Tor as a module over an exterior algebra

Abstract

Let $S$ be a regular local ring with residue field $k$ and let $M$ be a finitely generated $S$-module. Suppose that $f_1,\dots ,f_c\in S$ is a regular sequence that annihilates $M$, and let $E$ be an exterior algebra over $k$ generated by $c$ elements. The homotopies for the $f_i$ on a free resolution of $M$ induce a natural structure of graded $E$-module on Tor${}^S(M,k)$. In the case where $M$ is a high syzygy over the complete intersection $R:=S/(f_1,\dots ,f_c)$ we describe this $E$-module structure in detail, including its minimal free resolution over $E$.

Turning to Ext${}_R(M,k)$ we show that, when $M$ is a high syzygy over $R$, the minimal free resolution of Ext${}_R(M,k)$ as a module over the ring of CI operators is the Bernstein–Gel’fand–Gel’fand dual of the $E$-module Tor${}^S(M,k)$.

For the proof we introduce higher CI operators, and give a construction of a (generally non-minimal) resolution of $M$ over $S$ starting from a resolution of $M$ over $R$ and its higher CI operators.