  Massless and massive scalar fields and massless spinor fields are considered
at arbitrary temperatures in four dimensional ultrastatic curved spacetime.
Scalar models under consideration can be either conformal or nonconformal and
include selfinteraction. The one-loop nonlocal effective action at finite
temperature and free energy for these quantum fields are found up to the second
order in background field strengths using the covariant perturbation theory.
The resulting expressions are free of infrared divergences. Spectral
representations for nonlocal terms of high temperature expansions are obtained.
