In: Economics
Notice that d P
d qi = d P
d Q < 0. Consider the case of oligopoly and suppose that price is equal
to monopoly price. Monopoly price is such that the (4) holds exactly. The only di®erence
between (3) and (4)is that the latter has Q instead of qi. Since Q>qi, it follows that, for p
equal to monopoly price, the left-hand side of (3) is positive. Finally, if it is positive, each
¯rm has an incentive to increas output, which results in a lower price.
By a similar argument we can also show that price under Cournot competition is greater
than marginal cost.
7.7¤¤ Consider a duopoly for a homogenous product with demand Q = 10¡P=2.
Each ¯rm's cost function is given by C = 10 + q(q + 1). Determine the values of the
Cournot equilibrium.
Solution: Duopolist i's pro¯t is given by ¼i = qip(Q) ¡ C(qi) = qi[20 ¡ 2(qi + qj )] ¡ 10 ¡
qi(qi + 1). The ¯rst order condition for pro¯t maximization is given by:
20 ¡ 2(qi + qj ) ¡ 2qi ¡ 2qi ¡ 1=0: (5)
The problem of duopolist j is symmetric, therefore we have qi = qj = 2:375 and p = 10:5.
8.1 Explain why collusive pricing is di±cult in one-period competition and easier
when ¯rms interact over a number of periods.
Solution: In one-period competition each ¯rm has a strong incentive to deviate from
the pre-agreed collusive price, since the gains from deviating are higher than the losses. In
terms of the example in Section 8.1, had the duopolists interacted in only one period, the
gain would be given by one half of monopoly pro¯ts, while the loss from deviating would be
0. We would then be led to the usual Nash-Bertarand equilibrium when both ¯rms price at
marginal cost.
If, however, ¯rms interact over a number of periods, history, in the form of past pricing
behavior, becomes important. Deviation from the collusive price in one period can be met
by punishment (deviation) in future periods. Hence, the original defector must weigh shortterm gains against long-term losses, made possible exactly by multi-period interaction.
8.2 After several years of severe price competition that damaged Boeing's and
Airbus' pro¯ts, the two companies have recently pledged that they will not sink into
another price war. However, in June 1999, Boeing made an unusual o®er to sell 100
small aircraft to a leasing corporation at special discount prices. (Although customers