1. N. M. Krylov and N. N. Bogolyubov, Introduction to Nonlinear Mechanics [in Russian], Akad. Nauk Ukr. SSR, Kiev (1937).
2. V. I. Arnold, Mathematical Methods in Classical Mechanics [in Russian], Nauka, Moscow (1989).
3. V. M. Volosov and B. I. Morgunov, Averaging Method in the Theory of Nonlinear Oscillating Systems [in Russian], Moscow University, Moscow (1971).
4. N. N. Bogolyubov and Yu. A. Mitropol’skii, Asymptotic Methods in the Theory of Nonlinear Oscillations [in Russian], Fizmatgiz, Moscow (1963).
5. Yu. A. Mitropol’skii, Averaging Method in Nonlinear Mechanics [in Russian], Naukova Dumka, Kiev (1971).