THERMODYNAMICS OF 4-FERMION MODELS IN THE DELTA-EXPANSION

Using the linear delta expansion we investigate the free energy of the 1 + 1 and 2 + 1 dimensional Gross-Neveu model. In both cases the larg e-N result is exactly reproduced at first order in delta. However, whe n higher-order corrections are included our results for the phase tran sition differ both quantitatively and qualitatively from the large-N p redictions. In particular, our result for the critical temperature in 1 + 1 dimensions is in better agreement with a general theorem for pha se transitions in one space dimension.