Space-time directional Lyapunov exponents for cellular automata
  Space-time directional Lyapunov exponents are introduced. They describe the maximal velocity of propagation to the right or to the left of fronts of perturbations in a frame moving with a given velocity. The continuity of these exponents as function of the velocity and an inequality relating them to the directional entropy is proved. 
Maurice Courbage (MSC), Brunon Kaminski