where A ( T ) = A 0 ( T − − T 0 ) , and T 0 is some temperature at which A ( T ) changes sign. We denote this temperature by T 0 and not T c , since it will emerge below that it is not the temperature of the first-order transition, and since there is no critical point, the notion of a "critical temperature" is misleading to begin with. A 0 , B 0 , and C 0 are positive coefficients.