1. Appelquist, T., Chodos, A., Freund, P.G.O.: Modern-Klein Theories. Addison-Wesley, Reading (1987). arXiv:1006.5412 [gr-qc]
2. Beesham, A.: Class. Quantum Gravity 3, 481 (1986a)
3. Beesham, A.: Astrophys. Space Sci. 119, 415 (1986b)
4. Beesham, A.: Class. Quantum Gravity 3, 1027 (1986c)
5. Bennet, C.L., et al.: Astrophys. J. Suppl. Ser. 148, 19 (2003)