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# Dense Graphs Have Rigid Parts

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LIPIcs.SoCG.2020.65.pdf
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## Acknowledgements

The authors also thank Omer Angel and Ching Wong for several useful comments regarding the paper.

## Cite As

Orit E. Raz and József Solymosi. Dense Graphs Have Rigid Parts. In 36th International Symposium on Computational Geometry (SoCG 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 164, pp. 65:1-65:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)
https://doi.org/10.4230/LIPIcs.SoCG.2020.65

## Abstract

While the problem of determining whether an embedding of a graph G in ℝ² is infinitesimally rigid is well understood, specifying whether a given embedding of G is rigid or not is still a hard task that usually requires ad hoc arguments. In this paper, we show that every embedding (not necessarily generic) of a dense enough graph (concretely, a graph with at least C₀n^{3/2}(log n)^β edges, for some absolute constants C₀>0 and β), which satisfies some very mild general position requirements (no three vertices of G are embedded to a common line), must have a subframework of size at least three which is rigid. For the proof we use a connection, established in Raz [Discrete Comput. Geom., 2017], between the notion of graph rigidity and configurations of lines in ℝ³. This connection allows us to use properties of line configurations established in Guth and Katz [Annals Math., 2015]. In fact, our proof requires an extended version of Guth and Katz result; the extension we need is proved by János Kollár in an Appendix to our paper. We do not know whether our assumption on the number of edges being Ω(n^{3/2}log n) is tight, and we provide a construction that shows that requiring Ω(n log n) edges is necessary.

## Subject Classification

##### ACM Subject Classification
• Mathematics of computing → Combinatoric problems
• Mathematics of computing → Graph theory
##### Keywords
• Graph rigidity
• line configurations in 3D

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## References

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9. O. E. Raz. Distinct distances for points lying on curves in ℝ^d - the bipartite case. manuscript, 2020.
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