Julian Schwinger (1951년 6월 15일). “The Quantum Theory of Fields I”. 《Physical Review》 82 (6): 914–917. Bibcode:1951PhRv...82..914S. doi:10.1103/PhysRev.82.914.. The only difference between the argument in this paper and the argument presented here is that the operator "R" in Schwinger's paper is a pure time reversal, instead of a CPT operation, but this is the same for CP invariant free field theories which were all that Schwinger considered.
Julian Schwinger (1951년 6월 15일). “The Quantum Theory of Fields I”. 《Physical Review》 82 (6): 914–917. Bibcode:1951PhRv...82..914S. doi:10.1103/PhysRev.82.914.. The only difference between the argument in this paper and the argument presented here is that the operator "R" in Schwinger's paper is a pure time reversal, instead of a CPT operation, but this is the same for CP invariant free field theories which were all that Schwinger considered.