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3. B. van der Pol, Nonlinear Theory of Electrical Circuits [Russian translation], Svyaz’, Moscow (1935).
4. N. V. Butenin, Yu. I. Neimark, N. A. Fufaev, An Introduction to the Theory of Nonlinear Oscillations [in Russian], Nauka, Moscow (1976).
5. A. N. Sharkovskii, S. F. Kolyada, A. G. Sivak, and V. V. Fedorenko, Dynamics of One-Dimensional Mappings [in Russian], Naukova Dumka, Kiev (1989).