Zero-determinant (ZD) strategies are a novel class of strategies in the Repeated Prisoner's Dilemma (RPD) game discovered by Press and Dyson. This strategy set enforces a linear payoff relationship between a focal player and the opponent regardless of the opponent's strategy. In the RPD game, a discount factor and observation errors are both important because they often happen in society. However, they were not considered in the original discovery of ZD strategies. In some preceding studies, each of them were considered independently. Here, we analytically study the strategies that enforce linear payoff relationships in the RPD game considering both a discount factor and observation errors. As a result, we first revealed that the payoffs of two players can be represented by the form of determinants as shown by Press and Dyson even with the two factors. Then, we searched for all possible strategies that enforce linear payoff relationships and found that both ZD strategies and unconditional strategies are the only strategy sets to satisfy the condition. Moreover, we numerically derived minimum discount rates for the one subset of the ZD strategies in which the extortion factor approaches to infinity. For the ZD strategies whose extortion factor is finite, we numerically derived the minimum extortion factors above which such strategies exist. These results contribute to a deep understanding of ZD strategies in society.
Author summaryRepeated games where two players independently select cooperative or non-cooperative behavior have been used to model interactions of biological organisms. In a real situation, people sometimes cannot observe the direct behaviors that other people select. Instead, they receive signals that reflect other people's behaviors. Those signals are influenced by the environment. Therefore, people sometimes receive the wrong signals. As a result, people mistake other people's behaviors. Hence, in repeated games, assuming such observation errors is important to model biological phenomena close to reality. We mathematically derived that, in the repeated games with observation errors, there are only two types of strategies which enforce a linear payoff relationship to the opponent irrespective of the opponent's strategy. The subsets of the strategies can manipulate the opponent's payoff or enforce an unequal payoff relationship to the opponent. We further numerically revealed the conditions of error rates and a discount factor above which this strategy can exist. January 13, 2020 1/18 49 In this study, we search for ZD strategies under the situations that observation 50 errors and a discount factor are both incorporated. We search for the other possible 51 strategies, not just ZD strategies, that enforce a linear payoff relationship between the 52 January 13, 2020 2/18 two players. By formalizing the determinants for the expected payoffs in the RPD 53 game, we mathematically found that only ZD strategies [9] and unconditional 54 strategies [14,55] are the two types which enforce a linear payoff rela...