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2. Arnold, V. I., Small Denominators and Problems of Stability of Motion in Classical and Celestial Mechanics, Russian Math. Surveys, 1963, vol. 18, no. 6, pp. 85–191; see also: Uspekhi Mat. Nauk, 1963, vol. 18, no. 6, pp. 91–192.
3. Moser, J., On Invariant Curves of Area-Preserving Mappings of an Annulus, Nach. Akad. Wiss. Göttingen, Math. Phys. Kl. II, 1962, vol. 1962, no. 1, pp. 1–20.
4. Grundlehren Math. Wiss.;C L Siegel,1971
5. Markeev, A. P., On the Problem of Stability of Equilibrium Positions of Hamiltonian Systems, J. Appl. Math. Mech., 1970, vol. 34, no. 6, pp. 941–948; see also: Prikl. Mat. Mekh., 1970, vol. 34, no. 6, pp. 997–1004.