A proof of the molecular conjecture

• Authors: Naoki Katoh;Shin-ichi Tanigawa
• Venue: Proceedings of the twenty-fifth annual symposium on Computational geometry
• Year: 2009

A 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 graph independently by Tay and Whiteley as follows: A graph 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+1/2}-1)G contains {d+1/2} edge-disjoint spanning trees, where ({d+1/2}-1)G is the graph obtained from $G$ by replacing each edge by (d+1/2-1) 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 Jordán in 2006. In this paper we prove this long standing conjecture affirmatively for general dimension. Also, as a corollary, we obtain a combinatorial characterization of the 3-dimensional bar-and-joint rigidity matroid of the square of a graph.