# Ballentine 3.11

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## Main Question or Discussion Point

Top of page 69. I get six second order terms. Two of them cancel. Two are the ones on the right of 3.11. The other two involve KμKμ and KνKν. Why are there no such terms on the right of 3.11? I'm probably just missing something painfully obvious.

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Actually, there are more than 6 second-order terms. In order to see all of them, write each exponent as

$$e^{i \epsilon K_{\mu}} = 1 + i \epsilon K_{\mu} -1/2 \epsilon^2 K_{\mu}^2 + \ldots$$

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