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2. N. N. Moiseev, Asymptotic Methods of Nonlinear Mechanics [in Russian], Izd. Nauka, Moscow (1969).
3. G. N. Hapoport, A. K. Nemak, and V. A. Zhurakhovskii, Radiotekh. Élektron.,12, No. 4, 633 (1967).