Periods of Pseudo-Integrable Billiards

Abstract

Consider billiard desks composed of two concentric half-circles connected with two edges. We examine billiard trajectories having a fixed circle concentric with the boundary semicircles as the caustic, such that the rotation numbers with respect to the half-circles are ρ1 and ρ2 respectively. Are such billiard trajectories periodic, and what are all possible periods for given ρ1 and ρ2?.