Regularity of symbolic powers of square-free monomial ideals

Abstract

We study the regularity of symbolic powers of square-free monomial ideals. We prove that if $I = I_\Delta$ is the Stanley-Reisner ideal of a simplicial complex $\Delta$, then $\reg(I^{(n)}) \leqslant \delta(n-1) +b$ for all $n\geqslant 1$, where $\delta = \lim\limits_{n\to\infty} \reg(I^{(n)})/n$, and $b = \max\{\reg(I_\Gamma) \mid \Gamma \text{ is a subcomplex of } \Delta \text{ with } \F(\Gamma) \subseteq \F(\Delta)\}$. This bound is sharp for any $n$. When $I = I(G)$ is the edge ideal of a simple graph $G$, we obtain a general linear upper bound $\reg(I^{(n)}) \leqslant 2n + \ordmatch(G)-1$, where $\ordmatch(G)$ is the ordered matching number of $G$.