Rank Contributions of Vertices in Rigidity Matroids of Clique Covered Graphs

Abstract

The problems of characterizing the graphs $G$ which are generically rigid in ${\mathbb R}^d$, or more generally, determining the rank function of the $d$-dimensional rigidity matroid ${\cal R}_d(G)$ of an arbitrary graph $G$, have been solved when $d\leq 2$ but are major open problems in discrete geometry when $d\geq 3$. In this paper we shall concentrate on the case when $d=3$. We first revisit a conjecture of Dress from 1987 that the rank of the ${\cal R}_3$-closure of a graph $G$ is determined by its maximal complete subgraphs of size at least five. We show that his conjectured value for the rank of the closure gives an upper bound on the actual value. We also deduce that the truth of this conjecture would imply a good characterization of the rank of ${\cal R}_3(G)$ for all graphs $G$. The rank formula in Dress's conjecture leads us to consider the family of $K_t$-covered graphs, i.e., graphs in which every edge belongs to a complete subgraph $K_t$, for some $t\geq 3$. This family contains several well-studied graph classes such as body-pin graphs, combinatorial zeolites, and molecular graphs. We introduce a new notion of rank contributions of vertices in an arbitrary matroid on the edge set of a graph $G$, and use it to obtain lower bounds on the rank contributions of vertices in ${\cal R}_3(G)$ and ${\cal C}^1_2(G)$ when $G$ is $K_t$-covered. We use these bounds to show that a conjectured min-max formula for the rank of body-pin graphs in ${\cal R}_3$ holds for the $C_2^1$-cofactor matroid (which is conjectured by Whiteley to be equal to ${\cal R}_3$), and to obtain new sufficient connectivity conditions for the (global) rigidity of $K_4$- and $K_5$-covered graphs in ${\mathbb R}^3$.