There are many situations in which individuals will choose to act as a group, or coalition. Examples include social clubs, political parties, partnership formation, and legislative voting. Coalition formation games are a class of cooperative games where the aim is to partition a set of agents into coalitions, according to some criteria, such as coalitional stability or maximization of social welfare. We are particularly interested in a subclass of coalition formation games, hedonic games, which were first proposed by Drèze and Greenberg (1980) and later formalized by Banerjee et al. (2001) and Bogomolnaia and Jackson (2002). Hedonic sames are distinguished from general coalition formation games by the requirement that each agent's utility is wholly derived from the members of their own coalition.

This Dagstuhl Seminar brings multiple approaches and viewpoints to the study of coalition formation games, and in particular hedonic games, mainly from the perspective of computer science and economics. Particular topics to be discussed in talks and working groups include (but are not limited to):

• succinctly representable preferences over coalitions;
• the existence and verification of stable coalition structures (for various stability concepts);
• the computational complexity of finding or verifying stable or optimal partitions, or even determining whether such partitions exist;
• designing (if possible, efficient) algorithms for finding stable or optimal (or nearly so) coalition structures, or for verifying that a coalition structure is (nearly) stable or optimal;
• stability notions restricted to social networks or other networks;
• matching markets and matching under preferences, and their relation to hedonic games;
• dynamics of coalition formation; and
• group activity selection.

The overarching theme of this Dagstuhl Seminar is to bring together different communities working in coalition formation and hedonic games from various perspectives in computer science and economics and to bridge and bundle their research activities. Participants can form working groups that each will focus on exploring some of the challenging open questions of the field – this is coalition formation in action. Much of the great atmosphere of the seminars at Schloss Dagstuhl comes from informal meetings besides the official schedule, with participants doing leisure activities together and enjoying other joint undertakings – this is group activity selection in practice.

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Edith Elkind, Judy Goldsmith, Anja Rey, and Jörg Rothe