1. G. C. Duncan:Phys. Rey. A,6, 947 (1972).
2. S. S. Schweber:An Introduction to Relativistic Quantum Field Theory (Evanston, Ill., 1961).
3. The implications of these approximations have been investigated by several authors, notably byM. L. Goldberger andK. M. Watson:Collision Theory (New York, 1964).
4. However, one may show in the ease of a single mode the hierarchy of equations of motion may be solved to any desired order using a continued-fraction expansion. (G. C. Duncan: unpublished ealeulatiolts.)