1. ’t Hooft, G. (1993) in Aly, A., Ellis, J. and Randjbar-Daemi, S. (eds.), Salamfestschrifft, World Scientific, Singapore, gr-qc/9310026.
2. Susskind, L. (1995) J. Math. Phys., 36, 6377.
3. Bekenstein, J.D. (1974) Lett. Nuovo Cimento, 4, 737 and Phys. Rev. D, 9, 3292.
4. Wald, R.M. (2001) Living Reviews in Relativity, 2001-2006.
5. Bekenstein, J.D. (2000) Phys. LettersB, 481, 339; (2001) in Ruffini, R. (ed.), Proceedings of the Ninth Marcel Grossmann Meeting on General Relativity, World Scientific, Singapore.