ARPACK is a collection of Fortran77 subroutines designed to solve large
scale eigenvalue problems.

The package is designed to compute a few eigenvalues and corresponding
eigenvectors of a general n by n matrix A. It is most appropriate for
large sparse or structured matrices A where structured means that a
matrix-vector product w <- Av requires order n rather than the usual
order n**2 floating point operations. This software is based upon an
algorithmic variant of the Arnoldi process called the Implicitly
Restarted Arnoldi Method (IRAM).