A Rigidity Theorem for Ext

Abstract

The goal of this paper is to show that if $R$ is an unramified hypersurface, if $M$ and $N$ are finitely generated $R$ modules, and if $\operatorname{Ext}_{R}^{n}(M,N)=0$ for some $n\leq\operatorname{grade}{M}$, then $\operatorname{Ext}_{R}^{i}(M,N)=0$ for $i\leq n$. A corollary of this says that $\operatorname{Ext}_{R}^{i}(M,M)\neq 0$ for $i\leq\operatorname{grade}{M}$ and $M\neq 0$. These results are related to a question of Jorgensen and results of Dao.