1. Brown, L., (19T6). Private communication
2. Carroll, R., (1975). Phys. Rev. D12, 2344
3. Cvitanovic, P., and Kinoshita, T. (1974). Phys. Rev. D10, 4007.
4. Landau, L, and Lifschitz, E., (1965). Quantum Mechanics, Pergamon Press, § 111.
5. Rabi, I., (1928). Z. Physik 49, 507.