Acyclicity of Preferences, Nash Equilibria, and Subgame Perfect Equilibria: a Formal and Constructive Equivalence

Sequential game and Nash equilibrium are basic key concepts in game theory. In 1953, Kuhn showed that every sequential game has a Nash equilibrium. The two main steps of the proof are as follows: First, a procedure expecting a sequential game as an input is defined as “backward induction” in game theory. Second, it is proved that the procedure yields a Nash equilibrium. “Backward induction” actually yields Nash equilibria that define a proper subclass of Nash equilibria. In 1965, Selten named this proper subclass subgame perfect equilibria. In game theory, payoffs are rewards usually granted at the end of a game. Although traditional game theory mainly focuses on real-valued payoffs that are implicitly ordered by the usual total order over the reals, there is a demand for results dealing with non totally ordered payoffs. In the mid 1950’s, works of Simon or Blackwell already involved partially ordered payoffs. This paper further explores the matter: it generalises the notion of sequential game by replacing real-valued payoff functions with abstract atomic objects, called outcomes, and by replacing the usual total order over the reals with arbitrary binary relations over outcomes, called preferences. This introduces a general abstract formalism where Nash equilibrium, subgame perfect equilibrium, and “backward induction” can still be defined. Using a lemma on topological sorting, this paper proves that the following three propositions are equivalent: 1) Preferences over the outcomes are acyclic. 2) Every sequential game has a Nash equilibrium. 3) Every sequential game has a subgame perfect equilibrium. The result is fully computer-certified using the (highly reliable) constructive proof assistant called Coq. Beside the additional guarantee of correctness provided by the proof in Coq, the activity of formalisation also helps clearly identify the useful definitions and the main articulations of the proof.