Notes on Matlis reflexive modules

Abstract

Let $M$ denote a module over a local Noetherian ring $(R,\mathfrak{m})$ with residue field $\mathbb{k}$ and its injective hull $E_R$. Let $D_R(\cdot )={\mathrm{Hom}}_R(\cdot ,E_R)$ denote the Matlis duality functor. Then $M$ is called Matlis reflexive whenever the injection $M\to D_R^2(M)$ is an isomorphism. The following results are proved: (1) For an ideal of $R$ a reflexive module is quasi-complete, generalizing part of Belshoff's results (see \cite{Br}). (2) We improve Zöschinger's numerical criterion for Matlis reflexivity (see \cite{Zh2}). (3) As shown by Enochs (see \cite{Ee}) for a prime ideal and the injective hull $E_R(R/\mathfrak{p})$ it follows that $D_R(E_R(R/\mathfrak{p}))$ is the completion of a free $R_{\mathfrak{p}}$-module of rank ${\tau }_{\mathfrak{p}}$, the Enochs number of $\mathfrak{p}$. We characterize when ${\tau }_{\mathfrak{p}}=1$, extending author's result (see \cite{Sp16}).

11 pages