Is a zero-sum game always a two player game? Can a zero-sum game involve more than 2 players?
It does not seem to make sense to me, given that most books define a zero sum game as a game whereby the payoff matrices satisfy $$A + B = 0$$
Or $B = -A$.
Now, it is possible...to extend this to,
$$A + B + C= 0$$
But this definition is not intuitive, because it could be that $B = -A/2, C = -A/2$, but $B$ and $C$ players are not necessarily against each other.
There could be some other non-standard definition of a zero-sum game perhaps I am missing.
Can someone clarify this for me?
 A: A zero-sum game need not be a two-player game.
Consider, for example, a game involving three players $P, Q, R$.
A coin is tossed. $P$ predicts the outcome. If $P$'s prediction comes true, $Q$ and $R$ give him a dollar each, else he pays a dollar to each one of them.
A: The definition of zero-sum applies immediately to any number of players, as others have noted.
However, the nice properties of two-player zero-sum games, like the minmax theorem, unique value, interchangeability of equilibrium strategies, do not hold for 3 or more player zero-sum games unless extra conditions are imposed.
In particular note that one can take any two-player non-zero-sum (aka general-sum) game and add a third dummy player, whose actions have no effect on the original two players, and whose payoffs are set to make the game zero-sum. Now any equilibrium of this three-player game immediately gives an equilibrium of the original two-player game, so three-player zero-sum games are as hard to solve and at least as general as two-player general-sum games.
A class of many-player zero-sum games in which a number, but not all, of the nice properties persist is the case of zero-sum polymatrix games -- see:
Yang Cai, Ozan Candogan, Constantinos Daskalakis, Christos H. Papadimitriou:
Zero-Sum Polymatrix Games: A Generalization of Minmax. Math. Oper. Res. 41(2): 648-655 (2016).
A: A zero-sum game only requires the sum of the pay-offs to be zero. The number of players is not an issue. In fact, a game in which every time a player loses (or gains) something, the same is won (or lost) by another, is a zero-sum game.
