1. See the review article byR. Hofstadter:Rev. Mod. Phys.,28, 3, (1956).
2. M. N. Rosenbluth:Phys. Rev.,79, 615 (1950).
3. If one assumes invariance under time reversal, this term will not appear, i.e.,G 2=0. However this sort of invariance does not seem absolutely necessary. The authors would like to thank Prof.A. S. Wightman for illuminating discussions on this point.
4. E. Wigner:Ann. of Math.,40, 149 (1939) and subsequent papers.
5. Compare:G. Källén:Dan. Mat. Fys. Medd.,27, No. 12 (1953).