Belavin, A., Polyakov, A., Schwartz, A., and Tyupkin, Y., Phys. Lett. 59B, 85 (1975).
Belavin, A. and Zakharov, V., Phys. Lett. 73B, 53 (1978).
Atiyah, M., Drinfeld, V., Hitchin, N., and Manin, Y., Phys. Rev. Lett., 65A, 185 (1978).
Pohlmexer, K., Comm. Math. Phys. 74, 111 (1980).
Zakharov, V.E. and Shabat, A.B., Funkst. Analiz 13, N3, 13 (1979).
Zamolodchikov, A.B., ZhETF 79, N8 (1980).