Nicolas Bedaride, Periodic billiard tranjectories in polyhedra,
Forum Geometricorum, 8 (2008) 107--120.
Abstract: We consider the billiard map inside a polyhedron. We give a condition
for the stability of the periodic trajectories. We apply this result to the
case of the tetrahedron. We deduce the existence of an open set of
tetrahedra which have a periodic orbit of length four (generalization of
Fagnano's orbit for triangles), moreover we can study completely the orbit
of points along this coding.