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2. N. N. Bogolyubov, “Perturbations theory in nonlinear mechanics,” Sb. Tr. In-ta Stroitel’noy Mekhaniki AN USSR, 14, 9–34 (1950).
3. N. N. Bogolyubov and Yu. A. Mitropol’skiy, Asymptotic Methods of the Theory of Nonlinear Oscillations [in Russian], Fizmatlit, Moscow (1963).
4. V. Sh. Burd, The Method of Averaging over an Infinite Interval and Some Problems in the Theory of Oscillations [in Russian], YarGU, Yaroslavl’ (2013).
5. Yu. L. Daletskii and M. G. Kreyn, Stability of Solutions of Differential Equations in a Banach Space [in Russian], Nauka, Moscow (1970).