1. V. E. Zakharov et al., Theory of Solitons. The Method of Inverse Problem [in Russian], Nauka, Moscow (1980).
2. L. A. Takhtadzhyan and L. D. Faddeev, The Hamiltonian Method in the Theory of Solitons [in Russian], Nauka, Moscow (1986).
3. I. V. Cherednik, Teor. Mat. Fiz.,61, No. 1, 35?44 (1984).
4. V. G. Drinfel'd, Dokl. Akad. Nauk SSSR,283, No. 5, 1060?1064 (1985).
5. O. I. Bogoyavlenskii (Bogoyavlensky), Commun. Math. Phys.,51, No. 3, 201?210 (1976).