Engineering Economics
Oligopoly exists when there is/are:

Few sellers and many buyers
Many sellers and few buyers
Few sellers and few buyers
One seller and few buyers

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Engineering Economics
Perfect monopoly exists only if:

the single vendor can prevent the entry of all other vendors in the market
the single vendor gets the absolute franchise of the product
the single vendor is the only one who has the knowledge of the product
the single vendor is the only one who has the permit to sell

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