In thermodynamics, the Massieu function, symbol Ψ, is defined by the following relation: Ψ = Ψ {\displaystyle \Psi =\Psi {\big }\,} where for every system with degree of freedom r one may choose r variables, e.g., {\displaystyle {\big },} 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. Wikipedia