1. Andronov, A. A., Vitt, A. A. and Khaikin, S. E.:Theory of Oscillators, Dover edition 1987; 1st Russian edition 1937; 1st English edition, Pergamon Press, 1966.
2. Cartwright, M. L.: Forced oscillations in nearly-sinusoidal systems,J. Inst. Elec. Eng. 95 (1948), 88?96.
3. Falcone, M. and Israel, G.: Qualitative and numerical analysis of a class of prey-predators models,Acta Appl. Math. 4 (1984), 303?354.
4. Falcone, M., Israel, G. and Tedeschini-Lalli, L.: A class of prey-predator models with competition between predators, Nota interna, Dipartimento di Matematica, Università di Roma ?La Sapienza?, March 1984.
5. Gallavotti, G.:Meccanica Elementare, Boringhieri, 1981; English version:The Elements of Mechanics, Springer, 1984.