1. A. A. Kharkevich, Self-Induced Vibrations (Gostekhizdat, Moscow, 1953) [in Russian].
2. A. A. Andronov, A. A. Vitt, and S. E. Khaikin, Oscillation Theory (Fizmatgiz, Leningrad, 1959) [in Russian].
3. S. Lefschetz, Differential Equations: Geometric Theory (Wiley, New York, 1957).
4. N. N. Bogolyubov and Yu. A. Mitropol’skii, Asymptotical Methods in the Theory of Nonlinear Oscillations (Nauka, Moscow, 1974) [in Russian].
5. A. Blaquiere, Nonlinear System Analysis (Acad. Press, New York, 1966).