1. M. Gourdin andA. Martin:Nuovo Cimento,6, 757 (1957).
2. M. Gourdin andA. Martin:Nuovo Cimento,8, 699 (1958).
3. The case of tensor forces will be considered in a next paper. See alsoC.R.A.S.,245, 1597 (1957).
4. A preliminary report of this work has already been published inC.R.A.S.,246, 1513 (1958).
5. R. Jost andW. Kohn:Dans. Mat. Fys. Med.,27, 9 (1953).