Schwinger-Keldysh propagator
= Schwinger-Keldysh propagator
{c}
The closed time contour of the <in-in formalism> gives four bulk propagators: time-ordered $G_{RR}$, anti-time-ordered $G_{LL}$, and the two Wightman functions $G_{LR}$ and $G_{RL}$. Vertices on the two contour branches carry opposite factors of $i$.