1. Gardner, C. S., Green, J. M., Kruskal, M. D., and Miura, R. M.:Phys. Rev. Lett. 19 (1967), 1095.
2. For recent developments, see A. S. Fokas and V. E. Zakharov (eds),Important Developments in Soliton Theory, Springer-Verlag, New York, 1993.
3. Lax, P.:Comm. Pure Appl. Math. 21 (1968), 467.
4. Bona, J., Pritchard, W. G., and Scott, L. R.:Philos. Trans. Royal Soc. London, A 302 (1981), 457?510.
5. Fokas, A. S.:Proceedings of the III Potsdam-V Kiev International Workshop 1991, Springer-Verlag, Berlin, 1992;