ゲルマン＝ロウの定理
⟨ 0 | T φ ( x 1 ) φ ( x 2 ) ⋯ φ ( x n ) | 0 ⟩ = 0 ⟨ 0 | T φ I ( x 1 ) φ I ( x 2 ) ⋯ φ I ( x n ) exp  ( − i ħ ∫ V I ( t ) d t ) | 0 ⟩ 0 0 ⟨ 0 | T exp  ( − i ħ ∫ V I ( t ) d t ) | 0 ⟩ 0 {\displaystyle \langle 0|T\phi (x_{1})\phi (x_{2})\cdots \phi (x_{n})|0\rangle ={\frac {{}_{0}\langle 0|T\phi _{I}(x_{1})\phi _{I}(x_{2})\cdots \phi _{I}(x_{n})\exp {{\biggl (}-{\frac {i}{\hbar }}\int V_{I}(t)dt{\biggr )}}|0\rangle {}_{0}}{{}_{0}\langle 0|T\exp {{\biggl (}-{\frac {i}{\hbar }}\int V_{I}(t)dt{\biggr )}}|0\rangle {}_{0}}}}