[Octave-bug-tracker] [bug #46683] Need better configure test for bad ARPACK library

[Marco Caliari]

Follow-up Comment #20, bug #46683 (project octave):

Here my first example that I cannot reproduce in plain fortran. The matrix is
AA (taken from #31479), the initial vector opts.v0


A = [1,0,0,-1;0,1,0,0;0,0,1,0;0,0,2,1];
AA = sparse([A,zeros(4),zeros(4);zeros(4),A,zeros(4);zeros(4),zeros(4),A]);
opts.v0 =
[0.239;0.420472;0.358959;0.6824;0.483916;0.312213;0.845595;0.298945;0.38219;0.595434;0.01263;0.66212];


and I use eigs(AA,1,'lm',opts). I get  1.0000e+00 - 2.8107e-06i (Octave 4.0.0,
arpack 3.3.0). From line 296 in __eigs__.cc I understand have_sigma = false
and then EigsRealNonSymmetricMatrix should be used (see line 584). In
eigs_base.cc, line 1696, mode=1 (regular mode) and in line 1731, p=n-1=11. So,
it should correspond to dndrv1.f (attached). But here the result has imaginary
part exactly zero (as in Matlab, by the way).

(file #35998)
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Additional Item Attachment:

File name: dndrv1.f                       Size:14 KB


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