  In many economic, social and political situations individuals carry out
activities in groups (coalitions) rather than alone and on their own. Examples
range from households and sport clubs to research networks, political parties
and trade unions. The underlying game theoretic framework is known as coalition
formation.
  This survey discusses the notion of core stability in hedonic coalition
formation (where each player's happiness only depends on the other members of
his coalition but not on how the remaining players outside his coalition are
grouped). We present the central concepts and algorithmic approaches in the
area, provide many examples, and pose a number of open problems.
