Higher Dimensional Lattice Walks: Connecting Combinatorial and Analytic Behavior

We consider the enumeration of walks on the nonnegative lattice $\mathbb{N}^{d},$ with steps defined by a set $\mathcal{S}\subset \{-1, 0, 1\}^d\backslash\{{0}\}$. Previous work in this area has established asymptotics for the number of walks in certain families of models by applying the techniques of analytic combinatorics in several variables (ACSV), where one encodes the generating function of a lattice path model as the diagonal of a multivariate rational function. Melczer and Mishna obtained asymptotics when the set of steps $\mathcal{S}$ is symmetric over every axis; in this setting one can always apply the methods of ACSV to a multivariate rational function whose set of singularities is a smooth manifold (the simplest case). Here we go further, providing asymptotics for models with generating functions that must be encoded by multivariate rational functions having nonsmooth singular sets. In the process, our analysis connects past work to deeper structural results in the theory of ACSV. One application is a closed form for asymptotics of models defined by step sets that are symmetric over all but one axis. As a special case, we apply our results when $d=2$ to give a rigorous proof of asymptotics conjectured by Bostan and Kauers; asymptotics for walks returning to boundary axes and the origin are also given.