Locally Finite Knowledge Structures

Abstract: In a game of incomplete information, an infinite state space can create problems. When the space is uncountably large, the strategy spaces of the players may be unwieldly, resulting in a lack of measurable equilibria. When the knowledge of a player allows for an infinite number of possibilities, without conditions on the behavior of the other players, that player may be unable to evaluate and compare the payoff consequences of her actions. We argue that local finiteness is an important and desirable property, namely that at every point in the state space every player knows that only a finite number of points are possible. Local finiteness implies a kind of common knowledge of a countable number of points. Unfortunately its relationship to other forms of common knowledge is complex. In the context of the multi-agent propositional calculus, if the set of formulas held in common knowledge is generated by a finite set of formulas but a finite structure is not determined then there are uncountably many locally finite structures sharing this same set of formulas in common knowledge and likewise uncountably many with uncountable size. This differs radically from the infinite generation of formulas in common knowledge, and we show some examples of this. One corollary is that if there are infinitely many distinct points but a uniform bound on the number of points any player knows is possible then the set of formulas in common knowledge cannot be finitely generated.