1. Faddeev, L. D., Integrable Models in 1+1 Dimensional Quantum Field Theory, in Les Houch Lectures 1982, Elsevier, Amsterdam, 1984.
2. Kulish, P. P. and Sklyanin, E. K., Lecture Notes in Physics 151, 61 (1982).
3. Baxter, R. J., Ann. Phys. 70, 193 (1972).
4. Faddeev, L. D. and Takhatajan, L. A., Usp. Mat. Nauk 34, 13 (1979).
5. Kulish, P. P. and Reshetikhin, N. Yu., J. Phys. A 16, L591 (1983).