An oligopoly is a market dominated by a small number of vendors or service providers (oligopolists).
Because there are few participants in this type of market, each oligopolist is aware of the actions of its competitors.
Since the decisions of one company affect or cause influence in the decisions of the other, a situation of equilibrium is established in the group of bidders, with which there is no market competition.
Duopoly - two sellers
Tetrapoly - four sellers (in this example also possibility of chat to facilitate collusion)