Non-computable strategies and discounted repeated games

A number of authors have used formal models of computation to capture the idea of "bounded rationality" in repeated games. Most of this literature has used computability by a finite automaton as the standard. A conceptual difficulty with this standard is that the decision problem is not "closed." That is, for every strategy implementable by an automaton, there is some best response implementable by an automaton, but there may not exist any algorithm for finding such a best response that can be implemented by an automaton. However, such algorithms can always be implemented by a Turing machine, the most powerful formal model of computation. In this paper, we investigate whether the decision problem can be closed by adopting Turing machines as the standard of computability. The answer we offer is negative. Indeed, for a large class of discounted repeated games (including the repeated Prisoner's Dilemma) there exist strategies implementable by a Turing machine for which no best response is implementable by a Turing machine.