[petsc-users] Eigenvalues calculations in PETSc

Qin Lu lu_qin_2000 at yahoo.com
Wed Feb 26 17:16:57 CST 2014

The linear system is hard to solve and may be ill-conditioned. I got the condition number from PETSc, which does not look too bad (around 10^4), then the distribution of eigenvalues (spectrum) will be helpful for choosing suitable reordering/preconditioning types.

 From: Matthew Knepley <knepley at gmail.com>
To: Qin Lu <lu_qin_2000 at yahoo.com> 
Cc: "petsc-users at mcs.anl.gov" <petsc-users at mcs.anl.gov> 
Sent: Wednesday, February 26, 2014 4:46 PM
Subject: Re: [petsc-users] Eigenvalues calculations in PETSc

On Wed, Feb 26, 2014 at 2:41 PM, Qin Lu <lu_qin_2000 at yahoo.com> wrote:

>I need to find the spectrum of the a large linear system (over 100,000 unknowns). Option -ksp_compute_eigenvalues_explicitly ran out of memory since the matrix is too big, then -ksp_compute_eigenvalues only gave a small number of eigenvalues (around 30). My questions are:
>1. Is the number of computed eigenvalues determined by the number of GMRES iterations to converge?
>2. Can PETSc give the extreme eigenvalues now that the number of computed eigenvalues is limited?

It will help to understand why you want these.

Thanks a lot for your info,

What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
-- Norbert Wiener 
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