,
Romeo Rizzi
,
Florian Sikora
,
Stéphane Vialette
Creative Commons Attribution 4.0 International license
Multiple interval graphs are a well-known generalization of interval graphs introduced in the 1970s to deal with situations arising naturally in scheduling and allocation. A d-interval is the union of d intervals on the real line, and a graph is a d-interval graph if it is the intersection graph of d-intervals. In particular, it is a unit d-interval graph if it admits a d-interval representation where every interval has unit length. Whereas it has been known for a long time that recognizing 2-interval graphs and other related classes such as 2-track interval graphs is NP-complete, the complexity of recognizing unit 2-interval graphs remains open. Here, we settle this question by proving that the recognition of unit 2-interval graphs is also NP-complete. Our proof technique uses a completely different approach from the other hardness results of recognizing related classes. Furthermore, we extend the result for unit d-interval graphs for any d ⩾ 2, which does not follow directly in graph recognition problems -as an example, it took almost 20 years to close the gap between d = 2 and d > 2 for the recognition of d-track interval graphs. Our result has several implications, including that recognizing (x, …, x) d-interval graphs and depth r unit 2-interval graphs is NP-complete for every x ⩾ 11 and every r ⩾ 4.
@InProceedings{ardevolmartinez_et_al:LIPIcs.ISAAC.2023.8,
author = {Ard\'{e}vol Mart{\'\i}nez, Virginia and Rizzi, Romeo and Sikora, Florian and Vialette, St\'{e}phane},
title = {{Recognizing Unit Multiple Intervals Is Hard}},
booktitle = {34th International Symposium on Algorithms and Computation (ISAAC 2023)},
pages = {8:1--8:18},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-289-1},
ISSN = {1868-8969},
year = {2023},
volume = {283},
editor = {Iwata, Satoru and Kakimura, Naonori},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2023.8},
URN = {urn:nbn:de:0030-drops-193102},
doi = {10.4230/LIPIcs.ISAAC.2023.8},
annote = {Keywords: Interval graphs, unit multiple interval graphs, recognition, NP-hardness}
}