  The model of congestion games is widely used to analyze games related to
traffic and communication. A central property of these games is that they are
potential games and hence posses a pure Nash equilibrium. In reality it is
often the case that some players cooperatively decide on their joint action in
order to maximize the coalition's total utility. This is by modeled by
Coalitional Congestion Games. Typical settings include truck drivers who work
for the same shipping company, or routers that belong to the same ISP. The
formation of coalitions will typically imply that the resulting coalitional
congestion game will no longer posses a pure Nash equilibrium. In this paper we
provide conditions under which such games are potential games and posses a pure
Nash equilibrium.
