The planar Tree Lagrange Inversion Formula

Abstract

A planar tree power series over a field $K$ is a formal expression $$\sum c_T \cdot T$$ where the sum is extended over all isomorphism classes of finite planar reduced rooted trees $T$ and where the coefficients $c_T$ are in $K$. Mulitplications of these power series is induced by planar grafting of trees and turns the K-vectorspace $K\{x\}_\infty$ of those power series into an algebra, see [G]. If $f \in K \{x\}_\infty$ there is a unique $g(x) \in K \{x\}_\infty$ of order $> 0$ such that $$ g(x) = x \cdot f(g(x))$$ where $f(g(x))$ is obtained by substituting $g(x)$ for $x$ in $f(x).$ Formulas for the coefficients of $g$ in terms of the coefficients of $f$ are obtained by the use of the planar tree Lukaciewicz language. This result generalizes the classical Lagrange inversion formula, see [C],[R],[Sch].