Linear resolution of connected graph ideals and their powers

Abstract

For a finite simple graph $G$ and an integer $r\ge 1$, the $r$-connected ideal $I_r(G)$ is the squarefree monomial ideal generated by the vertex sets of connected induced subgraphs of size $r+1$, extending the classical edge ideal. We investigate the linearity of the minimal free resolutions of $I_r(G)$ via structural features of the associated clutter ${\mathcal{C}}_r(G)$. We introduce the class of co-chordal-cactus graphs and prove that $I_r(G)$ has a linear resolution for all $r\ge 2$ whenever $G$ lies in this family. The result further extends to $(2K_2,C_4)$-free graphs and co-grid graphs. For $r=1$, we show that the edge ideal $I_1(G)$ has Castelnuovo-Mumford regularity at most $3$ for all co-chordal-cactus and co-grid graphs. We also examine powers of connected ideals and establish that $I_r(G{)}^q$ has a linear resolution for every $q\ge 1$ in several natural graph families, including complements of trees with bounded degree, complete multipartite graphs, complements of cycles, graphs obtained by gluing complete graphs along cliques, and certain subclasses of split graphs.

29 pages, 2 figures