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arXiv · 2508.10136

Recognizing Penny and Marble Graphs is Hard for Existential Theory of the Reals

Anna Lubiw·Marcus Schaefer
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Abstract

We show that the recognition problem for penny graphs (contact graphs of unit disks in the plane) is $\exists\mathbb{R}$-complete, that is, computationally as hard as the existential theory of the reals, even if a combinatorial plane embedding of the graph is given. The exact complexity of the penny graph recognition problem has been a long-standing open problem. We lift the penny graph result to three dimensions and show that the recognition problem for marble graphs (contact graphs of unit balls in three dimensions) is $\exists\mathbb{R}$-complete. Finally, we show that rigidity of penny graphs is $\forall\mathbb{R}$-complete and look at grid embeddings of penny graphs that are trees.

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Anna Lubiw, Marcus Schaefer. 2026-09-09. Recognizing Penny and Marble Graphs is Hard for Existential Theory of the Reals. https://arxiv.org/abs/2508.10136

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