1. H.J. Bhabha, Proc. Roy. Soc. (London), A154 (1935) 195.
2. S.L. Glashow, Nucl. Phys. 22 (1961) 579.
3. S. Weinberg, Phys. Rev. Lett. 19 (1967) 1264.
4. A. Salam, Proceedings of the Eighth Nobel Symposium, May 1968, ed.: N. Svartholm (Wiley, 1968) 367.
5. S.L. Glashow, J. Iliopoulos, and L. Maiani, Phys. Rev. D2 (1970) 1285.