Counting quadrant walks via Tutte's invariant method

TL;DR

This paper adapts Tutte's invariant method to count quadrant lattice walks, proving algebraicity for known models, and introduces a complex analytic approach to analyze the nature of their generating functions.

Contribution

It extends Tutte's algebraic approach to all quadrant models, providing new proofs and integral-free expressions for their generating functions.

Findings

Proved algebraicity of all known or conjectured algebraic quadrant models.

Developed a complex analytic framework for non-D-finite models.

Established D-algebraicity for models with decoupling functions.

Abstract

In the 1970s, William Tutte developed a clever algebraic approach, based on certain "invariants", to solve a functional equation that arises in the enumeration of properly colored triangulations. The enumeration of plane lattice walks confined to the first quadrant is governed by similar equations, and has led in the past 20 years to a rich collection of attractive results dealing with the nature (algebraic, D-finite or not) of the associated generating function, depending on the set of allowed steps, taken in $\{-1, 0,1\}^2$. We first adapt Tutte's approach to prove (or reprove) the algebraicity of all quadrant models known or conjectured to be algebraic. This includes Gessel's famous model, and the first proof ever found for one model with weighted steps. To be applicable, the method requires the existence of two rational functions called invariant and decoupling function respectively. When they exist, algebraicity follows almost automatically. Then, we move to a complex analytic viewpoint that has already proved very powerful, leading in particular to integral expressions for the generating function in the non-D-finite cases, as well as to proofs of non-D-finiteness. We develop in this context a weaker notion of invariant. Now all quadrant models have invariants, and for those that have in addition a decoupling function, we obtain integral-free expressions for the generating function, and a proof that this series is D-algebraic (that is, satisfies polynomial differential equations).