> On 5 Aug 2022, at 5:04 AM, Patrick Alken <[email protected]> wrote:
>
> I am trying to solve a generalized symmetric eigensystem problem using SLEPc,
> and I am getting much different eigenvalues compared with the dense LAPACK
> solver DSYGVD. I have attached a minimal working example (MWE). The code
> relies on the external libraries GSL, Lapack and Lapacke.
>
> The matrices (A,B) come from discretizing the Laplacian in spherical
> coordinates using a radial basis function method. Here are the eigenvalues
> computed by the dense LAPACK solver:
>
> === LAPACK eigenvalues ===
> eval(0) = 1.309567289750e+00
> eval(1) = 1.648674331144e+00
> eval(2) = 1.709879393690e+00
> eval(3) = 1.709879393690e+00
> eval(4) = 2.142210251638e+00
> eval(5) = 2.142210251638e+00
> eval(6) = 2.145726457896e+00
> eval(7) = 2.216526311324e+00
> eval(8) = 2.216526311324e+00
> eval(9) = 2.695837030236e+00
>
>
> Here are the eigenvalues computed by SLEPc, using the command:
>
> $ ./mwe -eps_nev 10 -eps_smallest_magnitude -eps_tol 1e-8
>
> Number of requested eigenvalues: 10
> Linear eigensolve converged (10 eigenpairs) due to CONVERGED_TOL; iterations
> 196
> ---------------------- --------------------
> k ||Ax-kBx||/||kx||
> ---------------------- --------------------
> 0.094520 906.031
> 0.177268 363.291
> 1.324501 61.9577
> 1.786472 35.7082
> 2.187857 32.8364
> -2.886905 27.7314
> 2.899206 24.2224
> 4.195222 20.3007
> 4.787192 12.8346
> 7.221589 9.58513
> ---------------------- --------------------
>
> As we can see, the results are quite different than LAPACK and the error
> norms in the right column are quite large. Using a stricter tolerance
> (eps_tol) does not seem to help. Can anyone suggest how to diagnose and fix
> the problem?
You probably want to use a spectral transformation to find the smallest
magnitude eigenpairs.
I cannot try your code because of its dependencies, but I bet that if you
replace -eps_smallest_magnitude by -eps_target 0.0 -st_type sinvert, you’ll get
a match with the LAPACK eigenvalues.
Thanks,
Pierre
> <laplace3d.c><le_laplace3d.h><Makefile.txt><mwe.c>
