The Koszul complex in projective dimension one

Abstract

Let $R$ be a noetherian ring and $M$ a finite $R$-module. With a linear form $\chi$ on $M$ one associates the Koszul complex $K(\chi)$. If $M$ is a free module, then the homology of $K(\chi)$ is well-understood, and in particular it is grade sensitive with respect to $\Im\chi$. In this note we investigate the case of a module $M$ of projective dimension 1 (more precisely, $M$ has a free resolution of length 1) for which the first non-vanishing Fitting ideal $\I_M$ has the maximally possible grade $r+1$, $r=\rank M$. Then $h=\grade \Im\chi\le r+1$ for all linear forms $\chi$ on $M$, and it turns out that $H_{r-i}(K(\chi))=0$ for all even $i<h$ and $H_{r-i}(K(\chi))\iso \SS^{(i-1)/2}(C)$ for all odd $i<h$ where $\SS$ denotes symmetric power and $C=\Ext_R^1(M,R)$, in other words, $C=\Cok\psi^*$ for a presentation $$ 0\to F\stackrel{\psi}{\to} G \to M\to 0. $$ Moreover, if $h\le r$, then $H_{r-h}(K(\chi))$ is neither 0 nor isomorphic to a symmetric power of $C$, so that it is justified to say that $K(\chi)$ is grade sensitive for the modules $M$ under consideration. We furthermore show that the maximally possible value $\grade \Im\chi=r+1$ can only occur in two extreme cases: (i) $r=1$ or (ii) $\rank F=1$ and $r$ is odd.