En quête d'un indice de l'intensité de la concurrence. Réflexions sur l'indétermination de l'équilibre oligopolistique

The intensity of competition is viewed here in the specific form of the competitive toughness exhibited by a firm towards its competitors, as reflected in its market strategies (prices and quantities). The competitive environment, resulting in particular from former choices, concerning entry, location in the characteristics space or technology, is taken as given. A concept of equilibrium in a homogeneous or differentiated oligopoly is used, where simultaneous price and quantity choices must satisfy two constraints : an individual constraint on the market share and a collective constraint on market size. The relative weight of the multiplier which is associated with the former constraint may be taken as an index of the competitive toughness of any active competitor at a given equilibrium. We thus obtain a parametrisation of the equilibria, leading to a unified approach of the static theory of oligopolistic competition. The very number of active competitors within a population of potential producers is a supplementary index of competitive toughness characterizing an equilibrium. Some considerations on the macroeconomic use of such indices of competitive toughness conclude the paper. Classification JEL : D43, E3, L11, L13, L16