chseqr(3) on Linux
The text 20633dde69f4, as Debian 4.0 (Etch), Debian 3.1 (Sarge) shipped it.
compute the eigenvalues of a complex upper Hessenberg matrix H, and, optionally, the matrices T and Z from the Schur decomposition H = Z T Z**H, where T is an upper triangular matrix (the Schur form), and Z is the unitary matrix of Schur vectors