1. N. N. Bogolyubov and Yu. A. Mitropol’skii, ?? Asymptotic Methods in the Theory of Nonlinear Oscillations (Gostekhizdat, Moscow, 1955) [in Russian].
2. Yu. A. Mitropol’skii, Averaging Method in Nonlinear Mechanics (Naukova Dumka, Kiev, 1971) [in Russian].
3. V. M. Volosov and B. I. Morgunov, Averaging Method in the Theory of Nonlinear Oscillatory Systems (Izd.Moskov. Univ., Moscow, 1971) [in Russian].
4. Yu. A. Mitropol’skii, Nonstationary Processes in Nonlinear Oscillatory Systems, Ed. by N. N. Bogolyubov (AN USSR, Kiev, 1955) [in Russian].
5. Yu. S. Kolesov, V. S. Kolesov, and I. I. Fedik, Self-Oscillations in Distributed-Parameter Systems (Naukova Dumka, Kiev, 1979) [in Russian].