1. A. Stephenson, Mem. Proc. Manchester Literary and Philos. Soc. 52(8), 1 (1908).
2. A. Stephenson, Philos. Mag. 17, 765 (1909).
3. P. L. Kapitsa, Zh. Éksp. Teor. Fiz. 21(5), 588 (1951).
4. V. N. Chelomeĭ, Dokl. Akad. Nauk SSSR 110(3), 345 (1956).
5. N. N. Bogolyubov and Y. A. Mitropolsky, Asymptotic Methods in the Theory of Non-Linear Oscillations (Gordon and Breach, New York, 1961; Nauka, Moscow, 1974).