The Hessenberg form of a matrix is zero below the first
subdiagonal. If the matrix is symmetric or Hermitian,
the form is tridiagonal. [P,H] = HESS(A) produces a unitary
matrix P and a Hessenberg matrix H so that A = P*H*P' and
P'*P = EYE (P). By itself, HESS(A) returns H.