On minimal ring extensions

Abstract

Let $R$ be a commutative ring with identity. The ring $R\times R$ can be viewed as an extension of $R$ via the diagonal map $\Delta :R\hookrightarrow R\times R$, given by $\Delta (r)=(r,r)$ for all $r\in R$. It is shown that, for any $a,b\in R$, the extension $\Delta (R)[(a,b)]\subset R\times R$ is a minimal ring extension if and only if the ideal $<a-b>$ is a maximal ideal of $R$. A complete classification of maximal subrings of $R(+)R$ is also given. The minimal ring extension of a von Neumann regular ring $R$ is either a von Neumann regular ring or the idealization $R(+)R/\mathfrak{m}$ where $\mathfrak{m}\in \text{Max}(R)$. If $R\subset T$ is a minimal ring extension and $T$ is an integral domain, then $(R:T)=0$ if and only if $R$ is a field and $T$ is a minimal field extension of $R$, or $R_J$ is a valuation ring of altitude one and $T_J$ is its quotient field.