1. E. Noether,Gött. Nachr. 235 (1918). This reference is not easily available. The author is indebted to Prof. S. Bergia for his notes on the subject.
2. N. N. Bogoliubov and D. V. Shirkov,Introduction to the Theory of Quantized Fields (Interscience, New York, 1959).
3. S. S. Schweber,An Introduction to Relativistic Field Theory (Row & Peterson, Evanston, 1961).
4. A. O. Barut and A. J. Bracken,Phys. Rev. D 23, 2454 (1981).
5. R. E. Marshak and E. C. G. Sudarshan,Introduction to Elementary Particle Physics (Interscience, New York, 1961).