Rigidity condition for gluing two bar-joint rigid graphs embedded in $\mathbb{R}^d$

Abstract

How does one determine if a collection of bars joined by freely rotating hinges cannot be deformed without changing the length of any of the bars? In other words, how does one determine if a bar-joint graph is rigid? This question has been definitively answered using combinatorial rigidity theory in two dimensions via the Geiringer-Laman Theorem. However, it has not yet been answered using combinatorial rigidity theory in higher dimensions, given known counterexamples to the trivial dimensional extension of the Geiringer-Laman Theorem. To work towards a combinatorial approach in dimensions beyond two, we present a theorem for gluing two rigid bar-joint graphs together that remain rigid. When there are no overlapping vertices between the two graphs, the theorem reduces to Tay's theorem used to identify rigidity in body-bar graphs. When there are overlapping vertices, we rely on the notion of pinned rigid graphs to identify and constrain rigid motions. This theorem provides a basis for an algorithm for recursively constructing rigid clusters that can be readily adapted for computational purposes. By leveraging Henneberg-type operations to grow a rigid (or minimally rigid) graph and treating simplices-where every vertex connects to every other vertex-as fundamental units, our approach offers a scalable solution with computational complexity comparable to traditional methods. Thus, we provide a combinatorial blueprint for algorithms in multi-dimensional rigidity theory as applied to bar-joint graphs.