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The descriptive and empirical literature in Industrial Organization abounds with reference to economies of scale, product differentiation, and absolute advantages in cost or demand enjoyed by some firms over others. In particular, they feature prominently in Bain's list of barriers to entry in. However, textbook models of oligopoly often abstract away from all these features, and assume a homogeneous product with equal constant costs for all firms in the industry. The real-world complexities are confined to informal remarks, suggesting vague modifications of the formal results. This happens even in such outstanding treatments as those of Feller and Scherer.
Avinash Dixit (Mon,) studied this question.