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How to prove Wick's Theorem (Zee's eq. I.2 (16)) via Gaussian integration?
I'm working through Zee's QFT in a Nutshell but there's an integral [I.2 (16)] I couldn't quite derive.
The problem is to find
$$\langle x_i x_j ... x_k x_l\rangle=\frac{\int ... \int dx_1 ... dx_n ...
