Time-ordered product
= Time-ordered product
{title2=$T\{\mathcal O_1\cdots\mathcal O_n\}$}
A <time-ordered product> sorts operator insertions from latest time on the left to earliest time on the right. Reordering odd <fermionic operators> contributes the sign of their permutation. For bosonic $A,B$, $T[A(t)B(t')]=\theta(t-t')A(t)B(t')+\theta(t'-t)B(t')A(t)$. Vacuum expectations of free <time-ordered products> are evaluated by the <Wick theorem>.