1. H. Poincaré,On Curves Defined by Differential Equations (Selected Papers) [Russian translation], Gostekhizdat, Moscow-Leningrad (1947).
2. A. M. Lyapunov,Collected Papers, [in Russian], Vol. 2, Izd. AN SSSR, Moscow-Leningrad (1956).
3. Yu. I. Neimark, “Some cases of the dependence of periodic solutions on parameters,”Dokl. Akad. Nauk SSSR [Soviet Math. Dokl.],129, No. 4, 376–379 (1959).
4. J. K. Hale, “Integral manifolds of perturbed differential systems,”Ann. of Math.,73, 496–531 (1961).
5. V. I. Arnol’d, V. S. Afraimovich, Yu. S. Il’yashenko and L. P. Shil’nikov, “Bifurcation theory,” in:Contemporary Problems in Mathematics. Fundamental Directions [in Russian], Vol. 5, Itogi Nauki i Tekhniki, VINITI, Moscow (1986), pp. 5–218.