1. C. Rovelli and L. Smolin, “Discreteness of area and volume in quantum gravity,” Nucl. Phys. B, 442, 593–619 (1995); Erratum, 456, 753–754 (1995); arXiv:gr-qc/9411005v1 (1994)
2. A. Ashtekar and J. Lewandowski, “Quantum theory of geometry. I. Area operators,” Class. Q. Grav., 14, A55–A81 (1997); arXiv:gr-qc/9602046v2 (1996).
3. J. Ambjørn, J. Jurkiewicz, and R. Loll, “Nonperturbative Lorentzian path integral for gravity,” Phys. Rev. Lett., 924–927 (2000); arXiv:hep-th/0002050v3 (2000); “Emergence of a 4D world from causal quantum gravity,” Phys. Rev. Lett., 93, 131301 (2004);arXiv:hep-th/0404156v4 (2004); “Reconstructing the universe,” Phys. Rev. D, 72, 064014 (2005); arXiv:hep-th/0505154v2 (2005).
4. M. Bronstein, “Quantentheorie schwacher Gravitationsfelder,” Phys. Z. Sowjetunion, 9, 140–157 (1936).
5. G.’t Hooft, “Canonical quantization of gravitating point particles in 2+1 dimensions,” Class. Q. Grav., 10, 1653–1664 (1993); “Quantization of point particles in (2+1)-dimensional gravity and spacetime discreteness,” Class. Q. Grav., 13, 1023–1039 (1996).