Structural properties of admissible subgraphs and the depth of symbolic powers of cover ideals

Abstract

Let $G$ be a simple graph with cover ideal $J(G)$ in a polynomial ring $S = \k[x_1, \ldots, x_n]$. For each integer $t\ge 1$, let ${\alpha }_t(G)$ denote the maximum size of an ordered matching $M$ of $G$ with alternating path length $\ell (M)\le 2t-1$. Hang, Tam, and Vu established that \[ \operatorname{depth}\!\left(S/J(G)^{(t)}\right) \le n - 1 - α_t(G) \] for all $t\ge 1$, where $J(G{)}^{(t)}$ is the $t$-th symbolic power of $J(G)$. In this paper, we introduce a natural hereditary class of graphs $\mathcal{P}_\k$ for which this inequality holds with equality for every $t\ge 1$. We prove that $\mathcal{P}_\k$ contains all distance-hereditary graphs, and we show that within the class of chordal graphs, $\mathcal{P}_\k$ coincides with the class of distance-hereditary (or gem-free) graphs.