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This Demonstration offers a quick way to find the Nash equilibria of the Bertrand oligopoly model of price competition. Focus your attention on the blue piecewise curve, which shows firm 1's profits as a function of (the price this firm charges) and (the price all the other firms are charging). The blue curve has three parts: the first expresses the profits when (it follows the monopolist's profits curve, because when charging the lowest price, firm 1 sells to the entire market); the second expresses the profits when (the blue dot); it follows the green curve, which represents firm 1's profits when sharing the market with all the other firms (since they are charging the same price, they sell of the market demand each); the third expresses the profits when , which are zero since consumers will buy from the firms charging a cheaper price.

You can vary because it is endogenous to the problem. Since all the firms are identical, we just look at the symmetric Nash equilibria of this game (where all the firms charge the same price). They are given by the prices at which the blue dot (corresponding to firm 1's profits when matching the other firms' prices) is not below the blue curve (corresponding to firm 1's profits when charging a price different from the other firms). After getting some acquaintance with this Demonstration, by changing , you should see how the Nash equilibrium concept applies in this oligopolistic market.

When costs are linear, there is only one Nash equilibrium, where each firm charges a price equal to the marginal cost, irrespective of the number of firms. Profits are zero.

When costs are quadratic, there are many Nash equilibria, most of which induce positive profits for the firms. Price competition does not necessarily drive profits down to zero.

The equations for the model are:

demand: ,

total cost for each firm: ,

monopoly profits: ,

matching price profits: ,

where (the costs are linear or quadratic) and indicates the number of firms.

Firm 1's profits =