Regularity, depth and Serre depth of symbolic powers of complementary edge ideals

Abstract

Let $G$ be a finite simple graph and let $I_c(G)$ denote its complementary edge ideal. In this paper, we study the Castelnuovo-Mumford regularity, the depth and Serre depth for $(S_{2})$ of the symbolic powers of $I_c(G)$. We derive explicit formulas for $\operatorname{reg} I_c(G)^{(k)}$ and $\operatorname{reg} I_c(G)^{(k)}/I_c(G)^k$ for every $k\geq1$, in terms of the number and the sizes of the connected components of $G$. As applications, we determine their asymptotic behavior, and completely characterize the symbolic powers of complementary edge ideals having linear resolutions. We then investigate the symbolic depth function $k\mapsto\operatorname{depth} S/I_c(G)^{(k)}$. We determine explicitly its values for $k=2,3,4$, and establish bounds for the asymptotic depth. More generally, for an arbitrary squarefree monomial ideal $I$, we compare the local cohomology modules of $S/I^{(k)}$ for different symbolic powers, and derive corresponding inequalities for the depth and the Serre depth. Applying this to complementary edge ideals, we obtain $\operatorname{depth} S/I_{c}(G)^{(t)}\geq\operatorname{depth} S/I_{c}(G)^{(k)}$ for $k\geq 2t-1$, despite the fact that the symbolic depth function need not be non-increasing. Finally, we characterize complementary edge ideals satisfying Serre's condition $(S_r)$, and determine the Serre depth for $(S_{2})$ of all symbolic powers of complementary edge ideals. In particular, we obtain combinatorial characterizations of Serre's condition $(S_{2})$ and of the Cohen-Macaulayness of $S/I_{c}(G)^{(k)}$ for every $k\geq 1$.