1. Dipartimento Di Matematica, Politecnico Di Torino, Turin, Italy 2. Facultad de Ingenieria, UAQ, Santiago de Quer??taro, QRO, Mexico 3. Instituto de Matem??ticas, UNAM, Mexico City, Mexico
Abstract:
We prove that a compact minimal shadow boundary of a hypersurface in Euclidean space is totally geodesic. We show that shadow boundaries detect principal directions and umbilical points of a hypersurface. As application we deduce that every shadow boundary of a compact strictly convex surface contains at least two principal directions.