Dynamic game

A variant of a positional game distinguished by the fact that in such a game the players control the "motion of a point" in the state space $ X $. Let $ I = \{ i \} $ be the set of players. To each point $ x \in X $ corresponds a set $ S _ {i} ^ {(x)} $ of elementary strategies of player $ i \in I $ at this point, and hence, also, the set $ S ^ {(x)} = \prod _ {i} S _ {i} ^ {(x)} $ of elementary situations at $ x $. The periodic distribution functions

$$ F ( x _ {k} \mid x _ {1} , s ^ {( x _ {1} ) } \dots x _ {k - 1 } , s ^ {( x _ {k-1} ) } ) ,\ x _ {i} \in X ,\ s ^ {( x _ {i} ) } \in S ^ {( x _ {i} ) } , $$

representing the law of motion of the controlled point, which is known to all players, is defined on $ X $. If $ x _ {k} $ is fixed, the function $ F $ is measurable with respect to all the remaining arguments. A sequence $ P $ of successive states and elementary situations $ x _ {1} , s ^ {( x _ {1} ) } \dots x _ {k} , s ^ {( x _ {k} ) } \dots $ is a play of a general dynamic game. It is inductively defined as follows: Let there be given a segment of the play (an opening) $ x _ {1} , s ^ {( x _ {1} ) } \dots x _ {k-1} $( $ k \geq 2 $), and let each player $ i $ choose his elementary strategy $ s _ {i} ^ {( x _ {k-1} ) } \in S _ {i} ^ {( x _ {k-1} ) } $ so that the elementary situation $ s ^ {( x _ {k-1} ) } $ arises; the game then continues, at random, in accordance with the distribution $ F ( \cdot \mid x _ {1} , s ^ {( x _ {1} ) } \dots x _ {k-1} , s ^ {( x _ {k-1}) } ) $, into the state $ x _ {k} $. In each play $ P $ the pay-off $ h _ {i} ( P) $ of player $ i $ is defined. If the set of all plays is denoted by $ \mathfrak P $, the dynamic game is specified by the system

$$ \Gamma = < I , X , \{ S _ {i} ^ {(x)} \} _ {i \in I , x \in X } , F , \{ h _ {i} ( P) \} _ {i \in I , P \in \mathfrak P } > . $$

In a dynamic game it is usually assumed that, at the successive moments of selection of an elementary strategy, the players know the preceding opening. In such a case a pure strategy $ s _ {i} $ of player $ i $ is a selection of functions $ s _ {i} ^ {( x) } ( x _ {1} , s ^ {( x _ {1} ) } \dots s ^ {( x _ {k-1} ) } , x ) $ which put the opening ending in $ x $ into correspondence with the elementary strategy $ s _ {i} ^ {(x)} \in S _ {i} ^ {(x)} $. Dynamic games in which the preceding opening is only known partly to the players — e.g. games with "information lag" — have also been studied.

For a game to be specified, each situation $ s = \{ s _ {i} \} $ must induce a probability measure $ \mu _ {s} $ on the set of all plays, and the mathematical expectation $ {\mathsf E} h _ {i} ( P) $ with respect to the measure $ \mu _ {s} $ must exist. This mathematical expectation is also the pay-off of player $ i $ in situation $ s $.

In general, the functions $ h _ {i} ( P) $ are arbitrary, but the most frequently studied dynamic games are those with terminal pay-off (the game is terminated as soon as $ x _ {k} $ appears in a terminal set $ X ^ {T} \subset X $, and $ h _ {i} ( P) = h _ {i} ( x _ {k} ) $ where $ x _ {k} $ is the last situation in the game), and those with integral pay-off ( $ h _ {i} ( P) = \sum _ {k= 1 } ^ \infty h _ {i} ( x _ {k} , s ^ {( x _ {k} ) }) $).

Dynamic games are regarded as the game-like variant of a problem of optimal control with discrete time. It is in fact reduced to such a problem if the number of players is one. If, in a dynamic game, $ X \subset \mathbf R ^ {n} $, continuous time is substituted for discrete time and the random factors are eliminated, a differential game is obtained, which may thus be regarded as a variant of a dynamic game (see also Differential games).

Stochastic games, recursive games and survival games are special classes of dynamic games (cf. also Stochastic game; Recursive game; Game of survival).

References

[1]	N.N. Vorob'ev, "The present state of the theory of games" Russian Math. Surveys , 25 : 2 (1970) pp. 77–136 Uspekhi Mat. Nauk , 25 : 2 (1970) pp. 81–140