1. As usual we exclude the case of a zero-mass particle as well inH 0 as inH.
2. See Appendix for the properties of the operatorad().
3. H. Umezawa:Nuovo Cimento,40 A, 450 (1965).
4. Seee. g.:B. S. de Witt:Phys. Rev.,100, 905 (1955).
5. The Hamiltonian of the nonrelativistic Lee model, strictly speaking, has not the good properties we require ofH. But this will be unimportant for our purpose.