1. N. J. Zabusky and M. D. Kruskal, Phys. Rev. Lett. 15, 240 (1965).
2. J. K. Perring and T. H. R. Skyrme, Nucl. Phys. 31, 550 (1962).
3. A. C. Scott, F. Y. F. Chu, and D. W. McLaughlin, IEEE Proc. 61, 1443 (1973).
4. V. G. Makhankov, Phys. Rep. 35, 1 (1978).
5. S. R. Coleman, in New Phenomena in Subnuclear Physics, Proceedings of the International School of Subnuclear Physics, Erice, Sicily, July 11–Aug. 1, 1975, Subnucl. Ser. 13, 297 (1977).