Mean Field Games of Controls: on the convergence of Nash equilibria

In this paper, we investigate a class of mean field games where the mean field interactions are achieved through the joint (conditional) distribution of the controlled state and the control process. The strategies are of $openloop$ type, and the volatility coefficient $\sigma$ can be controlled. Using (controlled) Fokker-Planck equations, we introduce a notion of measure-valued solution of mean-field games of controls, and through convergence results, prove a relation between these solutions on the one hand, and the ${ϵ}_N$–Nash equilibria on the other hand. It is shown that ${ϵ}_N$–Nash equilibria in the $N$–player games have limits as $N$ tends to infinity, and each limit is a measure-valued solution of the mean-field games of controls. Conversely, any measure-valued solution can be obtained as the limit of a sequence of ${ϵ}_N$–Nash equilibria in the $N$–player games. In other words, the measure-valued solutions are the accumulating points of ${ϵ}_N$–Nash equilibria. Similarly, by considering an $ϵ$–strong solution of mean field games of controls which is the classical strong solution where the optimality is obtained by admitting a small error $ϵ,$ we prove that the measure-valued solutions are the accumulating points of this type of solutions when $ϵ$ goes to zero. Finally, existence of measure-valued solution of mean-field games of controls are proved in the case without common noise.