1. Rovelli, C.: Living Rev. Relativ. 1, 1 (1998). gr-qc/9710008
2. Thiemann, T.: Lect. Not. Phys. 631, 412003 (2003). gr-qc/0210094
3. Smolin, L.: hep-th/0408048 (2004)
4. Rovelli, C.: Quantum Gravity. Cambridge University Press, Cambridge (2004)
5. Ashtekar, A., Corichi, A., Singh, P.: Phys. Rev. D, 77, 024046 (2008). 0710.3565 [gr-qc]