A combinatorial characterization of S2 binomial edge ideals

Several algebraic properties of a binomial edge ideal JG 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, play an important role since they are in bijection with the minimal prime ideals of JG. In this paper we establish the first graph-theoretical characterization of binomial edge ideals JG satisfying Serre's condition (S2) by proving that this is equivalent to having G accessible, which means that JG is unmixed and the cut-point 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 JG, whose facets can be described in terms of cut-point 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 JG unmixed. This result, interesting on its own, provides the first relevant class of set systems for which the previous two notions are equivalent.