1. Green, M., Schwarz, J., Witten, E. (1987). Superstring Theory (Cambridge University Press, Cambridge).
2. de Vega, H. J., and Sánchez, N. (1994). String Theory in Cosmological Spacetimes (Proc. Erice School, Sicily), N. Sanchez, ed. (World Scientific, Singapore).
3. de Vega, H. J. (1992). Strings in Curved Spacetimes (Proc. Erice School, Sicily), N. Sanchez, ed. (World Scientific, Singapore).
4. 't Hooft, G. (1990). Nucl. Phys. B 335, 138.
5. Susskind, L., Thorlacius, L., and Uglum, J. (1993). Phys. Rev. D 48, 3743.