1. A.M. Polyakov and A.B. Zamolodchikov, unpublished.
2. A.M. Polyakov, Phys. Lett. 103B (1981) 207, 211.
3. H. Poincare, J. Math. Pure App. 5 se. 4 (1898) 157.
4. T.L. Curtright and C.B. Thorn, Phys. Rev. Lett. 48 (1982) 1309; E. Braaten, T. Curtright and C. Thorn, Phys. Lett. 118B (1982) 115; Ann. Phys. 147 (1983) 365; E. Braaten, T. Curtright, G. Ghandour and C. Thorn, Phys. Rev. Lett. 51 (1983) 19; Ann. Phys. 153 (1984) 147.
5. D. Friedan, unpublished.