H = H_{0} + V

i\frac{∂}{∂t}U(t, 0) = V_{H_{0}}(t)U(t, 0)

e^{iH_{0}t}e^{−iHt} = U(t, 0)

main result:

G(t, t′) = −i⟨Ψ|T (c_{H}(t)c_{H}^{†}(t′))|Ψ⟩
= −i⟨Ψ_{0}|U(−∞, +∞)T (U(+∞, −∞)c_{H}(t)c_{H}^{†}(t′))|Ψ_{0}⟩

with |Ψ⟩ = U(0, −∞)|Ψ_{0}⟩