Evolution of free-energy expansion parameters in the critical region. An exactly solvable model

The structure of the free-energy expansion of an anisotropic system is investigated in the framework of an exactly solvable model of phase transitions that preserves the interaction of fluctuations with momenta of equal magnitudes and opposite directions. It is shown that the coefficients of the fourth-order form vary with the temperature in the critical region, forming a phase portrait qualitatively similar to the one obtained by the renormalization-group method.