1. N. N. Bogolyubov and Yu. A. Mitropol’skii, Asymptotic Methods in the Theory of Nonlinear Oscillations (Nauka, Moscow, 1974) [in Russian].
2. Yu. A. Mitropol’skii, The Method of Averaging in Nonlinear Mechanics (Naukova Dumka, Kiev, 1971) [in Russian].
3. Yu. A. Mitropol’skii, Transient Processes in Nonlinear Oscillatory Systems (Izd. Akad. Nauk Ukrain. SSR, Kiev, 1955) [in Russian].
4. V. M. Volosov and B. I. Morgunov, The Method of Averaging in the Theory of Nonlinear Oscillatory Systems (Izd. Mosk. Univ., Moscow, 1971) [in Russian].
5. Yu. S. Kolesov, V. S. Kolesov and and I. I. Fedik, Self-Oscillations in Distributed-Parameter Systems (Naukova Dumka, Kiev, 1979) [in Russian].