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T. S. , Q. E. Feynman-r, and Q. , 18, footnote 9

C. E. , J. R. Helv, and . Phys, Acta 21 (1948) 183. The attack on the spin 1/2 problem was begun by D, Feldman and J. Schwinger, Phys. Rev, p.75, 1949.