Noncrossing arc diagrams and canonical join representations
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 has a canonical join representation: a unique lowest set of permutations joining to . 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.
Cite
@article{arxiv.1405.6904,
title = {Noncrossing arc diagrams and canonical join representations},
author = {Nathan Reading},
journal= {arXiv preprint arXiv:1405.6904},
year = {2026}
}
Comments
16 pages, 7 figures. Version 2: Changes only in Section 4. Now mentioning several more immediate consequences of the results, including the most general pattern-avoidance description of lattice quotients of the weak order on permutations. Version 3: Added figure showing the canonical join complex for S_4. Other very minor changes. Version 4: Minor expository changes