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Response Functions in Field Theory - Subtleties?
The definitions I saw of response functions, e.g. in Landau & Lifschitz (SP Sec. 125), or in Altland & Simons (Ch.7), are given in terms of expectation values of some physical quantity $\...
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Why the classical configuration always static when applying saddle point (semi-classical) approximation?
For an Green function/partition function:
$$\int D[\phi]e^{\frac{i S[\phi]}{\hbar}}$$
We can make saddle point approximation and gives classical configuration:
$$\delta \mathcal{S}=0\Longrightarrow \...
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