1. B. B. Kadomtsev and V. I. Petviashvili, Sov. Phys. Dokl., 15, 539–541 (1970).
2. A. V. Mikhailov, JETP Letters, 30, 414–418 (1979).
3. V. E. Zaharov, S. V. Manakov, S. P. Novikov, and L. P. Pitaevskii, Theory of Solitons: The Inverse Problem Method [in Russian], Nauka, Moscow (1980).
4. V. S. Dryuma, JETP Letters, 19, 387–388 (1974).
5. V. E. Zakharov and A. B. Shabat, Funct. Anal. Appl., 8, No. 3, 226–235 (1974).