I am working on Quantum Effective Action in Weinberg QFT vol2 (page 67).(adsbygoogle = window.adsbygoogle || []).push({});

In the last paragraph of page 67, the author said

"Equivalently, ## i \Gamma [ \phi _0 ] ## for some fixed field ........ with a shifted action ##I [ \phi + \phi_0 ]## :

[tex] i \Gamma [ \phi _0 ] = ∫_{1PI, CONNECTED} ∏_{r,x} d\phi^r (x) exp(iI[\phi+\phi_0])[/tex]

In this equation, I don't understand two things;

First one is why we have to use a shifted action ##I [ \phi + \phi_0 ]##.

Second one is why we only take into account of one-particle irreducible and connected terms to get Quantum Effective Action for some fixed field ##\phi^{r}_0 (x)##.

Thank you.

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# Question about Quantum Effective Action

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