[Octave-bug-tracker] [bug #44004] eig and eigs routine

[Lachlan Andrew]

Follow-up Comment #5, bug #44004 (project octave):

Thanks, Jordi, for pointing out that the initialization is random.  However,
that is not causing the problem here.

It seems that the call to the   dnaupd   function in ARPACK works reliably
(and that is where the randomness in the algorithm is).

The problem is in the call to  dneupd.  Most of the time, it correctly returns
the first  k  values of workl[iptr(5)] as expected.  However sometimes it
returns the *last* two (which are not valid eigenvalues) instead of the first
two.

Sometimes it replaces three or four, depending on the value of  k.

Note that the main difference between
  D = eigs(...)
and
  [V,D] = eigs(...)
is in the first parameter passed to  dneupd.

bug #41305 also suggests that there is a (different) bug in  dneupd, so I
wouldn't rule out that possibility.

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