{"type":"ScholarlyArticle","title":"Core Stability in Hedonic Games among Friends and Enemies: Impact of Neutrals","authors":[{"name":"Kazunori Ohta","slug":"kazunori-ohta","given":"Kazunori","family":"Ohta","affiliation":"Kyushu University , Fukuoka 819-0395 , Japan"},{"name":"Nathanaël Barrot","slug":"nathanael-barrot","given":"Nathanaël","family":"Barrot","affiliation":"Kyushu University , Fukuoka 819-0395 , Japan"},{"name":"Anisse Ismaili","slug":"anisse-ismaili","given":"Anisse","family":"Ismaili","affiliation":"Kyushu University , Fukuoka 819-0395 , Japan"},{"name":"Yuko Sakurai","slug":"yuko-sakurai","given":"Yuko","family":"Sakurai","affiliation":"National Institute of Advanced Industrial Science and Technology"},{"name":"Makoto Yokoo","slug":"makoto-yokoo","given":"Makoto","family":"Yokoo","affiliation":"Kyushu University , Fukuoka 819-0395 , Japan"}],"abstract":"We investigate hedonic games under enemies aversion and friends appreciation, where every agent considers other agents as either a friend or an enemy. We extend these simple preferences by allowing each agent to also consider other agents to be neutral. Neutrals have no impact on her preference, as in a graphical hedonic game.Surprisingly, we discover that neutral agents do not simplify matters, but cause complexity. We prove that the core can be empty under enemies aversion and the strict core can be empty under friends appreciation. Furthermore, we show that under both preferences, deciding whether the strict core is non-empty, is NP^NP-complete. This complexity extends to the core under enemies aversion. We also show that under friends appreciation, we can always find a core stable coalition structure in polynomial time.","doi":"10.24963/ijcai.2017/51","url":"https://ijcai.org/proceedings/2017/51","pdf":"https://ijcai.org/proceedings/2017/0051.pdf","markdown":"https://ijcai.org/proceedings/2017/51.md","bibtex":"https://ijcai.org/proceedings/2017/bibtex/51","volume":{"slug":"2017","title":"Proceedings of the Twenty-Sixth International Joint Conference on Artificial Intelligence","short_title":"IJCAI 2017","url":"https://ijcai.org/proceedings/2017/","year":2017,"location":"Melbourne, Australia","dates":"19-25 August 2017","editors":"Carles Sierra","isbn":"978-0-9992411-0-3","publisher":"International Joint Conferences on Artificial Intelligence Organization","doi_prefix":"10.24963/ijcai.2017"},"section":"Agent-based and Multi-agent Systems","track":"Main track","number":51,"pages":{"first":359,"last":365},"keywords":["Agent-based and Multi-agent Systems: Cooperative Games","Agent-based and Multi-agent Systems: Social Choice Theory"],"videos":[],"references":[{"position":1,"raw":"[Aziz and Brandl, 2012] Haris Aziz and Florian Brandl. 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