ztrsen(3) on Linux
The text 6c327886f255, as Debian 4.0 (Etch), Debian 3.1 (Sarge) shipped it.
reorder the Schur factorization of a complex matrix A = Q*T*Q**H, so that a selected cluster of eigenvalues appears in the leading positions on the diagonal of the upper triangular matrix T, and the leading columns of Q form an orthonormal basis of the corresponding right invariant subspace