  The running coupling of a generic field theory can be described through a
separable differential equation involving the corresponding $\beta$-function.
Only the first loop order can be solved analytically in terms of well-known
functions, all further loop orders lead to transcendental equations. While
obscure nowadays, many analytical methods have been devised to study them, most
specifically the Lagrange-B\"urmann formula. In this article we discuss the
structure of transcendental equations that take place at various loop orders.
Beyond the first two loop orders, these equations are simplified by applying an
optimal Pade approximant on the $\beta$-function. In general, these lead to
generalizations of Lambert's equation, the solutions of which are presented in
terms of a power series.
