Combinatorics of Castelnuovo-Mumford Regularity of Binomial Edge Ideals

Since the introduction of binomial edge ideals $J_{G}$ by Herzog et al. and independently Ohtani, there has been significant interest in relating algebraic invariants of the binomial edge ideal with combinatorial invariants of the underlying graph $G$. Here, we take up a question considered by Herzog and Rinaldo regarding Castelnuovo--Mumford regularity of block graphs. To this end, we introduce a new invariant $\nu(G)$ associated to any simple graph $G$, defined as the maximal total length of a certain collection of induced paths within $G$ subject to conditions on the induced subgraph. We prove that for any graph $G$, $\nu(G) \leq \text{reg}(J_{G})-1$, and that the length of a longest induced path of $G$ is less than or equal to $\nu(G)$; this refines an inequality of Matsuda and Murai. We then investigate the question: when is $\nu(G) = \text{reg}(J_{G})-1$? We prove that equality holds when $G$ is closed; this gives a new characterization of a result of Ene and Zarojanu, and when $G$ is bipartite and $J_{G}$ is Cohen-Macaulay; this gives a new characterization of a result of Jayanathan and Kumar. For a block graph $G$, we prove that $\nu(G)$ admits a combinatorial characterization independent of any auxiliary choices, and we prove that $\nu(G) = \text{reg}(J_{G})-1$. This gives $\text{reg}(J_{G})$ a combinatorial interpretation for block graphs, and thus answers the question of Herzog and Rinaldo.