A recursion on maximal chains in the Tamari lattices

Abstract

The Tamari lattices have been intensely studied since their introduction by Dov Tamari around 1960. However oddly enough, a formula for the number of maximal chains is still unknown. This is due largely to the fact that maximal chains in the $n$-th Tamari lattice $\mathcal{T}_{n}$ range in length from $n-1$ to ${n \choose 2}$. In this note, we treat vertices in the lattice as Young diagrams and identify maximal chains as certain tableaux. For each $i\geq-1$, we define $\mathcal{C}_{i}(n)$ as the set of maximal chains in $\mathcal{T}_{n}$ of length $n+i$. We give a recursion for $\#\mathcal{C}_{i}(n)$ and an explicit formula based on predetermined initial values. The formula is a polynomial in $n$ of degree $3i+3$. For example, the number of maximal chains of length $n$ in $\mathcal{T}_{n}$ is $\#\mathcal{C}_{0}(n)={n \choose 3}$. The formula has a combinatorial interpretation in terms of a special property of maximal chains.