PERIODIC ORBIT EXPANSIONS FOR THE LORENTZ GAS

We apply the periodic orbit expansion to the calculation of transport, thermodynamic. and chaotic properties of the finite-horizon triangula r Lorentz gas. We show numerically that the inverse of the normalized Lyapunov number is a good estimate of the probability of an individual periodic orbit. We investigate the convergence of the periodic orbit expansion and compare it with the convergence of the cycle expansions obtained from the Ruelle dynamical zeta-function. For this system with severe pruning we find that applying standard convergence acceleratio n schemes to the periodic orbit expansion is superior to the dynamical zeta-function approach. The averages obtained from the periodic orbit expansion are within 8% of the values obtained from direct numerical time and ensemble averaging. None of the periodic orbit expansions use d here is computationally competitive with the standard simulation app roaches for calculating averages. However, we believe that these expan sion methods are of fundamental importance, because they give a direct route to the phase space distribution function.