1. Beletsky, V .V.: 1969, Iskusstvennjie Sputniki Zenli Izd. ANSSSR, 3.
2. Beletsky, V.V. and Novikova, E.T.: 1969b, Kosmicheskiya Issledovania 7, 377.
3. Beletsky, V.V.: 1969a, KosmicheskiyaIssledovania 7, 827.
4. Bogoliubov, N.N. and Mitropolsky, Y.A.: 1961, Asymptotic Methods in the Theory of Non-linear Oscillations, Hindustan Publishing Corporation, Delhi, p. 6.
5. Das, S.K., Bhattacharya, P.K., and Singh, R.B.: 1976, Proc. Nat. Acad. Sci. India 46, 287.