1. Private communication to J. Weber and J. A. Wheeler; see Weber, J. & Wheeler, J. A. (1957). See also, Fierz, M. in Jubilee of relativity theory, edited by A. Mercier and M. Kervaire 1956;Fierz M.;Helv. Phys. Acta. Supplementum,1957
2. Lichnerowicz A. 1955 Theories relativistes de la gravitation et de Velectromagnetisme Chapters I III. Paris: Masson.
3. Proc. Roy;Marder L.;Soc. A,1958
4. Tolman R . C. 1934 Relativity thermodynamics and cosmology p. 235. Oxford: Clarendon Press.
5. Reality of the Cylindrical Gravitational Waves of Einstein and Rosen