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Everyone knows that one can calculate the S-Matrix with various tricks. One of them is to use traces to simplify the matrices.

Can someone tell me or point me to a place where I can find an explanation why I can do this?

I think I vaguely remember that I have seen once an equation showing why traces can be used, but I can't recall that source (book? lecture notes? websites?)

Thanks

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# QFT: Trace Theorem

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