Pairs of rings sharing their units
Gabriel Picavet, Martine Picavet L'Hermitte
Abstract
We are working in the category of commutative unital rings and denote by $\mathrm U(R)$ the group of units of a nonzero ring $R$. An extension of rings $R\subseteq S$, satisfying $\mathrm U(R)=R \cap\mathrm U(S)$ is usually called local. This paper is devoted to the study of ring extensions such that $\mathrm U(R)=\mathrm U(S)$, that we call strongly local. P. M. Cohn in a paper, entitled Rings with zero divisors, introduced some strongly local extensions. We generalized under the name Cohn's rings his definition and give a comprehensive study of these extensions. As a consequence, we give a constructive proof of his main result. Now Lequain and Doering studied strongly local extensions, where $S$ is semilocal, so that $S/\mathrm J(S)$, where $\mathrm J(S)$ is the Jacobson radical of $S$, is Von Neumann regular. These rings are usually called $J$-regular. We establish many results on $J$-regular rings in order to get substantial results on strongly local extensions when $S$ is $J$-regular. The Picard group of a $J$-regular ring is trivial, allowing to evaluate the group $\mathrm U(S)/\mathrm U(R)$ when $R$ is $J$-regular. We then are able to give a complete characterization of the Doering-Lequain context. A Section is devoted to examples. In particular, when $R$ is a field, the strongly local and weakly strongly inert properties are equivalent.