High-temperature massless scalar partition function on general stationary backgrounds

The high-temperature equilibrium partition function of a massless real scalar field nonminimally coupled to the scalar curvature is computed at second order in the derivative expansion on a generic stationary background. Using covariant perturbation theory, the expression of the thermal partition function at second order in powers of curvatures is also obtained, including its nonlocal contributions. For conformal coupling, the Weyl anomaly at fourth order in derivatives and second order in curvatures is evaluated using both expansions and the results found to be consistent.