Intervals in a family of Fibonacci lattices

TL;DR

This paper studies a family of Fibonacci lattice-based posets derived from Dyck paths avoiding specific patterns, providing enumeration, structural properties, and connections to other combinatorial objects.

Contribution

It introduces a new family of Fibonacci lattice posets, analyzes their lattice structure, and establishes links with Motzkin paths, Turán graphs, and compositions.

Findings

Posets are sublattices of the Stanley lattice.

Generated functions for interval counts and Möbius functions.

Bijections with pattern-avoiding bicolored Motzkin paths.

Abstract

We focus on a family of subsets $(\F^p_n)_{p\geq 2}$ of Dyck paths of semilength $n$ that avoid the patterns $DUU$ and $D^{p+1}$, which are enumerated by the generalized Fibonacci numbers. We endow them with the partial order relation induced by the well-known Stanley lattice, and we prove that all these posets are sublattices of the Stanley lattice. We provide generating functions for the numbers of linear and boolean intervals and we deduce the M\"obius function for every $p\geq 2$. We count meet-irreducible elements in $\FF_n^p$ which establishes a surprising link with the edges of the $(n,p)$-Tur\'an graph. We also prove that intervals are in one-to-one correspondence with bicolored Motzkin paths avoiding some patterns, which allows to enumerate intervals for $p=2$. Using a discrete continuity argument ($p\rightarrow \infty$), we present a similar enumerative study in a poset of some Dyck paths of semilength $n$ counted by $2^{n-1}$. Finally, we give bijections that transport the lattice structure on other combinatorial objects, proving that those lattices can be seen as the well-known dominance order on some compositions.