Or, in the interaction picture, i G ( x 1 , t 1 , x 2 , t 2 ) = ⟨ ⟨ n | T c ( e − − i ∫ ∫ c H ′ ( t ′ ) d t ′ ψ ψ ( x 1 , t 1 ) ψ ψ ( x 2 , t 2 ) ) | n ⟩ ⟩ . We can expand the exponential as a Taylor series to obtain the perturbation series