1. D eb ev er R . 1959 C.R. A cad. Sci. P aris 249 1324 1744.
2. G oldberg J . N . & Sachs R . K . 1961 (In p re p a ra tio n .)
3. N ew m an E . T . & T am b u rin o L . A. 1961 J . M a th . P h ys. (in th e P ress).
4. P en ro se R . i960 A n n . of Phys. 10 171.
5. R o b in so n I. 1959 R e p o rt to th e R o y a u m o n t Conference.