[petsc-users] [slepc] nan output for eigenvectors

[Barry Smith]

   You can run with -fp_trap to have the program stop as soon as the first Nan or Inf appears, this can help track down why it is happening. In a debugger you can also set the debugger to trap on floating point exceptions (syntax is debugger dependent) to focus in on where it first happens.

  Barry


> On Sep 7, 2022, at 3:20 AM, Jose E. Roman <jroman at dsic.upv.es> wrote:
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>> El 7 sept 2022, a las 6:18, Patrick Alken <patrick.alken at geomag.info> escribió:
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>> I sometimes get Nan output values in computed eigenvectors for the generalized symmetric eigenvalue problem produced by slepc. Is this a known issue, and is it related to the conditioning of the matrix pair (A,B)? Is there some way to compute a "condition number" of the matrix pair ahead of time to see if i have a good chance of getting stable eigenvectors out?
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> You should never get NaN. Can you send a reproducible example?
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>> 
>> In a possibly related issue, i am finding that petsc/slepc compiled with debugging vs optimization can produce very different eigenvectors for the same problem, while the eigenvalues are the same. The eigenvectors seem more accurate when I use the debugging version of the libraries. Could this be also a conditioning problem with the matrix pair?
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> What do you mean more accurate? The residual norm computed with EPSComputeError() should be below the tolerance in both debugging and optimized versions.
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> Jose
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