Finite homological dimension of Hom, vanishing of Ext, and applications to divisor class group

Abstract

For finitely generated modules $M$ and $N $ over a commutative Noetherian local ring $R$, we give various sufficient criteria for detecting freeness of $M$ or $N$ via vanishing of some finitely many Ext modules $\textrm{Ext}^i_R(M,N)$ and finiteness of certain homological dimension of $\textrm{Hom}_R(M,N)$. Some of our results provide partial progress towards answering a question of Ghosh-Takahashi and also generalize their main results in many ways, for instance, by reducing the number of vanishing. Certain special cases of our results allow us to address the Auslander-Reiten conjecture for modules whose (self-) dual has finite projective dimension. Along the way, we establish a new characterization of $I$-Ulrich modules of Dao-Maitra-Sridhar which we then apply to provide a negative answer to a question of Gheibi-Takahashi concerning characteristic modules. Among other techniques, we introduce and study certain generalizations of the notion of residually faithful modules of Brennan-Vasconcelos and Goto-Kumashiro-Loan, which play a crucial role in our study. As some applications of our results, we provide affirmative answers to two questions raised by Tony Se on $n$-semidualizing modules. Namely, we show that over a local ring of depth $t$, every $(t-1)$-semidualizing module of finite G-dimension is free. Moreover, we establish that for normal domains which satisfy Serre's condition $(S_3)$ and are locally Gorenstein in codimension two, the class of $1$-semidualizing modules forms a subgroup of the divisor class group. These two groups coincide when, in addition, the ring is locally regular in codimension two.