Lattice paths and submonoids of $\mathbb Z^2$

Abstract

We study a number of combinatorial and algebraic structures arising from walks on the two-dimensional integer lattice. To a given step set $X\subseteq\mathbb Z^2$, there are two naturally associated monoids: $\mathscr F_X$, the monoid of all $X$-walks/paths; and $\mathscr A_X$, the monoid of all endpoints of $X$-walks starting from the origin $O$. For each $A\in\mathscr A_X$, write $\pi_X(A)$ for the number of $X$-walks from $O$ to $A$. Calculating the numbers $\pi_X(A)$ is a classical problem, leading to Fibonacci, Catalan, Motzkin, Delannoy and Schroder numbers, among many other well-studied sequences and arrays. Our main results give relationships between finiteness properties of the numbers $\pi_X(A)$, geometrical properties of the step set $X$, algebraic properties of the monoid $\mathscr A_X$, and combinatorial properties of a certain bi-labelled digraph naturally associated to $X$. There is an intriguing divergence between the cases of finite and infinite step sets, and some constructions rely on highly non-trivial properties of real numbers. We also consider the case of walks constrained to stay within a given region of the plane. Several examples are considered throughout to highlight the sometimes-subtle nature of the theoretical results.