[Scipy-tickets] [SciPy] #1317: Arpack wrappers should tolerate odd # of complex eigenvalues

SciPy Trac scipy-tickets@scipy....
Sat Oct 23 09:51:42 CDT 2010

#1317: Arpack wrappers should tolerate odd # of complex eigenvalues
 Reporter:  pv                   |       Owner:  wnbell
     Type:  defect               |      Status:  new   
 Priority:  normal               |   Milestone:  0.9.0 
Component:  scipy.sparse.linalg  |     Version:  0.8.0 
 Keywords:                       |  
 Remaining item from #1313:

 III) This had been the hardest issue to find the reason for since I've
 never done anything with ARPACK. What I've got was an InvalidIndexError?
 here: arpack.py:264 z[:,i] = zr[:,i] + 1.0j * zr[:,i+1] called
 (indirectly) from test_nonsymmetric_modes. The reason is the following:
 I've got three eigenvalues in d with imaginary part nonzero. Why do I get
 it and you don't: If you don't give arpack a starting vector, it'll
 generate a random one (THIS SHOULD BE DOCUMENTED!) and this influences -
 which eigenvalues are returned by ARPACK - in which order they're
 returned. I've written two pure fortran test programs resembling python's
 ARPACK usage if you want to verify this (also happens on my Ubuntu Lucid
 Lynx x86_64 box). One should think about fixing that piece of code there
 in arpack.py (taking into account the possibility of an odd number of
 imaginary eigenvalues). But the testsuite will still fail due to the fact
 that the set of eigenvalues returned is not fixed. The solution is to fix
 them by giving starting vectors in the testsuite (see

Ticket URL: <http://projects.scipy.org/scipy/ticket/1317>
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