We apply the periodic orbit expansion to the calculation of transport, thermo-
dynamic, and chaotic properties of the finite-horizon triangular 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 con-
vergence of the cycle expansions obtained from the Ruelle dynamical (-function.
For this system with severe pruning we find that applying standard convergence
acceleration schemes to the periodic orbit expansion is superior to the dyna-
mical (-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 used here is com-
putationally competitive with the standard simulation approaches for calculat-
ing averages. However, we believe that these expansion methods are of
fundamental importance, because they give a direct route to the phase space
distribution function.