[Octave-bug-tracker] [bug #41305] eigs: error -14 in dneupd when requesting fewer eigenvalues

[Pantxo Diribarne]

Follow-up Comment #19, bug #41305 (project octave):

Sorry I have to moderate one of my comments :

Arpack routines return the number of converged eigen values and a warning is
emited by octave if it is not equal to the actual number of requested ones. So
it seems the convergence criterion is applied on a per value basis (so I was
completely wrong, I should learn to avoid guesses). 

In this example:

>There are cases, where  eigs(M,1,'LM')  fails, but
eigs(M,6,'LM')  successfully returns. 

we see no warning when requesting 6 values instead of one , so it looks like
Arpack satisfactorily converged for all individual values and it should be
able to return one value. 

Finally the above mentioned behavior, plus the fact that matlab has the same
issue, make me think that there is indeed a bug in Arpack, *not in octave*.

There is an obvious workaround for octave users: ask for more values than
needed when the error appears. 



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