-tracker] [bug #60924] DNAUPD did not find any eigenvalues to sufficient accuracy

[Rik]

Follow-up Comment #2, bug #60924 (project octave):

The code is using a function handle, in place of a matrix A, as the first
argument to eigs().  This is certainly allowed, but is an uncommon usage.

Does the code guarantee that the problem is unsymmetric?  The user is
requesting the Smallest Real ("sr") eigenvalues and according to the
documentation for eigs,


    "sr"
          Smallest Real part (valid only for complex or unsymmetric
          problems).


The problem is not complex because the entire output for the function handle
is wrapped in a call to real().


A = @(f) real(-ifft( -(eps/6)^(2/3)*ksq_eig .* fft(f)) -
alpha/(4*(eps/6)^(1/3))*u_eig .* f);


That would be my first thought.

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