Noncrossing arithmetic

TL;DR

This paper explores higher-order Kreweras complementation in noncrossing partitions, revealing its combinatorial core and its analogy to number divisibility, with new categorical and algebraic insights.

Contribution

It provides a simple, elementary account of higher-order Kreweras complementation, highlighting its combinatorial, algebraic, and categorical properties, and introduces novel viewpoints and computational methods.

Findings

Noncrossing partitions form a lattice as the decalage of a partial monoid.

Higher-order Kreweras complements relate to divisibility structures.

The approach offers efficient conceptual and computational tools.

Abstract

Higher-order notions of Kreweras complementation have appeared in the literature in the works of Krawczyk, Speicher, Mastnak, Nica, Arizmendi, Vargas, and others. While the theory has been developed primarily for specific applications in free probability, it also possesses an elegant, purely combinatorial core that is of independent interest. The present article aims at offering a simple account of various aspects of higher-order Kreweras complementation on the basis of elementary arithmetic, (co)algebraic, categorical and simplicial properties of noncrossing partitions. The main idea is to consider noncrossing partitions as providing an interesting noncommutative analogue of the interplay between the divisibility poset and the multiplicative monoid of positive integers. Just as the divisibility poset can be regarded as the decalage of the multiplicative monoid, we exhibit the lattice of noncrossing partitions as the decalage of a partial monoid structure on noncrossing partitions encoding higher-order Kreweras complements. While our results may be considered familiar, several of the viewpoints can be regarded as novel, offering an efficient approach both conceptually and computationally.