Torsion groups and the Bienvenu--Geroldinger conjecture

Abstract

Equipped with the operation of setwise multiplication induced by a (multiplicatively written) monoid $H$ on its parts, the collection of all finite subsets of $H$ containing the identity element is itself a monoid, denoted by ${\mathcal{P}}_{\text{fin},1}(H)$ and called the reduced finitary power monoid of $H$. One is naturally led to ask whether, for all $H$ and $K$ in a given class of monoids, ${\mathcal{P}}_{\text{fin},1}(H)$ and ${\mathcal{P}}_{\text{fin},1}(K)$ are isomorphic if and only if $H$ and $K$ are. The problem originates from a conjecture of Bienvenu and Geroldinger that was recently settled by the authors. Here, we provide a positive answer to the problem in the case where $H$ and $K$ are cancellative monoids, one of which is torsion. In particular, the answer is in the affirmative when $H$ and $K$ are torsion groups. Whether the conclusion extends to arbitrary groups remains open.

14 pages, no figures. Fixed a number of minor details. To appear in Bull. London Math. Soc