 leading N-by-N part of this
          array must contain the orthogonal matrix U from a real
          Schur factorization of Ac (see argument FACT).
          If LYAPUN = 'O' and FACT = 'N', then U is an output
          argument and on exit, if INFO = 0 or INFO = N+1, it
          contains the orthogonal N-by-N matrix from a real Schur
          factorization of Ac (see argument FACT).
          If LYAPUN = 'R', the array U is not referenced.

  LDU     INTEGER
          The leading dimension of the arra