          norm(Pi)*norm(G) ) / norm(X),

  where Omega, Theta and Pi are linear operators defined by

  Omega(W) = op(Ac)'*W*op(Ac) - W,
  Theta(W) = inv(Omega(op(W)'*X*op(Ac) + op(Ac)'X*op(W))),
     Pi(W) = inv(Omega(op(Ac)'*X*W*X*op(Ac))),

  and Ac = inv(I_n + G*X)*A (if TRANA = 'N'), or
      Ac = A*inv(I_n + X*G) (if TRANA = 'T' or 'C').

  Note that the Riccati equation (1) is equivalent to

      X = op(Ac)'*X*op(Ac) + op(Ac)'*X*G*X*op(Ac) + Q,           (2)

  and to
      _          _             