


Outer Billiards on Kites (AM171) 
Outer billiards is a basic dynamical system defined relative to a convex shape in the plane. B. H. Neumann introduced this system in the 1950s, and J. Moser popularized it as a toy model for celestial mechanics. All along, the socalled MoserNeumann question has been one of the central problems in the field. This question asks whether or not one can have an outer billiards system with an unbounded orbit. The MoserNeumann question is an idealized version of the question of whether, because of small disturbances in its orbit, the Earth can break out of its orbit and fly away from the Sun. In Outer Billiards on Kites, Richard Schwartz presents his affirmative solution to the MoserNeumann problem. He shows that an outer billiards system can have an unbounded orbit when defined relative to any irrational kite. A kite is a quadrilateral having a diagonal that is a line of bilateral symmetry. The kite is irrational if the other diagonal divides the quadrilateral into two triangles whose areas are not rationally related. In addition to solving the basic problem, Schwartz relates outer billiards on kites to such topics as Diophantine approximation, the modular group, selfsimilar sets, polytope exchange maps, profinite completions of the integers, and solenoidsconnections that together allow for a fairly complete analysis of the dynamical system. "This research is an exemplary sample of experimental mathematics."Serge L. Tabachnikov, Mathematical Review Another Princeton book authored or coauthored by Richard Evan Schwartz: Series:
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