Minimally rigid tensegrity frameworks

TL;DR

This paper establishes tight upper bounds on the number of edges in minimally infinitesimally rigid $d$-dimensional tensegrity frameworks, confirming a longstanding conjecture and advancing understanding of their combinatorial properties.

Contribution

It provides the first tight bounds on edges in minimally rigid tensegrity frameworks and confirms a conjecture of Whiteley from 1987.

Findings

Derived tight upper bounds on edges for minimally rigid frameworks.

Confirmed Whiteley's conjecture on three-dimensional rigidity matroids.

Provided stronger bounds for frameworks without bars in generic positions.

Abstract

A $d$-dimensional tensegrity framework $(T,p)$ is an edge-labeled geometric graph in ${\mathbb R}^d$, which consists of a graph $T=(V,B\cup C\cup S)$ and a map $p:V\to {\mathbb R}^d$. The labels determine whether an edge $uv$ of $T$ corresponds to a fixed length bar in $(T,p)$, or a cable which cannot increase in length, or a strut which cannot decrease in length. We consider minimally infinitesimally rigid $d$-dimensional tensegrity frameworks and provide tight upper bounds on the number of its edges, in terms of the number of vertices and the dimension $d$. We obtain stronger upper bounds in the case when there are no bars and the framework is in generic position. The proofs use methods from convex geometry and matroid theory. A special case of our results confirms a conjecture of Whiteley from 1987. We also give an affirmative answer to a conjecture concerning the number of edges of a graph whose three-dimensional rigidity matroid is minimally connected.