Noncrossing arc diagrams and canonical join representations

TL;DR

This paper links noncrossing arc diagrams with canonical join representations in lattice theory, revealing new combinatorial objects related to the weak order and Baxter numbers.

Contribution

It establishes a novel connection between noncrossing arc diagrams and canonical join representations, providing new combinatorial models for lattice quotients.

Findings

Noncrossing arc diagrams model canonical join representations.

New combinatorial objects are counted by Baxter numbers.

Bijections are established with generic rectangulations.

Abstract

We consider two problems that appear at first sight to be unrelated. The first problem is to count certain diagrams consisting of noncrossing arcs in the plane. The second problem concerns the weak order on the symmetric group. Each permutation $x$ has a canonical join representation: a unique lowest set of permutations joining to $x$. The second problem is to determine which sets of permutations appear as canonical join representations. The two problems turn out to be closely related because the noncrossing arc diagrams provide a combinatorial model for canonical join representations. The same considerations apply to more generally to lattice quotients of the weak order. Considering quotients produces, for example, a new combinatorial object counted by the Baxter numbers and an analogous new object in bijection with generic rectangulations.