2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/117881We obtain an upper bound of the number of collisions of any billiard trajectory in a polyhedral angle in terms of the minimal eigenvalue of a positive definite matrix which characterizes the angle. Elements of the matrix are scalar products between the unit normal vectors of faces of the angle.6 pagesDynamical Systems58F15, (28A65)text