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The Maximum Clique Problem in a Disk Graph Made Easy

Authors J. Mark Keil , Debajyoti Mondal

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ACM Subject Classification
  • Theory of computation → Computational geometry
  • Theory of computation → Parameterized complexity and exact algorithms
Keywords
  • Geometric Intersection Graphs
  • Disk Graphs
  • Ball Graphs
  • Maximum Clique

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Abstract

A disk graph is an intersection graph of disks in ℝ². Determining the computational complexity of finding a maximum clique in a disk graph is a long-standing open problem. In 1990, Clark, Colbourn, and Johnson gave a polynomial-time algorithm for computing a maximum clique in a unit disk graph. However, finding a maximum clique when disks are of arbitrary size is widely believed to be a challenging open problem. In this paper, we provide a new perspective to examine adjacencies in a disk graph that helps obtain the following results.  
- We design an 𝒪^*(n^{2k})-time algorithm, where 𝒪^* hides a polynomial factor, to find a maximum clique in a n-vertex disk graph with k different sizes of radii. This is polynomial for every fixed k, and thus settles the open question for the case when k = 2.
- Given a set of n unit disks, we show how to compute a maximum clique inside each possible axis-aligned rectangle determined by the disk centers in O(n⁵log n)-time. This is at least a factor of n^{4/3} faster than applying the fastest known algorithm for finding a maximum clique in a unit disk graph for each rectangle independently. 
- We give an 𝒪^*(n^{2rk})-time algorithm to find a maximum clique in a n-vertex ball graph with k different sizes of radii where the ball centers lie on r parallel planes. This is polynomial for every fixed k and r, and thus contrasts the previously known NP-hardness result for finding a maximum clique in an arbitrary ball graph.
- We design an 𝒪^*(n^{2k})-time algorithm to find a maximum clique in the intersection graph of a set S of n L-visible convex polygons, where k is the number of distinct shapes in S. This contrasts the known hardness result on finding a maximum clique in the intersection graph of unit disks and axis-aligned rectangles.

Cite As Get BibTex

J. Mark Keil and Debajyoti Mondal. The Maximum Clique Problem in a Disk Graph Made Easy. In 41st International Symposium on Computational Geometry (SoCG 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 332, pp. 63:1-63:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025) https://doi.org/10.4230/LIPIcs.SoCG.2025.63

Author Details

J. Mark Keil
  • Department of Computer Science, University of Saskatchewan, Saskatoon, Canada
Debajyoti Mondal
  • Department of Computer Science, University of Saskatchewan, Saskatoon, Canada

Funding

The work is supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC).

Acknowledgements

We thank Soichiro Yamazaki for useful feedback.

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