Abstract
Two firms offer product series from which multiple complementary pairs are formed. The firms engage in a price- or quantity-choosing game in the market. It is found that the integration of the two firms may not necessarily lower the equilibrium prices because it precludes “indirect competition” in the market. Therefore, the integration, which may appear as a vertical integration, could be an antitrust concern even in the absence of exclusionary purpose.
Funding source: Ministry of Education of China
Award Identifier / Grant number: 16JJD790002
Appendix A. Market Outcomes of the Quantity-Choosing Games
In the case of a quantity-choosing game, the (symmetric and linear) inverse demands for the products can be represented by:
with B > 0, η ≥ 0, κ ≥ 0, and λ ≥ 0, but the sign of μ not definite. The costs of production are still assumed to be zero. The product relationships are shown in Figure 2.

Product relationships (quantity-choosing game).
It can be checked that integration of the upstream firm U and downstream firm D lowers the equilibrium prices (i.e.,
The equilibrium outcome is
If the two firms integrate into a monopolist, the merged firm’s profits are
The equilibrium outcome is
Therefore, the integration lowers the equilibrium prices (i.e.,
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