Connections between unit-regularity, regularity, cleanness, and strong cleanness of elements and rings

TL;DR

This paper explores the relationships between various algebraic properties of elements and rings, providing new examples, characterizations, and connections between regularity, cleanness, and unit-regularity.

Contribution

It constructs a counterexample to a known open question and characterizes clean matrices with zero columns, linking unit-regular and clean elements.

Findings

Constructed a unit-regular ring not strongly clean

Characterized clean matrices with zero columns

Established conditions for powers of regular and unit-regular elements

Abstract

We construct an example of a unit-regular ring which is not strongly clean, answering an open question of Nicholson. We also characterize clean matrices with a zero column, and this allows us to describe an interesting connection between unit-regular elements and clean elements. It is also proven that given an element $a$ in a ring $R$, if $a,a^2,\ldots, a^k$ are all regular elements in $R$ (for some $k\geq 1$), then there exists $w\in R$ such that $a^{i}w^{i}a^{i}=a^{i}$ for $1\leq i\leq k$, and a similar statement holds for unit-regular elements. The paper ends with a large number of examples elucidating further connections (and disconnections) between cleanliness, regularity, and unit-regularity.