1. For a review see: Alessandrini, V., Amati, D., Le Bellac, M., Olive, D.: Phys. Rep.C 1, 269 (1971)
2. Kaku, M., Kikkawa, K.: Phys. Rev.D10, 110, 1823 (1974)
3. For a review, see Hoker, E. D., Phong, D.: The geometry of string peturbation theory. Rev. Mod. Phys.
4. Neveu, A., West, P.: Phys. Lett.168B, 192 (1985); Nucl. Phys.B278, 601 (1986),B293, 266 (1987); Witten, E.: Nucl. Phys.B268, 253 (1986); Hata, H., Itoh, K., Kugo, T., Ogawa, K.: Phys. Lett.172B, 186 (1986)
5. Neveu, A., West, P.: Nucl. Phys.B278, 601 (1986) Sect. 3