We discuss a connection between two well-known constructions in mathematics:
Cayley graphs and rational billiard surfaces. We describe a natural way to draw a
Cayley graph of a dihedral group on each rational billiard surface. Both of these
objects have the concept of “genus” attached to them. For the Cayley graph, the
genus is defined to be the lowest genus amongst all surfaces that the graph can be
drawn on without edge crossings. We prove that the genus of a Cayley graph
associated with a triangular billiard table is always 0 or 1. One reason this is
interesting is that there exist triangular billiard surfaces of arbitrarily high genus, so
the genus of the associated graph is often much lower than the genus of the billiard
surface.
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