1. M. B. Green, J. H. Schwarz and E. Witten, Superstring Theory. Vols. 1 & 2 (Cambridge University Press 1987); J. Polchinski, String Theory. Vols. 1 & 2 (Cambridge University Press 1998). See also, E. Alvarez, lectures in this School.
2. J. Ellis, N.E. Mavromatos and D.V. Nanopoulos, Phys. Lett. B293 (1992) 37; J. Chaos, Solitons and Fractals 10, 345 (eds. C. Castro and M.S. El Naschie, Elsevier Science, Pergamon 1999) [hep-th/9805120], and references therein.
3. see for instance L. Smolin, “How far are we from the quantum theory of gravity?,” arXiv:hep-th/0303185, and references therein.
4. L. Smolin, Lectures in this volume.
5. J. Kowalski-Glikman, lectures in this School, and references therein.