| Summary: | In this paper, we propose a perfect competition test which checks whether arbitrarily small coalitions of firms, which behave strategically on costs, are able to manipulate prices in their own benefit. We apply this test to economies with a continuum of differentiated producers. We show that, under thickness conditions on preferences, implying a high degree of substitutability among commodities, there is no monopolistic power neither in economies with constant marginal costs nor in economies with cubic costs. We also give an example of thin markets where monopolistic power prevails.
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