1. S.W. Hawking, Particle Creation by Black Holes, Commun. Math. Phys. 43 (1975) 199 [Erratum ibid. 46 (1976) 206-206] [ INSPIRE ].
2. J.D. Bekenstein, Black holes and entropy, Phys. Rev. D 7 (1973) 2333 [ INSPIRE ].
3. S.W. Hawking, Black Holes and Thermodynamics, Phys. Rev. D 13 (1976) 191 [ INSPIRE ].
4. R.M. Wald, The thermodynamics of black holes, Living Rev. Rel. 4 (2001) 6 [ gr-qc/9912119 ] [ INSPIRE ].
5. R. Sorkin, On the entropy of the vacuum outside a horizon, in Tenth International Conference on General Relativity and Gravitation, Contributed Papers, vol. II, B. Bertotti, F. de Felice and A. Pascolini eds., Consiglio Nazionale Delle Ricerche, 1983. http://www.perimeterinstitute.ca/personal/rsorkin/some.papers/31.padova.entropy.pdf .