# $N$-player games and mean-field games with smooth dependence on past absorptions

## TL;DR

This paper advances the theory of mean-field games with absorption by allowing more general state dynamics and costs, including infinite-dimensional dependence and linear growth, and proves existence, uniqueness, and approximate Nash equilibria.

## Contribution

It extends mean-field game models with absorption to more general, possibly infinite-dimensional settings with relaxed boundedness conditions, and establishes fundamental existence and uniqueness results.

## Key findings

- Proved existence of solutions in strict and relaxed feedback forms.
- Established uniqueness under monotonicity conditions.
- Showed approximate Nash equilibria for large N-player games.

## Abstract

Mean-field games with absorption is a class of games, that have been introduced in Campi and Fischer (2018) and that can be viewed as natural limits of symmetric stochastic differential games with a large number of players who, interacting through a mean-field, leave the game as soon as their private states hit some given boundary. In this paper, we push the study of such games further, extending their scope along two main directions. First, we allow the state dynamics and the costs to have a very general, possibly infinite-dimensional, dependence on the (non-normalized) empirical sub-probability measure of the survivors' states. This includes the particularly relevant case where the mean-field interaction among the players is done through the empirical measure of the survivors together with the fraction of absorbed players over time. Second, the boundedness of coefficients and costs has been considerably relaxed including drift and costs with linear growth in the state variables, hence allowing for more realistic dynamics for players' private states. We prove the existence of solutions of the MFG in strict as well as relaxed feedback form, and we establish uniqueness of the MFG solutions under monotonicity conditions of Lasry-Lions type. Finally, we show in a setting with finite-dimensional interaction that such solutions induce approximate Nash equilibria for the $N$-player game with vanishing error as $N\rightarrow \infty$.