Weakly $\sqrt{J}U$ Rings

TL;DR

This paper introduces and explores weakly U rings, a new class of rings generalizing several existing classes, and characterizes their properties, especially in group rings and rings of positive characteristic.

Contribution

It defines weakly U rings, investigates their properties, and provides a complete characterization for group rings with positive characteristic, extending previous work.

Findings

Weakly U rings are Dedekind-finite.

The matrix ring $M_n(R)$ is never weakly U for $n \u2265 2$.

When har(R)>0$, the ring's characteristic must be of the form $2^\u03b1 3^\u03b2$.

Abstract

We introduce and study the so-called {\it weakly $\sqrt{J}U$ rings} (hereafter abbreviated as {\it $W\sqrt{J}U$ rings} for short), in which every unit is of the form $j+1$ or $j-1$ for some $j$ in $\sqrt{J(R)} : = \{x \in R : x^n \in J(R) \text{ for some } n\ge 1\}$. This class of rings non-trivially generalizes the classes of $\sqrt{J}U$, $UU$, $JU$, $WUU$ and $WJU$ rings, respectively. We investigate their basic properties showing that they are Dedekind-finite, that $M_n(R)$ is never $W\sqrt{J}U$ for $n\ge 2$, and that when $\operatorname{char}(R)>0$ it must be equal to $2^\alpha 3^\beta$ for some $\alpha, \beta \in \mathbb{N} \cup \left\{ 0 \right\}$. Moreover, for group rings $RG$, we prove that if $RG$ is $W\sqrt{J}U$, then $R$ is $W\sqrt{J}U$ and $G$ is a torsion group. In addition, when $R$ has positive characteristic and $G$ is a locally finite $p$-group, we give a complete characterization like this: $RG$ is a $W\sqrt{J}U$ ring if, and only if, either $R$ is a $\sqrt{J}U$ ring and $G$ is a $2$-group, or $R$ is a $W\sqrt{J}U$ ring with $3\in J(R)$ and $G$ is a $3$-group, or $R\cong R_1\times R_2$ with $R_1$ a $\sqrt{J}U$ ring, $R_2$ a $W\sqrt{J}U$ ring and $G$ a trivial group. Our results substantially improve on recent achievements due to Saini and Udar in Czech. Math. J. (2025).