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Outer-string graphs, i.e., graphs that can be represented as intersection of curves in 2D, all of which end in the outer-face, have recently received much interest, especially since it was shown that the independent set problem can be solved efficiently in such graphs. However, the run-time for the independent set problem depends on N, the number of segments in an outer-string representation, rather than the number n of vertices of the graph. In this paper, we argue that for some outer-string graphs, N must be exponential in n. We also study some special string graphs, viz. monotone string graphs, and argue that for them N can be assumed to be polynomial in n. Finally we give an algorithm for independent set in so-called strip-grounded monotone outer-string graphs that is polynomial in n.
@InProceedings{biedl_et_al:LIPIcs.SWAT.2018.10,
author = {Biedl, Therese and Biniaz, Ahmad and Derka, Martin},
title = {{On the Size of Outer-String Representations}},
booktitle = {16th Scandinavian Symposium and Workshops on Algorithm Theory (SWAT 2018)},
pages = {10:1--10:14},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-068-2},
ISSN = {1868-8969},
year = {2018},
volume = {101},
editor = {Eppstein, David},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SWAT.2018.10},
URN = {urn:nbn:de:0030-drops-88360},
doi = {10.4230/LIPIcs.SWAT.2018.10},
annote = {Keywords: string graph, outer-string graph, size of representation, independent set}
}