Unions of sets of lengths

Abstract

Let $H$ be a Krull monoid such that every class contains a prime (this includes the multiplicative monoids of rings of integers of algebraic number fields). For $k \in \mathbb{N}$ let $\mathcal{V}_k (H)$ denote the set of all $m \in \mathbb{N}$ \ with the following property{\rm \,:} There exist atoms (irreducible elements) \ $u_1, \ldots, u_k, v_1, \ldots, v_m \in H$ with $u_1 \cdot \ldots \cdot u_k = v_1 \cdot \ldots \cdot v_m$. We show that the sets $\mathcal{V}_k (H)$ are intervals for all $k \in \mathbb{N}$. This solves Problem 37 in [4].