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3. R. Bishop, Oscillations [Russian translation], Izd. Nauka, Moscow (1968).
4. A. A. Kharkevich, Self-Oscillations [in Russian], Gostekhizdat, Moscow (1954).
5. A. I. Akhiezer and R. V. Polovin, Usp. Fiz. Nauk,104, No. 2, 185 (1971).