## Preprint ims94-14

M. Boshernitzan, G. Galperin, T. Kruger, & S. Troubetzkoy
Some Remarks on Periodic Billiard Orbits in Rational Polygons

Abstract: A polygon is called rational if the angle between each pair of sides is a rational multiple of $\pi.$ The main theorem we will prove is Theorem 1: For rational polygons, periodic points of the billiard flow are dense in the phase space of the billiard flow. This is a strengthening of Masur's theorem, who has shown that any rational polygon has many'' periodic billiard trajectories; more precisely, the set of directions of the periodic trajectories are dense in the set of velocity directions $\S^1.$ We will also prove some refinements of Theorem 1: the well distribution'' of periodic orbits in the polygon and the residuality of the points $q \in Q$ with a dense set of periodic directions.
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