A Proof of the Molecular Conjecture

Abstract

A $d$-dimensional body-and-hinge framework is a structure consisting of rigid bodies connected by hinges in $d$-dimensional space. The generic infinitesimal rigidity of a body-and-hinge framework has been characterized in terms of the underlying multigraph independently by Tay and Whiteley as follows: A multigraph $G$ can be realized as an infinitesimally rigid body-and-hinge framework by mapping each vertex to a body and each edge to a hinge if and only if ({d+1 \choose 2}-1)G$ contains ${d+1\choose 2}$ edge-disjoint spanning trees, where $({d+1 \choose 2}-1)G$ is the graph obtained from $G$ by replacing each edge by $({d+1\choose 2}-1)$ parallel edges. In 1984 they jointly posed a question about whether their combinatorial characterization can be further applied to a nongeneric case. Specifically, they conjectured that $G$ can be realized as an infinitesimally rigid body-and-hinge framework if and only if $G$ can be realized as that with the additional ``hinge-coplanar'' property, i.e., all the hinges incident to each body are contained in a common hyperplane. This conjecture is called the Molecular Conjecture due to the equivalence between the infinitesimal rigidity of ``hinge-coplanar'' body-and-hinge frameworks and that of bar-and-joint frameworks derived from molecules in 3-dimension. In 2-dimensional case this conjecture has been proved by Jackson and Jord{\'a}n in 2006. In this paper we prove this long standing conjecture affirmatively for general dimension.