Existence of Equilibria in Cournotian Games with Utility Functions that are Discontinuous at the Origin

We consider the Nash equilibrium existence problem for Cournotian games and we provide two results for it. The first is compatible with utility functions that are discontinuous at the origin, but requires the nonemptiness of best-replies at the origin (and in some sense is known); the second requires the discontinuity of utility functions at the origin and the emptiness of best-replies at the origin (and to the best of our knowledge is not known). Both results are proved by reducing the Nash equilibrium existence problem to a simple fixpoint existence problem for a real function defined on a real interval. The results are then applied to Cournot oligopolies and winner-take-all contest games; various examples illustrate the novelty of the results obtained.