[Octave-bug-tracker] [bug #53719] eigs delivers wrong result in case of complex valued general eigenvalue problem

[Marco Caliari]

Update of bug #53719 (project octave):

                  Status:                   Fixed => In Progress            

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Follow-up Comment #8:

I run the original test (asking for 4 eigenvalues) with a plain fortran file,
and arpack does not converge, which is better than silently returning the
wrong eigenvalues as octave.
Arpack calls several times znaupd which returns a vector x and ask for an
operation y = OP(x) (either y = B*x or y = A\x). I displayed the first element
of x and y before and after each of the operations. They coincide, in plain
fortran and octave, till the end of the first major iteration (about 30 calls
to znaupd). Then, eigenvalues are computed. They coincide, even if they are in
a different order. This is already strange, but maybe I missed a sorting
operation when Arpack directly displays results. Then the first residual has
to be computed. In order to do that, a B*x operation is needed. Well, at this
point, x is different in plain fortran and octave. At this point, resid, which
is an input/output argument for znaupd, is the same as input, but different as
output. It seems that znaupd behaves differently when called by the fortran
file or when called by octave. I have only one set of blas/lapack/arpack
libraries on my system.

I include a zip file, with the modified eigs-base.cc which produces output,
with zndrv4_53719.f (gfortran zndrv4_53719.f -lblas -llapack -larpack), and
with the output files, with an entry LOOK HERE to find the divergence point. I
still have no idea.

(file #44025)
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Additional Item Attachment:

File name: 53719.zip                      Size:20 KB


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