Uniform Harbourne-Huneke Bounds via Flat Extensions

Abstract

Over an arbitrary field $\mathbb{F}$, Harbourne conjectured that $$I^{(N (r-1)+1)} \subseteq I^r$$ for all $r>0$ and all homogeneous ideals $I$ in $S = \mathbb{F} [\mathbb{P}^N] = \mathbb{F} [x_0, \ldots, x_N]$. The conjecture has been disproven for select values of $N \ge 2$: first by Dumnicki, Szemberg, and Tutaj-Gasińska in characteristic zero, and then by Harbourne and Seceleanu in odd positive characteristic. However, the ideal containments above do hold when, for instance, $I$ is a monomial ideal in $S$. As a sequel to (arXiv:1510.02993), we present criteria for containments of type $I^{(N (r-1)+1)} \subseteq I^r$ for all $r>0$ and certain classes of ideals $I$ in a prodigious class of normal rings. Of particular interest is a result for monomial primes in tensor products of affine semigroup rings. Indeed, we explain how to give effective multipliers $N$ in several cases including: the $D$-th Veronese subring of any polynomial ring $\mathbb{F} [x_1, \ldots, x_n]$ $(n \ge 1)$; and the extension ring $\mathbb{F} [x_1, \ldots, x_n, z]/(z^D - x_1 \cdots x_n)$ of $\mathbb{F}[x_1, \ldots, x_n]$.