Bounded Gain of Energy on the Breathing Circle Billiard
  The Breathing Circle is a 2-dimensional generalization of the Fermi Accelerator. It is shown that the billiard map associated to this model has invariant curves in phase space, implying that any particle will have bounded gain of energy. 
Sylvie Oliffson Kamphorst and Sonia Pinto de Carvalho (Departamento de   Matematica, ICEx, UFMG. Brasil)