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3. A. A. Zevin, ?Qualitative analysis of stability of periodic oscillation and rotation in parametrically excitable nonlinear second-orer systems,? Mekh. Tverd. Tela, Akad. Nauk SSSR, No. 2, 38?44 (1983).
4. É. Kamke, Handbook on Ordinary Differential Equations [in Russian], Nauka, Moscow (1976).
5. B. G. Korenev, Introduction to the Theory of Bessel Functions [in Russian], Nauka, Moscow (1971).