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Concept
Minimax
Summary
Minmax (sometimes Minimax, MM or saddle point) is a decision rule used in artificial intelligence, decision theory, game theory, statistics, and philosophy for minimizing the possible loss for a worst case (maximum loss) scenario. When dealing with gains, it is referred to as "maximin" – to maximize the minimum gain. Originally formulated for several-player zero-sum game theory, covering both the cases where players take alternate moves and those where they make simultaneous moves, it has also been extended to more complex games and to general decision-making in the presence of uncertainty.
The maximin value is the highest value that the player can be sure to get without knowing the actions of the other players; equivalently, it is the lowest value the other players can force the player to receive when they know the player's action. Its formal definition is:
Where:
i is the index of the player of interest.
denotes all other players except player i.
is the action taken by player i.
denotes the actions taken by all other players.
is the value function of player i.
Calculating the maximin value of a player is done in a worst-case approach: for each possible action of the player, we check all possible actions of the other players and determine the worst possible combination of actions – the one that gives player i the smallest value. Then, we determine which action player i can take in order to make sure that this smallest value is the highest possible.
For example, consider the following game for two players, where the first player ("row player") may choose any of three moves, labelled T, M, or B, and the second player ("column" player) may choose either of two moves, L or R. The result of the combination of both moves is expressed in a payoff table:
(where the first number in each of the cell is the pay-out of the row player and the second number is the pay-out of the column player).
For the sake of example, we consider only pure strategies.
Official source
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EPFL2011