1. R. P. Feynman:Phys. Rev.,76, 769 (1949).
2. R. P. Fetnman:Phys. Rev.,80, 440 (1950) (Appendix A). See alsoY. Nambu:Prog. Theor. Phys.,5, 82 (1950);V. Fook:Phys. Zeits. Sowjetunion,12, 404 (1937);J. S. Schwinger:Phys. Rev.,82, 664 (1951);E. C. G. Stueckelbebg:Helv. Phys. Acta,14, 588 (1941);15, 23 (1942).
3. B. P. Fetnman:Rev. Mod. Phys.,20, 267 (1948).
4. L. P. Hokwitz andC. Piron:Helv. Phys. Ada,46, 316 (1973). The formulation of the theory for particles with spin is given byL. P. Horwitz,C. Piron andF. Reuse:Helv. Phys. Acta,48, 546 (1975). Related group theoretical studies, references inP. L. Huddleston,M. Lorente and P. Roman :Found. of Phys.,5, 75 (1975). The classical two-body problem was studied byC. Piron andF. Reuse (University of Genera, 1974).Y. S. Kim andM. B. Noz, and collaborators at Maryland have studied the relativistic two-body problem with harmonic oscillator potential in detail (references can be found inT. J. Karr: University of Maryland Technical Report No. 76-085 (January 1976), p. 76); for an earlier attempt (using unnormalizable wave functions) seeR. P. Feynman,M. Kislinger andF. Ravndal:Phys. Rev. V,3, 2706 (1971).
5. Y. Rabin: Tel Aviv University preprint TAUP-504-75, has discussed the relation between the approaches of ref. (2,4).