1. Some references are: J.Schwinger, Phys.Rev., 82 (1951) 914, 91 (1953) 713 Proc. Natl. Acad. Sci.(U.S.), 46 (1960) 883.
2. R.Feynman, Rev.Mod.Phys., 20 (1948) 36;
3. R.Feynman and A.Hibbs, Quantum Mechanics and Path Integrals, McGraw-Hill, New York, 1965.
4. In the first two papers cited in Ref.1 I have assumed space-reflection invariance and shown the equivalence between the spin-statistics relation and the invariance of the action principle under combined time reflection and complex conjugation. It was later remarked by Pauli that the separate hypothesis of space-reflection invariance was unnecessary, W.Pauli, Niels Bohr and the Development of Physics, McGraw-Hill, New York, 1955.
5. J.Schwinger, Phys.Rev., 127 (1962) 324.