Sylvie Oliffson Kamphorst and Sonia Pinto de Carvalho
Bounded Gain of Energy on the Breathing Circle Billiard
(1266K, postscipt)

ABSTRACT.  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.