Linear resolutions of powers of edge ideals of gap-free graphs with matching number four

Abstract

For a finite simple gap-free graph $G$, let $B(G)$ denote the vertices $v$ for which $G-N_G[v]$ is not co-chordal, and put $C(G)=V(G)\setminus B(G)$. We prove a sufficient condition for all ordinary powers of its edge ideal to have linear resolutions from the second power onward: every induced anticycle of length at least five must contain an edge with both endpoints in $C(G)$. The argument uses variable generators of augmented colon ideals to eliminate anticycles that may persist in the ordinary quadratic colons. It yields linear resolutions for all powers $I(G)^q$, $q\ge2$, whenever the ordinary matching number is at most four, which answers the first part of a question of Erey, Hà, Hibi, Kara and Morey. The criterion also applies to certain graphs with adjacent exceptional vertices and matching number five. The proofs are theoretical and valid over arbitrary fields. We do not assert linear quotients.