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2. A. C. Scott, F. Y. F. Chu, and D. W. McLaughlin, Proc. IEEE 61, 1442 (1973);
3. E. K. Sklyanin, L. A. Takhtadzhyan, and L. D. Faddeev, Theor.Math. Phys. 40, 688 (1979);
4. G. L. Lamb, Jr., Elements of Soliton Theory (Wiley, New York, 1980); H. B. Thacker, Rev. Mod. Phys. 53, 253 (1981)
5. See, e.g., Dynamical Problems in Soliton Systems, edited by S. Takeno (Springer, New York, 1985)