1. J. Schwinger:Phys, Rer.,82, 661 (1951).
2. M. Meraskis undK. Nisiiiiima:I’Ih/k. Her.,112, 331 (1961).
3. P. Sen:Ximrii cimenta,13, 122 (1959) is used here.
4. N. N. Bogoliubov andD. V. Smirkon :Introduction to the Theory of Quantised Fields. Appendix 1 (New York. l959).
5. E. C. Titchmarsii:Theory of Fourier Integrals. Second Edition, Theorem 95 (Oxford, 1948).