The total energy of the system is U ( S , X 1 , X 2 , … … ) where S is entropy, and the X i are the other extensive parameters of the system (e.g. volume, particle number, etc.). The entropy of the system may likewise be written as a function of the other extensive parameters as S ( U , X 1 , X 2 , … … ) . Suppose that X is one of the X i which varies as a system approaches equilibrium, and that it is the only such parameter which is varying. The principle of maximum entropy may then be stated as: