Combinatorial bounds for the v-number of binomial edge ideals

Source abstract

We study the local $\mathrm v$-numbers of binomial edge ideals from a combinatorial perspective. For each minimal prime $P_{\mathcal S}$ of $J_G$, indexed by a cutset $\mathcal S$ of $G$, we consider two combinatorial invariants, $\mathrm v^-_{\mathcal S}(G)$ and $\mathrm v^+_{\mathcal S}(G)$, defined in terms of transversals of suitable hypergraphs, and show that the local $\mathrm v$-number $\mathrm v_{P_{\mathcal S}}(J_G)$ satisfies $\mathrm v^-_{\mathcal S}(G)\leq \mathrm v_{P_{\mathcal S}}(J_G)\leq \mathrm v^+_{\mathcal S}(G)$. Under a suitable hypothesis on the connected components, we provide explicit graph-theoretic descriptions of these invariants and analyze when they coincide, giving a combinatorial way to determine the local $\mathrm v$-numbers. As an application, we derive a sufficient condition for this coincidence, which applies to a broad class of graphs. Furthermore, we identify a constant that controls the gap between $\mathrm v^-_{\mathcal S}(G)$ and $\mathrm v^+_{\mathcal S}(G)$, thereby showing that the local $\mathrm{v}$-number lies within a bounded interval. Applications to cycles and trees illustrate both the equality criterion and the resulting bounds.