Massieu function

In thermodynamics, Massieu function, symbol (Psi), is defined by the following relation:

where for every system with degree of freedom r one may choose r variables, e.g. , to define a coordinate system, where X and Y are extensive and intensive variables, respectively, and where at least one extensive variable must be within this set in order to define the size of the system. The (r + 1)-th variable, , is then called the Massieu function.