Approximations by maximal Cohen-Macaulay modules

TL;DR

This paper extends the theory of Cohen-Macaulay modules by establishing the existence of maximal CM preenvelopes and envelopes, characterizing rings with unique lifting properties, and analyzing cosyzygies in this context.

Contribution

It proves the existence of special maximal CM preenvelopes and envelopes, especially over henselian rings, and characterizes rings where maximal CM envelopes have the unique lifting property.

Findings

Existence of special maximal CM preenvelopes and envelopes.

Characterization of rings with maximal CM envelopes with unique lifting property.

Cosyzygies eventually become maximal CM modules.

Abstract

Auslander and Buchweitz have proved that every finitely generated module over a Cohen-Macaulay (CM) ring with a dualizing module admits a so-called maximal CM approximation. In terms of relative homological algebra, this means that every finitely generated module has a special maximal CM precover. In this paper, we prove the existence of special maximal CM preenvelopes and, in the case where the ground ring is henselian, of maximal CM envelopes. We also characterize the rings over which every finitely generated module has a maximal CM envelope with the unique lifting property. Finally, we show that cosyzygies with respect to the class of maximal CM modules must eventually be maximal CM, and we compute some examples.