There is also a possibility of a natural monopoly in contestable market. A model of oligopoly in which firms take their rivals' output as given. A model in which firms take their rival's prices as given. The production of a product that is unique but has many close substitutes. The theory of markets in which there are many similar but differentiated products.
Monopolistic competition contains some characteristic of monopoly and some of perfect competition.
The demand curve facing a monopolistically competitive firm is neither as steep as in monopoly nor as flat as in perfect competition.