1. S. P. Novikov, Usp. Mat. Nauk,39, No. 4, 97?98 (1984).
2. P. G. Grinevich and R. G. Novikov, Dokl. Akad. Nauk SSSR,286, No. 1, 19?22 (1986).
3. R. G. Novikov, Teor. Mat. Fiz.,66, No. 2, 234?240 (1986).
4. P. G. Grinevich and S. V. Manakov, Funkts. Anal. Prilozhen.,20, No. 2, 14?24 (1986).
5. L. D. Faddeev, Dokl. Akad. Nauk SSSR,165, No. 3, 514?517 (1965); ibid.,167, No. 1, 69?72 (1966).