The paint pot problem and common multiples in monoids

Abstract

Illustrated by a problem on paint pots that is easy to understand but hard to solve, we investigate whether particular monoids have the property of common right multiples. As one result we characterize generalized braid monoids represented by undirected graphs, being a subclass of Artin–Tits monoids. Stated in other words, we investigate to which graphs the old Garside result stating that braid monoids have the property of common right multiples, generalizes. This characterization also follows from old results on Coxeter groups and the connection between finiteness of Coxeter groups and common right multiples in Artin–Tits monoids. However, our independent presentation is self-contained up to some basic knowledge of rewriting, and also applies to monoids beyond the Artin–Tits format. The main new contribution is a technique to prove that the property of common right multiples does not hold, by finding a particular model, in our examples all being finite.