# An infinite beautiful mind

## Theorem identifies games with infinite choices having at least one Nash equilibrium

Life’s
a game, or at least treating it like a game mathematically can be a powerful
way to explain the choices people make. John Nash, the mentally troubled
mathematician depicted in the book and movie *A Beautiful Mind*, discovered one of the bedrock theories for
understanding competitive interactions (generically called “games”) in which
the players have a limited set of choices.

Now
mathematicians are expanding Nash’s ideas for cases when the players’ options
are infinite. Under certain conditions even infinite-choice games are
guaranteed to have at least one scenario for which each player gets the best deal possible (given everyone
else’s choices), according to a mathematical proof to be published in the
February 2009 *Nonlinear Analysis*.

Such a scenario — or set of choices for each player — is called a Nash equilibrium and is stable because no player can do any better by changing strategy (unless he or she forms a cartel to collude with other players, which isn’t allowed). Like a rock resting at the bottom of a valley, once the game reaches this stable scenario it will tend to stay that way. In a sense, it’s the fate of the game to end up at a Nash equilibrium, and this predictive power is why Nash’s ideas have become widely used in economics and other social sciences.

Nash proved that there is always at least one such equilibrium for games with a finite number of possible strategic choices. But not all imaginable games are so limited.

“There
are many economically important games in which the sets of pure strategies are
infinite,” comments Andrew McLennan, a mathematician and economist who studies
game theory at the

A
general theory that could always predict whether a game with infinite choices
will have a Nash equilibrium still eludes mathematicians. In the new work,
Jinlu Li, a mathematician at

“This paper is still far away from completely solving this problem,” Li says. The new rule doesn’t fully predict which infinite-choice games will have an equilibrium; some games that aren’t compact also will have one. “Our dream is we want to find the necessary and sufficient condition, a characteristic of the game that will always guarantee that it has an equilibrium.”

The work is also based on a simplified situation: A game with only two players in which, for one player to win, the other must lose. Nash’s theorem for finite-choice games works for much more complex games involving many players and the possibility of mutual benefit. Eventually mathematicians will have to expand Li and his colleagues’ work to include these more complicated games.

“It is part of a large and rapidly growing literature concerned with ... extensions of Nash’s theorem,” McLennan says.

Li, J., et al. 2008. On the existence of Nash equilibriums for infinite matrix games. Nonlinear Analysis: Real World Applications. 10(1):42. DOI: 10.1016/j.nonrwa.2007.08.012