1. S. Aubry, "The twist map, the extended Frenkel?Kontorova model and the devil's staircase," Physica,7D, 240?258 (1983).
2. S. Aubry and P. Y. Le Daeron, "The discrete Frenkel?Kontorova model and its extensions," Physica,8D, 381?422 (1983).
3. M. Herman, "Sur les courbes invariantes par les diffeomorphismes de l'anneau," Asterisque,103?104, 3?38 (1983).
4. H. M. Morse, "A fundamental class of geodesics on any closed surface of genus greater than one," Trans. Am. Math. Soc.,26, No. 1, 25?60 (1924).
5. G. Hedlund, "Geodesics on a two-dimensional Riemannian manifold with periodic coefficients," Ann. Math. Ser. 2,33, No. 4, 719?739 (1932).