A combinatorial characterization of $S_2$ binomial edge ideals

TL;DR

This paper characterizes when binomial edge ideals satisfy Serre's condition (S_2) using graph accessibility, linking algebraic properties to combinatorial graph structures and simplicial complexes.

Contribution

It provides the first graph-theoretical characterization of (S_2) binomial edge ideals via accessibility of cut sets, connecting algebraic conditions to combinatorial properties.

Findings

(S_2) condition equivalent to graph being accessible

Accessibility and strong accessibility are equivalent for cut sets in unmixed cases

Facets of the Stanley-Reisner complex relate to cut sets and accessibility

Abstract

Several algebraic properties of a binomial edge ideal $J_G$ can be interpreted in terms of combinatorial properties of its associated graph $G$. In particular, the so-called cut sets of a graph $G$, special sets of vertices that disconnect $G$ in a minimal way, play an important role since they are in bijection with the minimal prime ideals of $J_G$. In this paper we establish the first graph-theoretical characterization of binomial edge ideals $J_G$ satisfying Serre's condition $(S_2)$ by proving that this is equivalent to having $G$ accessible, which means that $J_G$ is unmixed and the cut sets of $G$ form an accessible set system. The proof relies on the combinatorial structure of the Stanley-Reisner simplicial complex of a multigraded generic initial ideal of $J_G$, whose facets can be described in terms of cut sets. Another key step in the proof consists in proving the equivalence between accessibility and strong accessibility for the collection of cut sets of $G$ with $J_G$ unmixed. This result, interesting on its own, provides the first relevant class of set systems for which the previous two notions are equivalent.