An Extension Theorem for Signotopes

Authors Helena Bergold , Stefan Felsner , Manfred Scheucher

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Author Details

Helena Bergold
  • Department of Computer Science, Freie Universität Berlin, Germany
Stefan Felsner
  • Institut für Mathematik, Technische Universität Berlin, Germany
Manfred Scheucher
  • Institut für Mathematik, Technische Universität Berlin, Germany


We thank the anonymous reviewers for valuable comments.

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Helena Bergold, Stefan Felsner, and Manfred Scheucher. An Extension Theorem for Signotopes. In 39th International Symposium on Computational Geometry (SoCG 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 258, pp. 17:1-17:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


In 1926, Levi showed that, for every pseudoline arrangement 𝒜 and two points in the plane, 𝒜 can be extended by a pseudoline which contains the two prescribed points. Later extendability was studied for arrangements of pseudohyperplanes in higher dimensions. While the extendability of an arrangement of proper hyperplanes in ℝ^d with a hyperplane containing d prescribed points is trivial, Richter-Gebert found an arrangement of pseudoplanes in ℝ³ which cannot be extended with a pseudoplane containing two particular prescribed points. In this article, we investigate the extendability of signotopes, which are a combinatorial structure encoding a rich subclass of pseudohyperplane arrangements. Our main result is that signotopes of odd rank are extendable in the sense that for two prescribed crossing points we can add an element containing them. Moreover, we conjecture that in all even ranks r ≥ 4 there exist signotopes which are not extendable for two prescribed points. Our conjecture is supported by examples in ranks 4, 6, 8, 10, and 12 that were found with a SAT based approach.

Subject Classification

ACM Subject Classification
  • Mathematics of computing → Discrete mathematics
  • Mathematics of computing → Solvers
  • Mathematics of computing → Enumeration
  • Hardware → Theorem proving and SAT solving
  • Theory of computation → Automated reasoning
  • Theory of computation → Computational geometry
  • arrangement of pseudolines
  • extendability
  • Levi’s extension lemma
  • arrangement of pseudohyperplanes
  • signotope
  • oriented matroid
  • partial order
  • Boolean satisfiability (SAT)


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