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Finding Geometric Representations of Apex Graphs is NP-Hard

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WALCOM: Algorithms and Computation (WALCOM 2022)

Part of the book series: Lecture Notes in Computer Science ((LNTCS,volume 13174))

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Abstract

Planar graphs can be represented as intersection graphs of different types of geometric objects in the plane, e.g., circles (Koebe, 1936), line segments (Chalopin & Gonçalves, SODA 2009), L-shapes (Gonçalves et al., SODA 2018). For general graphs, however, even deciding whether such representations exist is often \(\mathsf{NP}\)-hard. We consider apex graphs, i.e., graphs that can be made planar by removing one vertex from them. We show, somewhat surprisingly, that deciding whether geometric representations exist for apex graphs is \(\mathsf{NP}\)-hard.

More precisely, we show that for every positive integer g, recognizing every graph class \(\mathscr {G}\) such that \({\textsc {Pure}}\text {-}{\textsc {2}}\text {-}{\textsc {Dir}}\subseteq \mathscr {G}\subseteq {\textsc {1}}\text {-}{\textsc {String}}\) is \(\mathsf{NP}\)-hard, even if the inputs are apex graphs of girth at least g. Here, \({\textsc {Pure}}\text {-}{\textsc {2}}\text {-}{\textsc {Dir}}\) is the class of intersection graphs of axis-parallel line segments (where intersections are allowed only between horizontal and vertical segments), and \({\textsc {1}}\text {-}{\textsc {String}}\) is the class of intersection graphs of simple curves (where two curves cross at most once) in the plane. This partially answers an open question raised by Kratochvíl & Pergel (COCOON, 2007).

Most known \(\textsf {NP}\)-hardness reductions for these problems are from variants of 3-SAT. We reduce from the Planar Hamiltonian Path Completion problem, which uses the more intuitive notion of planarity. As a result, our proof is much simpler and encapsulates several classes of geometric graphs.

D. Chakraborty—This work was done when the author was a postdoctoral fellow at the Indian Institute of Science, Bangalore.

K. Gajjar—This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 682203-ERC-[Inf-Speed-Tradeoff].

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Notes

  1. 1.

    For a set of geometric objects \(\mathscr {C}\), its intersection graph, \(I(\mathscr {C})\), has \(\mathscr {C}\) as the vertex set and two vertices are adjacent if and only if the corresponding geometric objects intersect.

  2. 2.

    Formally, two closed disks are said to touch each other if they share exactly one point.

  3. 3.

    Formally, a simple curve is a subset of the plane which is homeomorphic to the interval [0, 1].

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Acknowledgements

This work was done when Kshitij Gajjar was a postdoctoral researcher at Technion, Israel. Kshitij Gajjar’s work is partially supported by NUS ODPRT Grant, WBS No. R-252-000-A94-133. Both authors thank the organisers of Graphmasters 2020 [18] for providing the virtual environment that initiated this research.

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Correspondence to Dibyayan Chakraborty .

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Chakraborty, D., Gajjar, K. (2022). Finding Geometric Representations of Apex Graphs is NP-Hard. In: Mutzel, P., Rahman, M.S., Slamin (eds) WALCOM: Algorithms and Computation. WALCOM 2022. Lecture Notes in Computer Science(), vol 13174. Springer, Cham. https://doi.org/10.1007/978-3-030-96731-4_14

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