1. H. Flashka, Progr. Theoret. Phys., 51, 703?716 (1974).
2. S. V. Manakov, JETP, 40, 269?274 (1975).
3. V. A. Marchenko, Sturm-Liouville Operators and Their Applications[in Russian], Naukova Dumka, Kiev (1977); English transl., Birkh¨auser, Basel (1986).
4. L. A. Takhtadzhyan and L. D. Faddeev, Hamiltonian Approach to the Theory of Solitons [in Russian], Nauka, Moscow (1986); English transl.: L. D. Faddeev and L. A. Takhtajan The Hamiltonian Methods in the Theory of Solitons, Berlin, Springer (1987).
5. M. Toda, Nonlinear Lattice Dynamics[in Japanese], Iwanami Shoten, Tokyo (1978); English transl.: Theory of Nonlinear Lattices, Springer, Berlin (1981).