books.google.ca - Mathematical billiards describe the motion of a mass point in a domain with elastic reflections off the boundary or, equivalently, the behavior of rays of light in a domain with ideally reflecting boundary. From the point of view of differential geometry, the billiard flow is the geodesic flow on a manifold...http://books.google.ca/books/about/Geometry_and_billiards.html?id=g8PdXL4ST4YC&q=Exercise&utm_source=gb-gplus-shareGeometry and billiards
