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In the case of price elasticity of demand, it also called lerner index. The formula can be expressed: $$L = \frac{P - MC}{P}$$ $$P$$ means monopoly price set by firms $$MC$$ means the marginal cost of production

The lerner index measures the level of market power and monopoly power that a firm owned.The higher lerner index indicated the more monopoly power allows a company have chance to establish prices that are higher than their marginal costs and then lead a higher monopoly price. In conclusion, a monopoly price is established by a monopolistic firm while they have no rivals in the market and fesible to raise price further above their marginal cost. In order to ensure a maximum economic return, the monopoly price is established at the point where marginal revenue equals marginal cost based on the firm's evaluation of the demand for its product. The lerner index can be used to measured the degree of monopoly power and monopoly price.