EISPACK

EISPACK is a historic software library of Fortran subroutines designed to compute eigenvalues and eigenvectors of matrices. It provided a portable, machine-independent collection for real and complex data, covering a range of matrix classes including general dense matrices and certain structured forms such as symmetric or Hermitian.

The package consists of modular subroutines intended to solve eigenvalue problems, returning eigenvalues and, where available, eigenvectors. It was widely used in scientific computing during the 1970s and 1980s and was distributed with documentation that described input/output conventions, error handling, and testing.

EISPACK is a predecessor to modern linear algebra libraries such as LAPACK. In the 1990s, LAPACK reimplemented many EISPACK routines with improved reliability and performance by leveraging the Basic Linear Algebra Subprograms (BLAS), and ultimately superseded EISPACK in many applications. Today, EISPACK remains of historical interest and is accessible from archival repositories for reference and education.