  We study the equation of state of a strongly interacting theory of
relativistic bosons and chiral fermions in the vicinity and above the chiral
phase transition temperature. Our model resembles presently used low-energy
models of QCD in many ways, but its simplicity allows us to study
systematically various approximation schemes by means of a derivative
expansion. In particular, we compare the results for the phase transition
temperature, equation of state and the thermal mass of the scalar fields
obtained in various approximations. We find that the large-N_c approximation,
being the zeroth order of our approximation, deviates significantly from
approximation schemes in which effects of (Pseudo-) Goldstone modes have been
taken into account, even at high temperatures and even if we allow for a finite
explicit symmetry breaking. The important role of the (Pseudo-) Goldstone modes
is also manifested in the phenomenon of "local order" which is missing in
large-N_c studies.
