1. V. E. Zakharov and A. B. Shabat, Zh. Eksp. Teor. Fiz.,61, 118 (1971).
2. V. E. Zakharov and A. B. Shabat, Zh. Eksp. Teor. Fiz.,64, 1627 (1973).
3. L. D. Faddeev, Preprint R2-12462 [in Russian], JINR, Dubna (1978).
4. E. K. Sklyanin, Differential Geometry, Lie Groups, and Mechanics. III [in Russian], Nauka, Leningrad (1980), p. 55.
5. F. A. Berezin, G. P. Pokhil, and V. M. Finkel'berg, Vestn. Mosk. Univ., Ser. 1, No.1, 21 (1964).