A Note on Endogenous Market Structure under Heterogeneous Firms*
放松了Sutton[1998]中对称研发成本的假设，推导出当进入与产品质量研发投资共同决定时市场集中度的下界，发现该下界取决于垂直差异化程度，这对实证研究有重要启示。
Industries, such as semiconductors and biotechnology, are characterized by heterogeneous research productivity and vertically differentiated products. In this note, I relax the assumption of symmetric R&D costs in Sutton [1998] and derive the lower bounds to market concentration when entry and R&D investments in product quality are jointly determined. I show that the lower bound to concentration depends on the degree of vertical differentiation, which is a function of market size and quality choices. My findings imply that empirical applications that fail to account for differences in quality will be biased towards the null hypothesis of exogenous entry costs otherwise.