See 'zgees.f' for all details.
ZGEES computes for an N-by-N complex non-symmetric matrix A, the
eigenvalues, the Schur form T, and, optionally, the matrix of Schur
vectors Q. This gives the Schur factorization A = Q*T*(Q**H).
Optionally, it also orders the eigenvalues on the diagonal of the
Schur form so that selected eigenvalues are at the top left.
The leading columns of Q then form an orthonormal basis for the
invariant subspace corresponding to the selected eigenvalues.
A complex matrix is in Schur form if it is upper triangular.