[petsc-users] Eigenvalues calculations in PETSc

Jed Brown jed at jedbrown.org
Thu Feb 27 12:33:31 CST 2014

Richard Mills <rtm at utk.edu> writes:

> Qin,
> Krylov methods will find the extreme eigenvalues first (unless you do
> something in particular to get at the interior values), so this is what you
> are getting from PETSc.
> Another thing to keep in mind is that the eigenvalue estimates using the
> Hessenberg matrix from GMRES can be pretty terrible.  If you want something
> better, you may want to use something like Arnoldi/Lanczos or
> Jacobi-Davidson.  Yousef Saad's book at
> http://www-users.cs.umn.edu/~saad/eig_book_2ndEd.pdf offers some great
> explanations of these methods, which you can use via the PETSc-based SLEPc
> package.

Agreed.  Use SLEPc if you want actual eigenvalues or eigenvectors.  You
can target wherever you want.
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