1. Nelson, P.: Phys. Rep.149, 337 (1987) and references therein
2. D'Hoker, E., Phong, D.H.: Rev. Mod. Phys.60, 917 (1988) and references therein
3. Seiberg, N.: Lecture at the 1990 Yukawa International Seminar on Common Trends in Mathematics and Quantum Field Theories, Kyoto, Japan (1990) and at the Cargese meeting Random surfaces, quantum gravity and strings. Cargese, France (1990) and references therein
4. D'Hoker, E.: Preprint UCLA/91/TEP/35 (1991) and references therein
5. Polyakov, A.: Phys. Lett.B103, 207 (1981); Phys. Lett.B103, 211 (1981)