1. V. E. Zakharov and E. A. Kuznetsov, Zh. Éksp. Teor. Fiz. 113, 1892 (1998) [JETP 86, 1035 (1998)].
2. Y. S. Kivshar and B. Luther-Davies, Phys. Rep. 298, 81 (1998).
3. F. T. Hioe, Phys. Rev. Lett. 82, 1152 (1999).
4. B. A. Kalinikos, M. M. Scott, and C. E. Patton, Phys. Rev. Lett. 84, 4697 (2000).
5. L. D. Faddeev and L. A. Takhtajan, Hamiltonian Methods in the Theory of Solitons (Nauka, Moscow, 1986; Springer-Verlag, Berlin, 1987).