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2. Birkhoff, G. D.: Surface transformations and their dynamical applications. Acta Mathematicae,43, 44?47. (1920) Reprinted In: Collected Mathematical Papers of G. D. Birkhoff, Vol. II. New York: Dover 1968 pp. 195?202
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4. Birkhoff, G. D.: Sur quelques courbes fermées remarquables Bull. S.M.F.60, (1932), reprinted in Collected Math. Papers, Vol. II, pp. 418?443
5. Birkhoff, G. D., Sur l'existence de régions d'instabilité en dynamique. Ann Inst. Henri Poincaré2, (1932), reprinted in Collected Math. Papers, Vol. II, pp. 444?461