Bounds on depth of tensor products of modules

Abstract

Let $R$ be a local complete intersection ring and let $M$ and $N$ be nonzero finitely generated $R$-modules. We employ Auslander's transpose in the study of the vanishing of Tor and obtain useful bounds for the depth of the tensor product $M\otimes_{R}N$. An application of our main argument shows that, if $M$ is locally free on the the punctured spectrum of $R$, then either $\depth(M\otimes_{R}N)\geq \depth(M)+\depth(N)-\depth(R)$, or $\depth(M\otimes_{R}N)\leq \cod(R)$. Along the way we generalize an important theorem of D. A. Jorgensen and determine the number of consecutive vanishing of $\Tor_i^R(M,N)$ required to ensure the vanishing of all higher $\Tor_i^R(M,N)$.