In quantum field theory Feynman has invented a diagrammatic method to encode various terms in the Taylor decomposition of integrals of the following form below which I will write in a baby version as finite dimensional integral rather than path integral (and using “imaginary time”): $ $ Z(j_1,\dots,j_n):=\int_{\mathbb{R}^n} \exp\{B(x_1,\dots,x_n)+\sum_{i=1}^nj_ix_i\}dx/ \int_{\mathbb{R}^n} \exp\{B(x_1,\dots,x_n)\}dx,$ $ where $ B$ is a positive definite quadratic form on $ \mathbb{R}^n$ . Furthermore one can write $ Z(j)=exp\{ W(j)\}$ and it is shown that $ W(j)$ is a sum of terms corresponding only to **connected** diagrams.

In the context of path integrals there is a notion of effective action which in this context is defined as follows. Let $ \phi_i:=\frac{\partial W(j)}{\partial j_i}$ . Define the Legendre transform $ $ \Gamma(\phi):=\sum_{i=1}^n \phi_ij_i-W(j).$ $

In QFT it is claimed that the Taylor decomposition of $ \Gamma(\phi)$ is the sum of terms corresponding to

connected one particle irreduciblediagrams. I am wondering if a finite dimensional (baby) version of this claim is true, and if this is the case whether there is a reference to a detailed discussion of the finite dimensional case.