DECAY OF CORRELATIONS AND CONTROL OF CHAOTIC BILLIARDS

We consider a two-dimensional square Sind billiard with a centered dis c, a computer simulation of which shows that a specific correlation fu nction displays an initial nonexponential decay. The initial nonexpone ntial era is larger when the Lyapounov exponent is smaller. The onset of the exponential era corresponds to the onset of chaos in the system , and the initial nonexponential era can be understood as the preparat ion time for the manifestation of chaos. By suitable perturbation of t he radius of the central disc, the initial nonexponential era increase s 8 times in length, thus giving rise to a particular parametric contr ol of the system.