_{0}^{t}H(t')\,dt'}\right)} ; 其中, T {\displaystyle T} 是時間排序算符。 必須用戴森級數(英语:Dysonseries)來表示, U ( t ) = 1 + ∑ n = 1 ∞ ( − i ℏ ) n ∫ 0 t d t 1 ∫ 0 t 1 d t 2 …
Quantum Field Theory. QED and the Men who Made it: Dyson, Feynman, Schwinger, and Tomonaga. Princeton series in physics. Princeton University Press. 1994: