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I am unable to prove exactly one trace identity that appears in the appendix of Peskin and Schroeder's QFT book. Can someone help me?

The theorem [Appendix A.4 eqn (A.28)] says that the order of $\gamma$ matrices inside of a trace can be reversed:


Can someone help me with this trivial thing...?

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up vote 2 down vote accepted

Please see (the derivation leading to Eq. 23)

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