Thermodynamic formalism for dispersing billiards

For any finite horizon Sinai billiard map T on the two-torus, we find t_*>1 such that for each t in (0,t_*) there exists a unique equilibrium state $μ_t$ for $- t\log J^uT$, and $μ_t$ is T-adapted. (In particular, the SRB measure is the unique equilibrium state for $- \log J^uT$.) We show that $μ_t$ is exponentially mixing for Holder observables, and the pressure function $P(t)=\sup_μ\{h_μ-\int t\log J^uT d μ\}$ is analytic on (0,t_*). In addition, P(t) is strictly convex if and only if $\log J^uT$ is not $μ_t$ a.e. cohomologous to a constant, while, if there exist $t_a\ne t_b$ with $μ_{t_a}= μ_{t_b}$, then P(t) is affine on (0,t_*). An additional sparse recurrence condition gives $\lim_{t\to 0} P(t)=P(0)$.

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