Abstract
A strong clique in a graph is a clique intersecting all inclusion-maximal stable sets. Strong cliques play an important role in the study of perfect graphs. We study strong cliques in the class of diamond-free graphs, from both structural and algorithmic points of view. We show that the following five NP-hard or co-NP-hard problems remain intractable when restricted to the class of diamond-free graphs: Is a given clique strong? Does the graph have a strong clique? Is every vertex contained in a strong clique? Given a partition of the vertex set into cliques, is every clique in the partition strong? Can the vertex set be partitioned into strong cliques?
On the positive side, we show that the following two problems whose computational complexity is open in general can be solved in linear time in the class of diamond-free graphs: Is every maximal clique strong? Is every edge contained in a strong clique? These results are derived from a characterization of diamond-free graphs in which every maximal clique is strong, which also implies an improved Erdős-Hajnal property for such graphs.
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Notes
- 1.
In [25], the authors state that the two problems are NP-hard, but their proof actually shows co-NP-hardness.
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Acknowledgements
The authors are grateful to Ademir Hujdurović for helpful discussions and the anonymous reviewers for constructive remarks. The second named author has been supported by the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 734922, by Mexico’s CONACYT scholarship 254379/438356, by Erasmus+ for practices (SMT) Action KA103 - Project 2017/2019, by MICINN from the Spanish Government under project PGC2018-095471-B-I00, and by AGAUR from the Catalan Government under project 2017SGR1087. The first and third named authors are supported in part by the Slovenian Research Agency (I0-0035, research programs P1-0285 and P1-0404, and research projects J1-9110, N1-0102, N1-0160). Part of this work was done while the third named author was visiting LAMSADE, University Paris-Dauphine; their support and hospitality is gratefully acknowledged.
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Chiarelli, N., Martínez-Barona, B., Milanič, M., Monnot, J., Muršič, P. (2020). Strong Cliques in Diamond-Free Graphs. In: Adler, I., Müller, H. (eds) Graph-Theoretic Concepts in Computer Science. WG 2020. Lecture Notes in Computer Science(), vol 12301. Springer, Cham. https://doi.org/10.1007/978-3-030-60440-0_21
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