On asymptotic vanishing behavior of local cohomology

Abstract

Let $R$ be a standard graded algebra over a field $k$, with irrelevant maximal ideal $\fm$, and $I$ a homogeneous $R$-ideal. We study the asymptotic vanishing behavior of the graded components of the local cohomology modules $\{\HH{i}{\fm}{R/I^n}\}_{n\in \NN}$ for $i<\dim R/I$. We show that, when $\chara k= 0$, $R/I$ is Cohen-Macaulay, and $I$ is a complete intersection locally on $\Spec R \setminus\{\fm\}$, the lowest degrees of the modules $\{\HH{i}{\fm}{R/I^n}\}_{n\in \NN}$ are bounded by a linear function whose slope is controlled by the generating degrees of the dual of $I/I^2$. Our result is a direct consequence of a related bound for symmetric powers of locally free modules. If no assumptions are made on the ideal or the field $k$, we show that the complexity of the sequence of lowest degrees is at most polynomial, provided they are finite. Our methods also provide a result on stabilization of maps between local cohomology of consecutive powers of ideals.

13 pages. To appear in Math. Z