1. G. Builder. “Austr. Journal of Physics”, Vol. II, No.3 (1958).
2. S.J. Prokhovnik. “Proc. Cambr. Phil. Soc.” 60, 265 (1964).
3. M. Robinson. “The cosmological distance ladder”, W.H. Freeman (1985).
4. M. Robinson. “Cosmology”, Oxford Physic series (1977).
5. S.J. Prokhovnik. “Light in Einstein universe”, Reidel (1985).