1. Ashtekar, A.: Phys. Rev. Lett. 57, 2244 (1986)
2. Rovelli, C.: Quantum Gravity. Cambridge University Press, Cambridge (2004)
3. Engle, J., Livine, E. R., Pereira R., Rovelli, C.: Nucl. Phys. B 799, 136 (2008). arXiv:0711.0146
4. Freidel, L., Krasnov, K.: Class. Quant. Grav. 25, 125018 (2008). arXiv:0708.1595
5. Barrett, J. W., Dowdall, R. J., Fairbairn, W. J., Hellmann, F., Pereira, R.: Lorentzian spin foam amplitudes: graphical calculus and asymptotics. arXiv:0907.2440