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

[Jose E. Roman]

> El 7 sept 2022, a las 6:18, Patrick Alken <patrick.alken at geomag.info> escribió:
> 
> 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?

You should never get NaN. Can you send a reproducible example?

> 
> 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?

What do you mean more accurate? The residual norm computed with EPSComputeError() should be below the tolerance in both debugging and optimized versions.

Jose