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2. R. L. Ingraham:Nuovo Cimento,27, 303 (1963).
3. J. M. Canfield:Ph. D. Thesis, New Mexico State University (1963).
4. S. S. Schweber:An Introduction to Relativistic Quantum Field Theory (Evanston. III., 1961), Sect. l0c.
5. N. Bogoliubov andD. Shirkov:Introduction to the Theory of Quantized Fields (New York, 1959), Sect.18.