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Eigenvalues broken?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg102827] Eigenvalues broken?
  • From: "vadim.oganesyan at gmail.com" <vadim.oganesyan at gmail.com>
  • Date: Fri, 28 Aug 2009 00:47:04 -0400 (EDT)
  • References: <200907310952.FAA19221@smc.vnet.net> <h50sjj$6pl$1@smc.vnet.net>

I have been doing full diagonalization of sparse symmetric real
matrices (Hamiltonians) of different sizes and just noticed that
Eigenvalues (and Eigensystem) break once the matrices are 256by256
(maybe smaller, but that's where i first saw it).

i first noticed the problem when checking the quality of the solutions
by comparing eigenvalues with expectation of the matrix in each of
eigenvectors.

even more dramatically, simply rerunning Eigenvalues twice on the same
matrix produces somewhat different results!!!

If I chose to get only few eigenvales (thus forcing it to use Arnoldi
method) the problem goes away.

I was recently told that the version of Lapack used by Mathemica was
recompiled by people at Wolfram for added functionality -- did they
break it too?
My trusty C++ code has no problems with these matrices....

thoughts? suggestions?
thanks!
PS
i can send the matrix in a file to anybody willing to look into this


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