1. S. Weinberg, Phys. Rev. Lett. 19, 1264 (1967)
2. A. Salam, Elementary Particle Theory, ed. by N. Svartholm (Almquist Forlag AB, 1968)
3. S.L. Glashow, Nucl. Phys. 22, 579 (1961)
4. B. Pontecorvo, Zh. Eksp. Teor. Fiz. 33, 549 (1957)
5. B. Pontecorvo, Zh. Eksp. Teor. Fiz. 53, 1717 (1967)