1. L. I. Schiff, “Quantum Mechanics,” McGraw-Hill, New York (1968), sect. 57.
2. J. D. Bjorken and S. Drell, “Relativistic Quantum Fields,” McGraw-Hill, New York (1965), sect. 15.2.
3. I. Bialyncki-Birula, in this volume.
4. See e. g. D. Lurie, “Particles and Fields,” Interscience, New York (1968), chap. 6.
5. M. Berrondo and R. Jáuregui, Kinam5: 163 (1983).