Exact statistics of three-hard-disk system in two-dimensional space

A system with three hard particles in two-dimensional rectangle container is investigated. The system motion is chaotic and ergodic in energy surface. The partition function of the system in its energy surface is analytically calculated based on a suitable configuration classification. Theoretical results are fully confirmed by numerical simulations. A van der Waals-like behavior, i.e., increasing system size results in increasing pressure on the container wall, is revealed in some region of the container size. This generalizes the observation of the two-hard-disk system of Phys. Rev. E 65 (2002) 066104.