Succinct game
{{Short description|Game in algorithmic game theory}}
{{see also|Game theory#Alternative game representations}}
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|colspan="5"|Consider a game of three players, I,II and III, facing, respectively, the strategies {T,B}, {L,R}, and {l,r}. Without further constraints, 3*23=24 utility values would be required to describe such a game. |
!align="center"|L, l
!align="center"|L, r !align="center"|R, l !align="center"|R, r |
align="center"|T
|align="center"|{{fontcolor|red|4}}, {{fontcolor|green|6}}, {{fontcolor|blue|2}} |align="center"|{{fontcolor|red|5}}, {{fontcolor|green|5}}, {{fontcolor|blue|5}} |align="center"|{{fontcolor|red|8}}, {{fontcolor|green|1}}, {{fontcolor|blue|7}} |align="center"|{{fontcolor|red|1}}, {{fontcolor|green|4}}, {{fontcolor|blue|9}} |
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align="center"|B
|align="center"|{{fontcolor|red|8}}, {{fontcolor|green|6}}, {{fontcolor|blue|6}} |align="center"|{{fontcolor|red|7}}, {{fontcolor|green|4}}, {{fontcolor|blue|7}} |align="center"|{{fontcolor|red|9}}, {{fontcolor|green|6}}, {{fontcolor|blue|5}} |align="center"|{{fontcolor|red|0}}, {{fontcolor|green|3}}, {{fontcolor|blue|0}} |
colspan="5"|For each strategy profile, the utility of the first player is listed first ({{fontcolor|red|red}}), and is followed by the utilities of the second player ({{fontcolor|green|green}}) and the third player ({{fontcolor|blue|blue}}). |
In algorithmic game theory, a succinct game or a succinctly representable game is a game which may be represented in a size much smaller than its normal form representation. Without placing constraints on player utilities, describing a game of players, each facing strategies, requires listing utility values. Even trivial algorithms are capable of finding a Nash equilibrium in a time polynomial in the length of such a large input. A succinct game is of polynomial type if in a game represented by a string of length n the number of players, as well as the number of strategies of each player, is bounded by a polynomial in n (a formal definition, describing succinct games as a computational problem, is given by Papadimitriou & Roughgarden 2008).
Types of succinct games
=Graphical games=
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|Say that each player's utility depends only on his own action and the action of one other player - for instance, I depends on II, II on III and III on I. Representing such a game would require only three 2x2 utility tables, containing in all only 12 utility values. {|border="1" cellpadding="3" cellspacing="0" style="margin:0.5em; border:1px #aaa solid; border-collapse:collapse; display:inline-table;" class="wikitable" | !align="center"|L !align="center"|R |
align="center"|T
|align="center"|{{fontcolor|red|9}} |align="center"|{{fontcolor|red|8}} |
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align="center"|B
|align="center"|{{fontcolor|red|3}} |align="center"|{{fontcolor|red|4}} |
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| !align="center"|l !align="center"|r |
align="center"|L
|align="center"|{{fontcolor|green|6}} |align="center"|{{fontcolor|green|8}} |
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align="center"|R
|align="center"|{{fontcolor|green|1}} |align="center"|{{fontcolor|green|3}} |
border="1" cellpadding="3" cellspacing="0" style="margin:0.5em; border:1px #aaa solid; border-collapse:collapse; display:inline-table;" class="wikitable"
| !align="center"|T !align="center"|B |
align="center"|l
|align="center"|{{fontcolor|blue|4}} |align="center"|{{fontcolor|blue|4}} |
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align="center"|r
|align="center"|{{fontcolor|blue|5}} |align="center"|{{fontcolor|blue|7}} |
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Graphical games are games in which the utilities of each player depends on the actions of very few other players. If is the greatest number of players by whose actions any single player is affected (that is, it is the indegree of the game graph), the number of utility values needed to describe the game is , which, for a small is a considerable improvement.
It has been shown that any normal form game is reducible to a graphical game with all degrees bounded by three and with two strategies for each player. Unlike normal form games, the problem of finding a pure Nash equilibrium in graphical games (if one exists) is NP-complete. The problem of finding a (possibly mixed) Nash equilibrium in a graphical game is PPAD-complete. Finding a correlated equilibrium of a graphical game can be done in polynomial time, and for a graph with a bounded treewidth, this is also true for finding an optimal correlated equilibrium.
{{Clear}}