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We investigate the Euclidean 𝖽-Dimensional Stable Roommates problem, which asks whether a given set V of 𝖽⋅ n points from the 2-dimensional Euclidean space can be partitioned into n disjoint (unordered) subsets Π = {V₁,…,V_{n}} with |V_i| = 𝖽 for each V_i ∈ Π such that Π is {stable}. Here, {stability} means that no point subset W ⊆ V is blocking Π, and W is said to be {blocking} Π if |W| = 𝖽 such that ∑_{w' ∈ W}δ(w,w') < ∑_{v ∈ Π(w)}δ(w,v) holds for each point w ∈ W, where Π(w) denotes the subset V_i ∈ Π which contains w and δ(a,b) denotes the Euclidean distance between points a and b. Complementing the existing known polynomial-time result for 𝖽 = 2, we show that such polynomial-time algorithms cannot exist for any fixed number 𝖽 ≥ 3 unless P=NP. Our result for 𝖽 = 3 answers a decade-long open question in the theory of Stable Matching and Hedonic Games [Iwama et al., 2007; Arkin et al., 2009; Vladimir G. Deineko and Gerhard J. Woeginger, 2013; Vladimir G. Deineko and Gerhard J. Woeginger, 2013; David F. Manlove, 2013].
@InProceedings{chen_et_al:LIPIcs.ESA.2022.36,
author = {Chen, Jiehua and Roy, Sanjukta},
title = {{Multi-Dimensional Stable Roommates in 2-Dimensional Euclidean Space}},
booktitle = {30th Annual European Symposium on Algorithms (ESA 2022)},
pages = {36:1--36:16},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-247-1},
ISSN = {1868-8969},
year = {2022},
volume = {244},
editor = {Chechik, Shiri and Navarro, Gonzalo and Rotenberg, Eva and Herman, Grzegorz},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2022.36},
URN = {urn:nbn:de:0030-drops-169741},
doi = {10.4230/LIPIcs.ESA.2022.36},
annote = {Keywords: stable matchings, multidimensional stable roommates, Euclidean preferences, coalition formation games, stable cores, NP-hardness}
}