[SciPy-User] Eigenvectors of sparse symmetric matrix

Lutz Maibaum lutz.maibaum@gmail....
Mon Oct 25 20:31:08 CDT 2010

```On Oct 25, 2010, at 5:39 PM, Lutz Maibaum wrote:
> On Oct 25, 2010, at 3:27 PM, Pauli Virtanen wrote:
>> The wrappers are currently written to print a warning if the iteration
>> does not converge, and I do not see yet why that doesn't happen.
>
> Interesting, I don't get a warning. I see that there is a convergence check in _SymmetricArpackParams.iterate(), but not in _UnsymmetricArpackParams.iterate(). Could that be the reason?

The test is actually there, the symmetric case just has an additional test.

However, I noticed that the warning is triggered when the returned "info" status is -1. Is it possible that it should be compared to 1? From http://www.caam.rice.edu/software/ARPACK/UG/node137.html:

c  INFO    Integer.  (INPUT/OUTPUT)
c          If INFO .EQ. 0, a randomly initial residual vector is used.
c          If INFO .NE. 0, RESID contains the initial residual vector,
c                          possibly from a previous run.
c          Error flag on output.
c          =  0: Normal exit.
c          =  1: Maximum number of iterations taken.
c                All possible eigenvalues of OP has been found. IPARAM(5)
c                returns the number of wanted converged Ritz values.
c          =  2: No longer an informational error. Deprecated starting
c                with release 2 of ARPACK.
c          =  3: No shifts could be applied during a cycle of the
c                Implicitly restarted Arnoldi iteration. One possibility
c                is to increase the size of NCV relative to NEV.
c                See remark 4 below.
c          = -1: N must be positive.
c          = -2: NEV must be positive.
c          = -3: NCV-NEV >= 2 and less than or equal to N.
c          = -4: The maximum number of Arnoldi update iteration
c                must be greater than zero.
c          = -5: WHICH must be one of 'LM', 'SM', 'LR', 'SR', 'LI', 'SI'
c          = -6: BMAT must be one of 'I' or 'G'.
c          = -7: Length of private work array is not sufficient.
c          = -8: Error return from LAPACK eigenvalue calculation;
c          = -9: Starting vector is zero.
c          = -10: IPARAM(7) must be 1,2,3,4.
c          = -11: IPARAM(7) = 1 and BMAT = 'G' are incompatable.
c          = -12: IPARAM(1) must be equal to 0 or 1.
c          = -9999: Could not build an Arnoldi factorization.
c                   IPARAM(5) returns the size of the current Arnoldi
c                   factorization.

Thanks,

Lutz

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