1. V. M. Muzafarov, Teor. Mat. Fiz.,70, 30 (1987).
2. F. D. Gakhov, Boundary-Value Problems [in Russian], Nauka, Moscow (1972).
3. L. A. Takhtadzhyan and L. D. Faddeev, The Hamiltonian Approach in Soliton Theory [in Russian], Nauka, Moscow (1986).
4. R. G. Newton, J. Math. Phys.,23, 2257 (1982).
5. V. E. Zakharov, S. V. Manakov, S. P. Novikov, and L. P. Pitaevskii, The Theory of Solitons: The Inverse Scattering Method [in Russian], Nauka, Moscow (1980).