On the basis of this (finite) group equation and its scaling property, Gell-Mann and Low could then focus on infinitesimal transformations, and invented a computational method based on a mathematical flow function ψ (g) = G d /(∂ G /∂ g) of the coupling parameter g, which they introduced. Like the function h (e) of Stueckelberg and Petermann, their function determines the differential change of the coupling g (μ) with respect to a small change in energy scale μ through a differential equation, the renormalization group equation :