Remarks on modules of finite projective dimension

TL;DR

This paper explores the homological properties of modules with finite projective dimension over Noetherian local rings, establishing conditions for freeness, analyzing Ext-modules, and addressing related homological conjectures.

Contribution

It generalizes classical results to non-regular rings, provides bounds on Ext-module dimensions, and investigates tensor product projective dimensions and prime ideal properties.

Findings

Tensor products of modules impose structural constraints, often forcing modules to be free.

Sharp bounds on the Krull dimensions of Ext-modules are established.

Results relate to the grade conjecture, homological conjectures, and projective dimension questions.

Abstract

We investigate homological and depth-theoretic properties of finitely generated modules of finite projective dimension over Noetherian local rings. A central theme is the study of criteria for freeness and reflexivity derived from the torsion-freeness or reflexivity of tensor products of the form \( M \otimes_R M \) and \( M \otimes_R M^* \). Under mild homological assumptions, we prove that such properties of these tensor products impose strong structural constraints on \( M \), often forcing it to be free. These results generalize classical theorems of Auslander beyond the regular case. The second part of the paper is devoted to the dimension and support of Ext-modules, particularly \( \operatorname{Ext}^i_R(M, R) \) for critical values of \( i \), when \( M \) has finite projective dimension. We establish sharp bounds on their Krull dimensions, analyze their behavior for prime and equidimensional modules, and relate these findings to the grade conjecture and other homological conjectures, i.e., whenever $\operatorname{grade}(M) = \operatorname{ht}(\operatorname{Ann}(M))$ where Gdim$(M)<\infty$. We consider the problem that asks whenever is \( \pd_R(M \otimes_R N) = 1 \)? Applications include new cases of a question of Jorgensen, which asks whether \( \operatorname{pd}(M) < i \) whenever \( \operatorname{Ext}^i_R(M, M) = 0 \) and \( M \) has finite projective dimension over a complete intersection ring. Finally, we examine the projective dimension of prime ideals in rings that fail chain conditions.