Graph continuous function

Consider a game with ${\displaystyle N}$ agents with agent ${\displaystyle i}$ having strategy ${\displaystyle A_{i}\subseteq \mathbb {R} }$; write ${\displaystyle \mathbf {a} }$ for an N-tuple of actions (i.e. ${\displaystyle \mathbf {a} \in \prod _{j=1}^{N}A_{j}}$) and ${\displaystyle \mathbf {a} _{-i}=(a_{1},a_{2},\ldots ,a_{i-1},a_{i+1},\ldots ,a_{N})}$ as the vector of all agents' actions apart from agent ${\displaystyle i}$.

Let ${\displaystyle U_{i}:A_{i}\longrightarrow \mathbb {R} }$ be the payoff function for agent ${\displaystyle i}$.

A game is defined as ${\displaystyle [(A_{i},U_{i});i=1,\ldots ,N]}$.

Function ${\displaystyle U_{i}:A\longrightarrow \mathbb {R} }$ is graph continuous if for all ${\displaystyle \mathbf {a} \in A}$ there exists a function ${\displaystyle F_{i}:A_{-i}\longrightarrow A_{i}}$ such that ${\displaystyle U_{i}(F_{i}(\mathbf {a} _{-i}),\mathbf {a} _{-i})}$ is continuous at ${\displaystyle \mathbf {a} _{-i}}$.

Dasgupta and Maskin named this property "graph continuity" because, if one plots a graph of a player's payoff as a function of his own strategy (keeping the other players' strategies fixed), then a graph-continuous payoff function will result in this graph changing continuously as one varies the strategies of the other players.

The property is interesting in view of the following theorem.

If, for ${\displaystyle 1\leq i\leq N}$, ${\displaystyle A_{i}\subseteq \mathbb {R} ^{m}}$ is non-empty, convex, and compact; and if ${\displaystyle U_{i}:A\longrightarrow \mathbb {R} }$ is quasi-concave in ${\displaystyle a_{i}}$, upper semi-continuous in ${\displaystyle \mathbf {a} }$, and graph continuous, then the game ${\displaystyle [(A_{i},U_{i});i=1,\ldots ,N]}$ possesses a pure strategy Nash equilibrium.

• Partha Dasgupta and Eric Maskin 1986. "The existence of equilibrium in discontinuous economic games, I: theory". The Review of Economic Studies, 53(1):1–26