1. H. A. Wilson:Modern Physics (Blackie, London, 1937), p. 381.
2. V. Fock:The Theory of Space, Time and Gravitation (Pergamon, Oxford, 1964), p. 219.
3. J. Ehlkrs:Relativity Theory and Astrophysics. - I:Relativity and Cosmology (American Math. Soc, Providence, 1967), p. 90.
4. H. Aspden:Phys. Lett. A,110, 113 (1985).