We extend the models in ("Competition in two-sided markets" of Armstrong (2006, Rand Journal of Economics) by adding within-group externalities. In the monopoly and duopoly cases, positive within-group externalities reduce the price of the own group. Negative externalities have an opposite price...

Even under antitrust enforcement, firms may still form a cartel in an infinitely-repeated oligopoly model when the discount factor is sufficiently close to one. We present a linear oligopoly model where the profit-maximizing cartel price converges to the competitive equilibrium price as the...