The zero four-momentum and equal mass limits are taken for the bubble diagram
of scalar fields. It is seen that RTF and ITF are in complete agreement.
However contributions from this diagram to both retarded and time-ordered
functions do depend on the order of the limits and can be infinite in some
cases. This shows explicitly that the relation between the free energy and a
derivative expansion of a thermal effective action is generally much more
complicated that is the case at zero temperature.Comment: 13 pages, LaTeX (3 PS figures appended), Imperial/TP/92-93/4
