  We consider the 3-dimensional massive Gross-Neveu model at finite temperature
as an effective theory for strong interactions. Using the Matsubara imaginary
time formalism, we derive a closed form for the renormalized $T$-dependent
four-point function. This gives a singularity, suggesting a phase transition.
Considering the free energy we obtain the $T$-dependent mass, which goes to
zero for some temperature. These results lead us to the conclusion that there
is a second-order phase transition.
