reading Zee's QFT Theory in a Nutshell and on p 13 on the explanation of the Gaussian integral he loses me going from (9) to (10) by jumping from a relatively straightforward derivation of [itex]\int e^{-1/2ax^2}dx [/itex](adsbygoogle = window.adsbygoogle || []).push({});

by then saying "acting on this with -2(d/da)

to get [itex]<x^{2n}> ={\int e^{-1/2ax^2}x^{2n}dx}/{\int e^{-1/2ax^2}dx}= \frac{1}{a^n}(2n-1)!![/itex]

not sure how he gets to the double factorial expression - how do you know this integral would not diverge?

also not clear on the meaning of -2(d/da) as an operator

assuming the notation <x> means the expectation of some arbitrary variable

the wiki article and other internet sources on Wick's theorem refers to ordering of creation and annihilation operators, which Zee has not mentioned yet

sorry for the rambling post, but any help would be greatly appreciated

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# Confused about Wick & Zee

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