1. See, for instance,K. Huang:Quarks, Leptons and Gauge Fields (World Scientific, 1982).
2. P. C. W. Davies andJ. Brown:Superstrings. A Theory of Everything (Cambridge University Press, Cambridge, 1988).
3. M. B. Green, J. H. Schwarz andE. Witten:Superstring Theory, Vol. I, II (Cambridge University Press, Cambridge, 1987).
4. See, for instance,P. Langacker:Phys. Rep.,72, 185 (1981).
5. M. Göckeler andT. Schücker:Differential Geometry, Gauge Theories, and Gravity (Cambridge University Press, Cambridge, 1987).