[1] E. Alvarez, arXiv:hep-th/0501146.
[2] J. J. van der Bij, H. van Dam and Y. J. Ng, “Theory Of Gravity And The Cosmological Term: The Little Group Viewpoint,” Physica 116A, 307 (1982).
[3] W. Buchmuller and N. Dragon, Phys. Lett. B 207 (1988) 292.
[4] A. Einstein, (1919) Siz. Preuss. Acad. Scis., english translation in “The principle of relativity”, by A. Einstein et al (Dover).
[5] M. Henneaux and C. Teitelboim, Phys. Lett. B 222 (1989) 195.