  In a strategic form game a strategy profile is an equilibrium if no viable
coalition of agents (or players) benefits (in the Pareto sense) from jointly
changing their strategies. Weaker or stronger equilibrium notions can be
defined by considering various restrictions on coalition formation. In a Nash
equilibrium, for instance, the assumption is that viable coalitions are
singletons, and in a super strong equilibrium, every coalition is viable.
Restrictions on coalition formation can be justified by communication
limitations, coordination problems or institutional constraints. In this paper,
inspired by social structures in various real-life scenarios, we introduce
certain restrictions on coalition formation, and on their basis we introduce a
number of equilibrium notions. As an application we study our equilibrium
notions in resource selection games (RSGs), and we present a complete set of
existence and non-existence results for general RSGs and their important
special cases.
