1. S. Fubini, G. Furlan andC. Rossetti:Nuovo Cimento,40A, 117 (1965);S. Fubini:Nuovo Cimento,43 A, 475 (1966).
2. C. G. Bollini andJ. J. Giambiagi:Equal-time commutation relations and sum rules, Universidad de Buenos Aires (preprint), to be published inNucl. Phys.
3. S. Gasiorowicz:Phys. Rev.,146, 1071 (1966).
4. It should be emphasized thata 1(k 2, p·k) in (6)-(9) are integrated with respect tok o which appears both ink 2 (=k0/2−k 2) andp·k (=p o k o −p·k).
5. The prime on the integration signs means an integration over all intermediate states except the one-nucleon.