Concurrence de Bertrand

In a Bertrand oligopoly, firms compete by undercutting each other's prices until prices fall to marginal cost. Each firm anticipates its rival's reaction and adjusts prices accordingly. Once prices reach marginal cost, further reductions are impossible without losses, and both firms stabilize at that price level.

Firms earn zero economic profit because price equals marginal cost at equilibrium. When price reaches marginal cost, firms cannot reduce prices further without incurring losses. This outcome occurs because customers prefer lower prices, forcing firms to compete until all profit margins disappear.

Delta and United compete on the New York to Los Angeles route with identical $250 marginal costs. Delta sets $300, United undercuts at $290, Delta responds with $280, and United lowers to $270. This back-and-forth continues until prices reach $250, demonstrating how firms progressively undercut each other toward marginal cost.

Bertrand oligopoly requires that firms have identical products, identical constant marginal costs, and unlimited capacity to satisfy demand at the current price. Consumers must choose based solely on price. The firm with the lowest price captures the entire market until it reaches capacity constraints.

Firms can prevent zero-profit outcomes through product differentiation or building customer loyalty. When products differ or customers show brand preference, firms can maintain price premiums above marginal cost. Under strict Bertrand conditions with homogeneous products, however, firms can only compete on price.

Bertrand equilibrium represents a Nash equilibrium where each firm's price strategy is optimal given its competitor's price. No firm can improve profit by unilaterally changing its price. At this equilibrium, both firms charge marginal cost and earn zero economic profit, making further strategic adjustments unprofitable.

Consumers prefer lower prices and will purchase from the cheapest supplier when products are identical. The firm with the lowest price attracts all customers until it reaches capacity. This customer preference drives continuous price undercutting, as each firm seeks to undercut rivals and capture market share.