= Time-ordered correlation function
The vacuum expectation value of a <time ordering> of <quantum field> insertions, for example $\langle0|T\{\phi(x)\phi(y)\}|0\rangle$. The vacuum <path integral> with the <Feynman i-epsilon prescription> computes these functions. For <fermion> fields the ordering includes a minus sign for each odd permutation. Differentiating a <time-ordered correlation function> can differentiate its ordering step functions and produce <Ward identity contact terms>.
Back to article page
