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2. B. P. Demidovich, Lectures on Mathematical Theory of Stability (Nauka, Moscow, 1967) [in Russian].
3. V. I. Kalenova and V. M. Morozov, Nonstationary Linear Systems and Their Application to Mechanics (Fizmatlit, Moscow, 2010) [in Russian].
4. J. J. Stoker, Nonlinear Vibrations in Mechanical and Electrical Systems (Wiley, New York, 1950; Inostrannaya Literatura, Moscow, 1952).
5. N. N. Bogolyubov and Yu. A. Mitropol’skii, Asymptotic. Methods in the Theory of Nonlinear Oscillations (Nauka, Moscow, 1974; Gordon and Breach, New York, 1964).