Chaotic Boltzmann's Billiard Systems at positive energy

This paper deals with the so-called Boltzmann billiard, that is, a billiard subjected to a central force of the type $V(r)=-\alpha \mathrm{/}r-\beta \mathrm{/}{r}^2$, $\alpha$ and $\beta$ being positive constants, and with a straight reflection table. In the particular case of $\alpha$ and $\beta$ positive, we prove the presence of a symbolic dynamics, and hence of positive topological entropy, at positive energy and for $\beta$ sufficiently small.