Abstract 3-Rigidity and Bivariate $C_2^1$-Splines II: Combinatorial Characterization

TL;DR

This paper provides a combinatorial characterization of independence in a specific 3-rigidity matroid, advancing understanding of rigidity in 3D frameworks and confirming related conjectures.

Contribution

It offers the first combinatorial characterization of independence in the generic $C_2^1$-cofactor matroid, a key step in understanding 3D rigidity.

Findings

Verified conjectures of Dress, Lovász, and Yemini for this matroid.

Established the matroid as the unique maximal 3-rigidity matroid.

Provided tools for analyzing rigidity in 3-dimensional frameworks.

Abstract

We showed in the first paper of this series that the generic $C_2^1$-cofactor matroid is the unique maximal abstract $3$-rigidity matroid. In this paper we obtain a combinatorial characterization of independence in this matroid. This solves the cofactor counterpart of the combinatorial characterization problem for the rigidity of generic 3-dimensional bar-joint frameworks. We use our characterization to verify that the counterparts of conjectures of Dress (on the rank function) and Lov\'{a}sz and Yemini (which suggested a sufficient connectivity condition for rigidity) hold for this matroid.