Kolmogorov–Sinai Entropy, Lyapunov Exponents, and Mean Free Time in Billiard Systems

We perform new experiments on the Kolmogorov–Sinai entropy, Lyapunov exponents, and the mean free time in billiards. We study their dependence on the geometry of the scatterers made up of two interpenetrating square lattices, each one with circular scatterers with different radius. We find, in particular, that the above quantities are continuous functions of the ratio of the scatterer radius. However, it seems that their derivative is discontinuous around the radius ratio which separates the diffusive and nondiffusive types of geometries.