Stability of a system with unilateral variations

System S will be said to be of unilateral variations if the disturbance cannot take only a single positive or negative sign. Thus, we can take the origin =0 for the extent. 

Let us choose, for example, 0 (we could also have made the opposite choice and the conclusions drawn would have been the same). In view of the inequality [2.52], system S will be stable if  

Note 2.4.- These conditions can, of course, very easily be expressed in terms of chemical potentials by using expressions [1.23] and [1.42]. 

Note 2.5 - These conditions can also be expressed in tenns of characteristic functions. Then by using expression [1.20], if the system is at thermodynamic equilibrium, condition [2.58] is written  

The concavity of the curve is turned toward positive values of / and 

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