          if INFO = 0 or INFO = N+1, the leading N-by-N upper
          Hessenberg part of this array contains the upper quasi-
          triangular matrix T in Schur canonical form from a Schur
          factorization of Ac (see argument FACT).

  LDT     INTEGER
          The leading dimension of the array T.  LDT &gt;= max(1,N).

  U       (input or output) DOUBLE PRECISION array, dimension
          (LDU,N)
          If LYAPUN = 'O' and FACT = 'F', then U is an input
          argument and on entry, the