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3. Bjorken J., Drell S., Relativistic Quantum Fields. McGraw-Hill, New York 1965.
4. Abers E. S., Lee B. W., Phys. Rep.9 (1973), 60. According to these authors the path integral for the kernel must be written from the Hamiltonian and not the Lagrangian.
5. Barnes Ted, Ghandour G. I., ICTP Trieste preprint IC/78/80.