A tropical approach to rigidity : Counting realisations of frameworks

Clarke, Oliver and Dewar, Sean and Tripp, Daniel Green and Maxwell, James and Nixon, Anthony and Ren, Yue and Smith, Ben (2026) A tropical approach to rigidity : Counting realisations of frameworks. Journal of the London Mathematical Society, 113 (2): e70438. ISSN 0024-6107

Text (jlms.70438.pdf)
jlms.70438.pdf - Published Version
Available under License Creative Commons Attribution.
Download (1MB)

Abstract

A realisation of a graph in the plane as a bar‐joint framework is rigid if there are finitely many other realisations, up to isometries, with the same edge lengths. Each of these finitely many realisations can be seen as a solution to a system of quadratic equations prescribing the distances between pairs of points. For generic realisations, the size of the solution set depends only on the underlying graph so long as we allow for complex solutions. We provide a characterisation of the realisation number — that is the cardinality of this complex solution set — of a minimally rigid graph. Our characterisation uses tropical geometry to express the realisation number as an intersection of Bergman fans of the graphic matroid. As a consequence, we derive a combinatorial upper bound on the realisation number involving the Tutte polynomial. Moreover, we provide computational evidence that our upper bound is usually an improvement on the mixed volume bound.