Integer realizations of disk and segment graphs

  • Authors:
  • Colin Mcdiarmid;Tobias MüLler

  • Affiliations:
  • University of Oxford, United Kingdom;Utrecht University, Netherlands

  • Venue:
  • Journal of Combinatorial Theory Series B
  • Year:
  • 2013

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Abstract

A disk graph is the intersection graph of disks in the plane, a unit disk graph is the intersection graph of same radius disks in the plane, and a segment graph is an intersection graph of line segments in the plane. Every disk graph can be realized by disks with centers on the integer grid and with integer radii; and similarly every unit disk graph can be realized by disks with centers on the integer grid and equal (integer) radius; and every segment graph can be realized by segments whose endpoints lie on the integer grid. Here we show that there exist disk graphs on n vertices such that in every realization by integer disks at least one coordinate or radius is 2^2^^^@W^^^(^^^n^^^) and on the other hand every disk graph can be realized by disks with integer coordinates and radii that are at most 2^2^^^O^^^(^^^n^^^); and we show the analogous results for unit disk graphs and segment graphs. For (unit) disk graphs this answers a question of Spinrad, and for segment graphs this improves over a previous result by Kratochvil and Matousek.