On the maximal number of real embeddings of spatial minimally rigid graphs

Evangelos Bartzos 1 Ioannis Emiris 1 Jan Legerský 2 Elias Tsigaridas 3
1 AROMATH - AlgebRe, geOmetrie, Modelisation et AlgoriTHmes
CRISAM - Inria Sophia Antipolis - Méditerranée , National and Kapodistrian University of Athens
3 PolSys - Polynomial Systems
Inria de Paris, LIP6 - Laboratoire d'Informatique de Paris 6
Abstract : The number of embeddings of minimally rigid graphs in $\mathbb{R}^D$ is (by definition) finite, modulo rigid transformations, for every generic choice of edge lengths. Even though various approaches have been proposed to compute it, the gap between upper and lower bounds is still enormous. Specific values and its asymptotic behavior are major and fascinating open problems in rigidity theory. Our work considers the maximal number of real embeddings of minimally rigid graphs in $\mathbb{R}^3$. We modify a commonly used parametric semi-algebraic formulation that exploits the Cayley-Menger determinant to minimize the {\em a priori} number of complex embeddings, where the parameters correspond to edge lengths. To copewith the huge dimension of the parameter space and find specializations of the parameters that maximize the number of real embeddings, we introduce a method based on coupler curves that makes the sampling feasible for spatial minimally rigid graphs. Our methodology results in the first full classification of the number of real embeddings of graphs with 7 vertices in $\mathbb{R}^3$, which was the smallest open case. Building on this and certain 8-vertex graphs, we improve the previously known general lower bound on the maximum number of real embeddings in $\mathbb{R}^3$.
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Evangelos Bartzos, Ioannis Emiris, Jan Legerský, Elias Tsigaridas. On the maximal number of real embeddings of spatial minimally rigid graphs. ISSAC '18 International Symposium on Symbolic and Algebraic Computation, ACM, Jul 2018, New York, United States. pp.55-62, ⟨10.1145/3208976.3208994⟩. ⟨hal-01710518v2⟩



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