1. Chattopadhyay K, Lele S, Ranganathan S, Subbanna G N and Thangaraj N 1985Curr. Sci. 54 895
2. de Bruijn N G 1981Ned. Akad. Weten. Proc. Ser. A43 39, 53
3. Duneau M and Katz A 1985Phys. Rev. Lett. 54 2688
4. Elser V 1985a AT and T Bell Laboratories Preprint
5. Elser V 1985b AT and T Bell Laboratories Preprint