For any specific volume ν ν , denote p ν ν ( T ) the function that describes how the pressure varies with the temperature T , as allowed by the equation of state, when the specific volume of the material is forcefully kept constant at ν ν . Analogously, for any pressure P , let ν ν P ( T ) be the function that describes how the specific volume varies with the temperature, when the pressure is kept constant at P . Namely, those functions are such that