Hello, I'm looking at the following computation from the Peskin and Schroeder's book:(adsbygoogle = window.adsbygoogle || []).push({});

See file attached

In the second page, the second term that's being integrated, I don't understand why it has a negative i in the exponential, that'll keep the energy term the same, but will swap the sign of the spatial terms, wouldn't it?

Thank you beforehand

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# Klein-Gordon Propagator

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