1. A.H. Nayfeh, D.T. Mook, Nonlinear Oscillations (Wiley-Interscience, New York, 1979)
2. M.I. Dykman, M.A. Krivoglaz, Soviet Scientific Reviews Volume (Harwood Academic, New York, 1984) pp. 265
3. L.D. Landau, E.M. Lifshitz, Mechanics, 3rd edn. (Pergamon, New York, 1976)
4. A.H. Nayfeh, Introduction to Perturbation Techniques. (Wiley, New York, 1981)
5. V.I. Arnold, Geometrical Methods in the Theroy of Ordinaty Differential Equations, Volume 250 of Grundlehren der mathematischen Wissenschaften, 2nd edn. (Springer, New York, 1988)