  In momentum space the Feynman propagator $D_{F}(k)$ at non-zero temperature
is defined by a simple dispersion relation with the familiar property of being
an even function of $k^{0}$ and analytic for Re$(k^{0})^{2}>0$. The coordinate
space form of the propagator $D_{F}(x)$ is expressed directly in terms of
matrix elements of the field operator and requires a new type of operator
ordering.

High Energy Physics - Phenomenology