1. The existing literature on the subject is extremely wide. Here we merely quote some reviews.H. Abarbanel, R. L. Sugar andH. H. Islam: Lectures at the14th Annual Institute for Theoretical Physics (Boulder, Colo., 1972);C. Itzykson:Ann. der Phys.,7, 59 (1972);
2. H. M. Fried:Functional Methods and Models in Quantum Field Theory (Cambridge, Mass., 1972); and more recentlyC. A. Orzalesi: IC/73/148 International Centre for Theoretical Physics preprint, to appear inAbn. of Phys.
3. See in particularE. S. Fradkin:Proc. P. N. Lebedev Phys. Inst., Vol. 29 (1965); and, more available,E. S. Fradkin, U. Esposito andS. Termini:Rivista Nuovo Cimento,2, 498 (1970); more recently, Orzalesi (1). A different expansion was given byR. Blankenbecler andR. L. Sugar:Phys. Rev. D,2, 3024 (1970). See also the recent work on potential theory byS. J. Wallace:Ann. of Phys.,78, 190 (1973).
4. J. Schwinger:Proc. Nat. Acad. Sci.,37, 452 (1951);K. Symanzik:Zeits. Naturf.,9 a, 809 (1954);E. S. Fradkin (2).
5. H. Abarbanel andC. Itzykson:Phys. Rev. Lett.,23, 53 (1969).