Billiard dynamics near an invariant horizontal circle
  Sufficiently differentiable oval billiards always have invariant rotational curves, but there are only two types of ovals with an invariant horizontal circle in its phase-space: the constant width ovals and some very special symmetric curves.   In this work we study the dynamics near the horizontal circle for the billiard map of those two types of ovals, and show that the horizontal circle is approached, from both sides, by other invariant rotational curves. We will also describe some dynamical consequences. 
Geraldo Cesar Goncalves Ferreira, Sylvie Oliffson Kamphorst and Sonia   Pinto-de-Carvalho
math.DS nlin.CD