Three-Period Orbits in Billiards on the Surfaces of Constant Curvature
  An approach due to Wojtkovski [9], based on the Jacobi fields, is applied to study sets of 3-period orbits in billiards on hyperbolic plane and on two-dimensional sphere. It is found that the set of 3-period orbits in billiards on hyperbolic plane, as in the planar case, has zero measure. For the sphere, a new proof of Baryshnikov's theorem is obtained which states that 3-period orbits can form a set of positive measure provided a natural condition on the orbit length is satisfied. 
Victoria Blumen, Ki Yeun Kim, Joe Nance, Vadim Zharnitsky
math.DS