3.2 Equilibrium Finite Temperature GREEN's Function

In Section 3.1 the GREEN's function for a system under equilibrium
at zero temperature has been introduced. Furthermore, the GREEN's function was
readily expressed as perturbation expansion in the interaction picture. In this
section, the GREEN's function for a system under equilibrium at finite
temperature is presented. The GREEN's function at finite temperature has a
simple perturbation expansion similar to that for the GREEN's function at zero
temperature and also enables us to evaluate the properties of the system.
Subsections