I'm considering the Euclidean Klein-Gordon theory, with action S0[ϕ]=12∫ d4x ϕ(x)[−∂2x+m2]ϕ(x).
Z0[J]=∫D[ϕ] exp(−S0[ϕ]+∫ d4x ϕ(x)J(x)).
I'm supposed to derive the following equation of motion, involving the two-point function: [−∂2y+m2]⟨ϕ(y)ϕ(x)⟩=δ(4)(y−x).
I've been told to do this by starting with the following definition of the one-point function:
⟨ϕ(x)⟩=1Z0[0]δZ0[J]δJ(x)|J = 0
I'm supposed to use the invariance of the functional integration under field re-defintions; IE. if we replace ϕ(x) with ϕ′=ϕ(x)+ϵ(x).
I've been looking online and the usual approach that I've seen involves looking at the equality ∫D[ϕ]exp(−S0[ϕ])ϕ(x)=∫D[ϕ′]exp(−S0[ϕ′])ϕ′(x)
How would you do this starting with ⟨ϕ(x)⟩?
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