1. R. Hofstadter andW. R. McAllister:Phys. Rev.,98, 217 (1955).
2. M. Cini andS. Fubini:Nuovo Cimento,3, 764 (1956) quoted in the following as I.
3. See e.g.G. Wentzel:Quantum Theory of Fields (New York, 1949). This author uses a convention opposite to ours for the isotopic spin of the nucleons.
4. M. Cini, S. Fubini andA. Stanghellini:Nuovo Cimento,3, 1379 (1956).
5. W. Thirring : comunication at the Rochester Conference, 1956.