1. H.B. Callen, Thermodynamics. J. Wiley, New York, 1960.
2. Kang S. Lee, M.J. Rhoades-Brown and U. Heinz, Phys. Lett.,B174, 123, 1986.
3. B. Lukács, J. Zimányi and N.L. Balazs, Phys. Lett.,B183, 27, 1987.
4. J. Ehlers, in: Relativity, Astrophysics and Cosmology, ed. W. Israel D. Reidel Publ. Co., Dordrecht, 1973.
5. B. Lukács, Acta Phys. Pol.,B14, 33, 1983.