where ε is an infinitesimal number and ϕ I is the field operator under the free theory. Here, the exponential should be understood as its power series expansion. For example, in ϕ ϕ 4 -theory, the interacting term of the Hamiltonian is H I ( t ) = ∫ ∫ d 3 x λ λ 4 ! ϕ ϕ I ( x ) 4 , and the expansion of the two-point correlator in terms of λ λ becomes ⟨ ⟨ Ω Ω | T { ϕ ϕ ( x ) ϕ ϕ ( y ) } | Ω Ω ⟩ ⟩ = ∑ ∑ n = 0 ∞ ∞ ( − − i λ λ ) n ( 4 ! ) n n ! ∫ ∫ d 4 z 1 ⋯ ⋯ ∫ ∫ d 4 z n ⟨ ⟨ 0 | T { ϕ ϕ I ( x ) ϕ ϕ I ( y ) ϕ ϕ I ( z 1 ) 4 ⋯ ⋯ ϕ ϕ I ( z n ) 4 } | 0 ⟩ ⟩ ∑ ∑ n = 0 ∞ ∞ ( − − i λ λ ) n ( 4 ! ) n n ! ∫ ∫ d 4 z 1 ⋯ ⋯ ∫ ∫ d 4 z n ⟨ ⟨ 0 | T { ϕ ϕ I ( z 1 ) 4 ⋯ ⋯ ϕ ϕ I ( z n ) 4 } | 0 ⟩ ⟩ . This perturbation expansion expresses the interacting two-point function in terms of quantities ⟨ ⟨ 0 | ⋯ ⋯ | 0 ⟩ ⟩ that are evaluated in the free theory.