The Stable Tamari Lattice

Abstract

We introduce the stable Tamari lattice $\mathcal{ST}_n$, an infinite lattice on $\mathbb{N}^n$. The $r$-Tamari lattices on $r$-Dyck paths of height $n+1$ occur as a nested family of principal lower order ideals whose union is $\mathcal{ST}_n$. Partition-indexed principal lower order ideals recover the $ν$-Tamari lattices associated with partition boundaries. We give an explicit coordinate description of the stable Tamari order in terms of interval vectors determined by the lower endpoint. These coordinates characterize comparability and covering relations, and their sum gives the maximum length of a chain between two comparable elements. Analogous coordinates determined by the upper endpoint give an explicit formula for the Möbius function. We also construct an EL-labeling of every interval and show that every interval in $\mathcal{ST}_n$ is isomorphic to an interval in a classical Tamari lattice. We classify the possible $h$-vectors by Dyck paths and show that each $h$-vector fiber is parametrized by a unimodular basis of interval vectors. Combining this parametrization with the lower-endpoint coordinates, we obtain a finite rational formula for the multivariate generating function of the height generating functions of all principal lower order ideals. Under the $r$-Tamari realization, these height generating functions recover the interval generating functions of Bergeron and Préville-Ratelle.