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\subsection{Oligopoly}  An oligopoly market structure is one that is composed of a small number of firms having control of a large proportion of a certain market. An oligopoly is similar to a monopoly, but in the case of an oligopoly, two or more firms own a large market share, as opposed to a monopoly where only one firms owns a large market share. The market share owned by the oligopoly can be expressed as a concentration ratio, as shown below:  CRx = Y. ratio. This is written in the form of $CR_x$ where  CR refers to represents  the concentration ratio, x refers to represents  the number of firms, andYpercent is equal to  the market percentage represents the  share of the market  that the firm owns. oligopoly has control of.  An oligopoly may be defined as the concentration ratio of less than eight firms having a market share of between 25 to 80per. More than 80per would be classified as a monopolistic market structure, and less than 25per would be indicative of the firms not owning a sufficient market share to influence price levels and quantities of demand and supply. \subsubsection{Indicators of an Oligopoly}  There are four dominant indicators to see if an oligopoly exists within a market. The first of these is what is known as concentration ratio. This is written in the form of $CR_x$ CR represents the concentration where firm  interdependence where  and collusion. price rigidity.  In an ideal situation, from the point of view of firms a firm  operating within an oligopoly, they a firm  would know the price at which other operating firms plan to set their products. products at.  This is because firms are interdependent, and set their prices according to prices of the effect the price  of other operating firms. This firms' product will have on the quantity produced and sold of an individual firm. TThe kinked demand curve shown below will indicate as to why this  is because the case. In order for the following explanation to be coherent and concise, definitions  of the kinked following terms will prove beneficial. Price elasticity of  demand curve, can be defined as the level of responsiveness of demand based on a change in price. Marginal revenue can be defined as the additional revenue gained from one additional unit of output. Marginal cost can therefore be defined  as the cost of producing one additional unit of output. Average revenue may be defined as the revenue gained from each unit that is sold. It can be expressed as the formula: $TR/Q$ As  shown below. Due on the kinked demand curve, the demand curve above Price 1 is very elastic. This means that there is a greater than proportional change in quantity demanded for a change in price. Practically, this means that if a firm in an oligopoly raises its price for a good, the quantity consumed will rapidly decrease. This is due  to the substitution effect, effect;  if the price of a good increases, consumers will instead  choose to buy similar goods at a lower price. price from other firms who have kept their prices constant (i.e. at a lower price than the aforementioned firm's new price). Therefore, not only will the average revenue decrease (as quantity demanded has decreased at a higher rate than the price units sold at has increased), but also total revenue, (expressed as $P*Q$) as the quantity demanded has has decreased at a much more rapid rate than price has increased.  Abnormal Profit. Therefore the price elasticity of demand above point PL1 will be very elastic, i.e. the responsiveness of demand to a change in price is very high. Collusion of firms for a stable of price. Barriers to entry. \subsubsection{Types of Inefficiency}