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(The prisoner's dilemma is a special game between two arrested prisoners, explaining why it is difficult to maintain cooperation even when cooperation is beneficial to both parties. The prisoner's dilemma is a representative example of the non-zero-sum game of game theory, reflecting that the best individual choice is not the best choice for the group. Although the dilemma itself is only a model nature, similar situations occur frequently in terms of price competition, environmental protection, and interpersonal relationships.)