 is only just stable (or convergent) then the Lyapunov
  equation will be ill-conditioned.  A symptom of ill-conditioning
  is "large" elements in U relative to those of A and B, or a
  "small" value for scale.

  SB03OZ routine can be also used for solving "unstable" Lyapunov
  equations, i.e., when matrix A has all eigenvalues with positive
  real parts, if DICO = 'C', or with moduli greater than one,
  if DICO = 'D'. Specifically, one may solve for X = op(U)**H*op(U)
  either the continuous-time Lyapunov