A counterpart to Nagata idealization

Abstract

Idealization of a module $K$ over a commutative ring $S$ produces a ring having $K$ as an ideal, all of whose elements are nilpotent. We develop a method that under suitable field-theoretic conditions produces from an $S$-module $K$ and derivation $D:S\rightarrow K$ a subring $R$ of $S$ that behaves like the idealization of $K$ but is such that when $S$ is a domain, so is $R$. The ring $S$ is contained in the normalization of $R$ but is finite over $R$ only when $R = S$. We determine conditions under which $R$ is Noetherian, Cohen-Macaulay, Gorenstein, a complete intersection or a hypersurface. When $R$ is local, then its ${\bf m}$-adic completion is the idealization of the ${\bf m}$-adic completions of $S$ and $K$.