1. Sinai, Ya. G.: Dynamical systems with elastic reflections. Ergodic properties of dispersing billiards. Usp. Mat. Nauk25, 141?192 (1970); Russ. Math. Surv.25, 137?189 (1970)
2. Bunimovich, L. A., Sinai, Ya. G.: On a fundamental theorem in the theory of dispersing billiards. Mat. Sb.90, 415?431 (1973); Math. USSR Sb.19, 407?424 (1973)
3. Arnold, V.J.: Small denominators and problems of stability of motion in classical and celectial mechanics. Usp. Mat. Nauk18, 91?192 (1963); Russ. Math. Surv.18, 85?191 (1963)
4. Hopf, E.: Statistik der Lösungen geodätischer Probleme vom unstabilen Typus. II. Math. Ann.117, 590?608 (1940)
5. Bunimovich, L. A.: On billiards close to dispersing. Mat. Sb.94, 49?73 (1974); Math. USSR Sb.23, 45?67 (1974)