dtrsen(3) on Linux
The text 5c9cd7051c10, as Debian 4.0 (Etch), Debian 3.1 (Sarge) shipped it.
reorder the real Schur factorization of a real matrix A = Q*T*Q**T, so that a selected cluster of eigenvalues appears in the leading diagonal blocks of the upper quasi-triangular matrix T,