1. Burdet, G., and Perrin, M. (1990). “Radiation Structures on Isotropic Hypersurfaces.” Preprint CPT-90/P.2479.
2. Burdet, G., and Perrin, M. (1992).Lett. in Math. Physics 25, 39.
3. Cahen, M., Debever, R., Defrise, L. (1967).Jour. Math. Mechanics 16, 7.
4. Debever, R., McLenaghan, R. G. (1981).J. Math. Phys. 22, 1711.
5. Newman, E., and Penrose, R. (1962).J. Math. Phys. 3, 3.