shseqr(3) on Linux
The text a71dc8db4f93, as Debian 4.0 (Etch), Debian 3.1 (Sarge) shipped it.
compute the eigenvalues of a real upper Hessenberg matrix H and, optionally, the matrices T and Z from the Schur decomposition H = Z T Z**T, where T is an upper quasi-triangular matrix (the Schur form), and Z is the orthogonal matrix of Schur vectors