English

A counterexample to an isomorphism problem for power monoids

Commutative Algebra 2025-09-30 v1 Combinatorics

Abstract

Let HH be a (multiplicatively written) monoid. The family Pfin,1(H)\mathcal{P}_{\text{fin},1}(H) of finite subsets of HH containing the identity element is itself a monoid when endowed with setwise multiplication induced by HH. Tringali and Yan proved that two monoids H1H_1 and H2H_2 contained in a special class of commutative, cancellative monoids are isomorphic if and only if Pfin,1(H1)\mathcal{P}_{\text{fin},1}(H_1) and Pfin,1(H2)\mathcal{P}_{\text{fin},1}(H_2) are. Moreover, they raised the question whether the same holds in the general setting of cancellative monoids. We show that if H1H_1 and H2H_2 are (commutative) valuation monoids with trivial unit groups and isomorphic quotient groups, then Pfin,1(H1)Pfin,1(H2)\mathcal{P}_{\text{fin},1}(H_1)\simeq\mathcal{P}_{\text{fin},1}(H_2). This provides a negative answer to Tringali and Yans question already within the class of valuation submonoids of the additive group Z2\mathbb{Z}^2.

Keywords

Cite

@article{arxiv.2509.23818,
  title  = {A counterexample to an isomorphism problem for power monoids},
  author = {Balint Rago},
  journal= {arXiv preprint arXiv:2509.23818},
  year   = {2025}
}
R2 v1 2026-07-01T06:02:26.934Z