1. Gamow, G. (1946). Nature158, 549.
2. Gödel, K. (1949). Rev. Mod. Phys.21, 447.
3. Gödel, K. (1990). In Kurt Gödel Collected Works, Volume II, S. Feferman et al., eds. (Oxford University Press, Oxford), p. 208.
4. Ellis, G. F. R. (1971). In Proc. International School of Physics “Enrico Fermi,” XLVII — General Relativity and Cosmology (Varenna, 30 June-12 July 1969), B. K. Sachs, ed. (Academic Press, New York), p. 104.
5. Obukhov, Yu. N. (1992). Gen. Rel. Grav.24, 121.