Periodic Orbits of Oval Billiards on Surfaces of Constant Curvature
  In this paper we define and study the billiard problem on bounded regions on surfaces of constant curvature. We show that this problem defines a 2-dimensional conservative and reversible dynamical system, defined by a Twist diffeomorphism, if the boundary of the region is an oval. Using these properties and defining good perturbations for billiards, we show, in this new version, that having only a finite number of nondegenerate periodic orbits for each fixed period is an open property for billiards on surfaces of constant curvature and a dense one on the Euclidean and the hyperbolic planes. For the proof of the density, the techniques we use for the Euclidean and hyperbolic cases, do not work for the spherical case, due to a constraint (the perimeter of the polygonal trajectory being a multiple of {\pi}). We finish this paper studying the stability of these nondegenerate orbits. 
Luciano Coutinho dos Santos, Sonia Pinto-de-Carvalho
math.DS