In general, there are higher order terms present in the free energy, but it is a reasonable approximation to consider the series to fourth order in the order parameter, as long as the order parameter is small. For the system to be thermodynamically stable (that is, the system does not seek an infinite order parameter to minimize the energy), the coefficient of the highest even power of the order parameter must be positive, so b ( T ) > 0 . For simplicity, one can assume that b ( T ) = b 0 , a constant, near the critical temperature. Furthermore, since a ( T ) changes sign above and below the critical temperature, one can likewise expand a ( T ) ≈ ≈ a 0 ( T − − T c ) , where it is assumed that a > 0 for the high-temperature phase while a < 0 for the low-temperature phase, for a transition to occur. With these assumptions, minimizing the free energy with respect to the order parameter requires