Intersection Graphs of Rays and Grounded Segments
Vol. 22, no. 2, pp. 273-295, 2018. Regular paper.
Abstract We consider several classes of intersection graphs of line segments in the plane and prove new equality and separation results between those classes. In particular, we show that:
  • intersection graphs of grounded segments and intersection graphs of downward rays form the same graph class,
  • not every intersection graph of rays is an intersection graph of downward rays, and
  • not every outer segment graph is an intersection graph of rays.
The first result answers an open problem posed by Cabello and Jejčič. The third result confirms a conjecture by Cabello. We thereby completely elucidate the remaining open questions on the containment relations between these classes of segment graphs. We further characterize the complexity of the recognition problems for the classes of outer segment, grounded segment, and ray intersection graphs. We prove that these recognition problems are complete for the existential theory of the reals. This holds even if a 1-string realization is given as additional input.
Submitted: October 2017.
Reviewed: February 2018.
Revised: March 2018.
Accepted: May 2018.
Final: June 2018.
Published: August 2018.
Communicated by Csaba D. Tóth
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