A Ridge-Saturation Characterization of $α$-Critical $\mathbf {W}_p$ Graphs

Abstract

We characterize the graphs which are simultaneously $α$-critical and members of the class $\mathbf W_p$. The characterization is stated in three equivalent languages. In the graph itself, such a graph is a well-covered graph whose codimension-one localization fibers all have size at least $p$ and whose edges are exactly covered by the cliques induced by those fibers. In the independence complex, it is a pure flag complex in which every ridge has degree at least $p$ and every missing edge is generated by the link of a ridge. In the complement, it is a $K_{r+1}$-saturated graph, where $r=α(G)$, all maximal cliques have size $r$, and the minimum $(r-1)$-clique-codegree is at least $p$. This gives an exact formula for the largest $p$ for which a well-covered graph belongs to $\mathbf W_p$. We make this complement correspondence explicit, record saturation-theoretic consequences including dense-complement rigidity and $p$-sensitive edge and order bounds, and give a family of sharp examples showing that the local sufficient condition from the recent work of Hoang, Levit and Mandrescu is not necessary outside the locally triangle-free setting, for all $p\ge2$.