Complexes of C-Unbalanced Subsets and Balanced-Subset Posets

Abstract

Let \(V=\{v_1,\dots,v_m\}\subset\mathbb{R}^d\). We study simplicial complexes arising from subsets of \(V\) whose convex hulls avoid a prescribed convex set. Given a convex set \(C\subset\mathbb{R}^d\), let \(\mathcal K(V,C)\) be the simplicial complex consisting of all subsets \(S\subset V\) such that \(\operatorname{conv}(S)\cap C=\emptyset\). We prove that \(\mathcal K(V,C)\) is homotopy equivalent to the union of the faces of \(\operatorname{conv}(V)\) that are disjoint from \(C\). In the special case \(C=\{r\}\), subsets \(S\) satisfying \(r\in\operatorname{conv}(S)\) are called weakly \(r\)-balanced. If \(r\in\operatorname{relint}\operatorname{conv}(V)\) and the poset of proper weakly \(r\)-balanced subsets is nonempty, then its order complex is homotopy equivalent to the sphere \(S^{m-k-2}\), where \(k=\dim\operatorname{aff}(V)\).