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{|align="right" border="1" cellpadding="3" cellspacing="0" width="230px" style="margin: 1em 1em 1em 1em; background: #f9f9f9; border: 1px #aaa solid; border-collapse: collapse; font-size: 70%;"
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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*2<sup>3</sup>=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}}
|-
!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 game|normal form]] representation. Without placing constraints on player utilities, describing a game of <math>n</math> players, each facing <math>s</math> [[strategy (game theory)|strategies]], requires listing <math>ns^n</math> utility values. Even trivial algorithms are capable of finding a [[Nash equilibrium]] in a time [[time complexity#Polynomial 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''<ref name="Papadimitriou2007"/> (a formal definition, describing succinct games as a [[computational problem]], is given by Papadimitriou & Roughgarden 2008<ref name="Papadimitriou2008"/>).
{{Clear}}
 
==Types of succinct games==
===Graphical games===
{|align="right" border="1" cellpadding="3" cellspacing="0" width="230px" style="margin: 1em 1em 1em 1em; background: #f9f9f9; border: 1px #aaa solid; border-collapse: collapse; font-size: 70%;"
|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.
    <div style="text-align:center;">
        {|border="1" cellpadding="3" cellspacing="0" style="margin:0.5em; border:1px #aaa solid; border-collapse:collapse; display:inline-table;"
            |
            !align="center"|''L''
            !align="center"|''R''
            |-
            !align="center"|''T''
            |align="center"|{{fontcolor|red|9}}
            |align="center"|{{fontcolor|red|8}}
            |-
            !align="center"|''B''
            |align="center"|{{fontcolor|red|3}}
            |align="center"|{{fontcolor|red|4}}
        |}
        {|border="1" cellpadding="3" cellspacing="0" style="margin:0.5em; border:1px #aaa solid; border-collapse:collapse; display:inline-table;"
            |
            !align="center"|''l''
            !align="center"|''r''
            |-
            !align="center"|''L''
            |align="center"|{{fontcolor|green|6}}
            |align="center"|{{fontcolor|green|8}}
            |-
            !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;"
            |
            !align="center"|''T''
            !align="center"|''B''
            |-
            !align="center"|''l''
            |align="center"|{{fontcolor|blue|4}}
            |align="center"|{{fontcolor|blue|4}}
            |-
            !align="center"|''r''
            |align="center"|{{fontcolor|blue|5}}
            |align="center"|{{fontcolor|blue|7}}
        |}
    </div>
|}
[[Graphical game (game theory)|Graphical game]]s are games in which the utilities of each player depends on the actions of very few other players. If <math>d</math> is the greatest number of players by whose actions any single player is affected (that is, it is the [[Indegree#Indegree and outdegree|indegree]] of the game graph), the number of utility values needed to describe the game is <math>ns^{d+1}</math>, which, for a small <math>d</math> is a considerable improvement.
 
It has been shown that any normal form game is [[reduction (complexity)|reducible]] to a graphical game with all degrees bounded by three and with two strategies for each player.<ref name="Goldberg2006"/> Unlike normal form games, the problem of finding a pure Nash equilibrium in graphical games (if one exists) is [[NP-complete]].<ref name="Gottlob2005"/> The problem of finding a (possibly mixed) Nash equilibrium in a graphical game is [[PPAD (complexity)|PPAD]]-complete.<ref name="Daskalakis2006"/> 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.<ref name="Papadimitriou2008"/>
{{Clear}}
 
===Sparse games===
{|align="right" border="1" cellpadding="4" cellspacing="0" width="230px" style="margin: 1em 1em 1em 1em; background: #f9f9f9; border: 1px #aaa solid; border-collapse: collapse; font-size: 70%;"
|colspan="5"|When most of the utilities are 0, as below, it is easy to come up with a succinct representation.
|-
|
!align="center"|''L'', ''l''
!align="center"|''L'', ''r''
!align="center"|''R'', ''l''
!align="center"|''R'', ''r''
|-
!align="center"|''T''
|align="center"|{{fontcolor|red|0}}, {{fontcolor|green|0}}, {{fontcolor|blue|0}}
|align="center"|{{fontcolor|red|2}}, {{fontcolor|green|0}}, {{fontcolor|blue|1}}
|align="center"|{{fontcolor|red|0}}, {{fontcolor|green|0}}, {{fontcolor|blue|0}}
|align="center"|{{fontcolor|red|0}}, {{fontcolor|green|7}}, {{fontcolor|blue|0}}
|-
!align="center"|''B''
|align="center"|{{fontcolor|red|0}}, {{fontcolor|green|0}}, {{fontcolor|blue|0}}
|align="center"|{{fontcolor|red|0}}, {{fontcolor|green|0}}, {{fontcolor|blue|0}}
|align="center"|{{fontcolor|red|2}}, {{fontcolor|green|0}}, {{fontcolor|blue|3}}
|align="center"|{{fontcolor|red|0}}, {{fontcolor|green|0}}, {{fontcolor|blue|0}}
|}
[[Sparse game]]s are those where most of the utilities are zero. Graphical games may be seen as a special case of sparse games.
 
For a two player game, a sparse game may be defined as a game in which each row and column of the two payoff (utility) matrices has at most a constant number of non-zero entries. It has been shown that finding a Nash equilibrium in such a sparse game is PPAD-hard, and that there does not exist a fully [[polynomial-time approximation scheme]] unless PPAD is in [[P (complexity)|P]].<ref name="Chen2006"/>
{{Clear}}
 
===Symmetric games===
{|align="right" border="1" cellpadding="3" cellspacing="0" width="230px" style="margin: 1em 1em 1em 1em; background: #f9f9f9; border: 1px #aaa solid; border-collapse: collapse; font-size: 70%;"
|Suppose all three players are identical (we'll color them all {{fontcolor|purple|purple}}), and face the strategy set {T,B}. Let #TP and #BP be the number of a player's peers who've chosen T and B, respectively. Describing this game requires only 6 utility values.
    <div style="text-align:center;">
        {|border="1" cellpadding="3" cellspacing="0" style="margin:0.5em; border:1px #aaa solid; border-collapse:collapse; display:inline-table;"
            |
            !align="center"|#TP=2<br/>#BP=0
            !align="center"|#TP=1<br/>#BP=1
            !align="center"|#TP=0<br/>#BP=2
            |-
            !align="center"|''T''
            |align="center"|{{fontcolor|purple|5}}
            |align="center"|{{fontcolor|purple|2}}
            |align="center"|{{fontcolor|purple|2}}
            |-
            !align="center"|''B''
            |align="center"|{{fontcolor|purple|1}}
            |align="center"|{{fontcolor|purple|7}}
            |align="center"|{{fontcolor|purple|2}}
        |}
    </div>
|}
In [[symmetric game]]s all players are identical, so in evaluating the utility of a combination of strategies, all that matters is how many of the <math>n</math> players play each of the <math>s</math> strategies. Thus, describing such a game requires giving only <math>s\tbinom{n+s-2}{s-1}</math> utility values.
 
In a symmetric game with 2 strategies there always exists a pure Nash equilibrium – although a ''symmetric'' pure Nash equilibrium may not exist.<ref name="Cheng2004"/> The problem of finding a pure Nash equilibrium in a symmetric game (with possibly more than two players) with a constant number of actions is in [[AC0|AC<sup>0</sup>]]; however, when the number of actions grows with the number of players (even linearly) the problem is NP-complete.<ref name="Brandt2009"/> In any symmetric game there exists a [[symmetric equilibrium]]. Given a symmetric game of ''n'' players facing ''k'' strategies, a symmetric equilibrium may be found in polynomial time if k=<math>O(\log n/\log \log n)</math>.<ref name="Papadimitriou2005"/> Finding a correlated equilibrium in symmetric games may be done in polynomial time.<ref name="Papadimitriou2008"/>
{{Clear}}
 
===Anonymous games===
{|align="right" border="1" cellpadding="3" cellspacing="0" width="230px" style="margin: 1em 1em 1em 1em; background: #f9f9f9; border: 1px #aaa solid; border-collapse: collapse; font-size: 70%;"
|If players were different but did not distinguish between other players we would need to list 18 utility values to represent the game - one table such as that given for "symmetric games" above for each player.
    <div style="text-align:center;">
        {|border="1" cellpadding="3" cellspacing="0" style="margin:0.5em; border:1px #aaa solid; border-collapse:collapse; display:inline-table;"
            |
            !align="center"|#TP=2<br/>#BP=0
            !align="center"|#TP=1<br/>#BP=1
            !align="center"|#TP=0<br/>#BP=2
            |-
            !align="center"|''T''
            |align="center"|{{fontcolor|red|8}}, {{fontcolor|green|8}}, {{fontcolor|blue|2}}
            |align="center"|{{fontcolor|red|2}}, {{fontcolor|green|9}}, {{fontcolor|blue|5}}
            |align="center"|{{fontcolor|red|4}}, {{fontcolor|green|1}}, {{fontcolor|blue|4}}
            |-
            !align="center"|''B''
            |align="center"|{{fontcolor|red|6}}, {{fontcolor|green|1}}, {{fontcolor|blue|3}}
            |align="center"|{{fontcolor|red|2}}, {{fontcolor|green|2}}, {{fontcolor|blue|1}}
            |align="center"|{{fontcolor|red|7}}, {{fontcolor|green|0}}, {{fontcolor|blue|6}}
        |}
    </div>
|}
In [[anonymous game]]s, players have different utilities but do not distinguish between other players (for instance, having to choose between "go to cinema" and "go to bar" while caring only how crowded will each place be, not who'll they meet there). In such a game a player's utility again depends on how many of his peers choose which strategy, and his own, so <math>sn\tbinom{n+s-2}{s-1}</math> utility values are required.
 
If the number of actions grows with the number of players, finding a pure Nash equilibrium in an anonymous game is [[NP-hard]].<ref name="Brandt2009"/> An optimal correlated equilibrium of an anonymous game may be found in polynomial time.<ref name="Papadimitriou2008"/> When the number of strategies is 2, there is a known [[polynomial-time approximation scheme|PTAS]] for finding an [[epsilon-equilibrium|ε-approximate Nash equilibrium]].<ref name="Daskalakis2007"/>
{{Clear}}
 
===Polymatrix games===
{|align="right" border="1" cellpadding="3" cellspacing="0" width="230px" style="margin: 1em 1em 1em 1em; background: #f9f9f9; border: 1px #aaa solid; border-collapse: collapse; font-size: 70%;"
|If the game in question was a polymatrix game, describing it would require 24 utility values. For simplicity, let us examine only the utilities of player I (we would need two more such tables for each of the other players).
    <div style="text-align:center;">
        {|border="1" cellpadding="2" cellspacing="0" style="margin:0.5em; border:1px #aaa solid; border-collapse:collapse; display:inline-table;"
            |
            !align="center"|''L''
            !align="center"|''R''
            |-
            !align="center"|''T''
            |align="center"|{{fontcolor|red|4}}, {{fontcolor|green|6}}
            |align="center"|{{fontcolor|red|8}}, {{fontcolor|green|7}}
            |-
            !align="center"|''B''
            |align="center"|{{fontcolor|red|3}}, {{fontcolor|green|7}}
            |align="center"|{{fontcolor|red|9}}, {{fontcolor|green|1}}
        |}
        {|border="1" cellpadding="2" cellspacing="0" style="margin:0.5em; border:1px #aaa solid; border-collapse:collapse; display:inline-table;"
            |
            !align="center"|''l''
            !align="center"|''r''
            |-
            !align="center"|''T''
            |align="center"|{{fontcolor|red|7}}, {{fontcolor|blue|7}}
            |align="center"|{{fontcolor|red|1}}, {{fontcolor|blue|6}}
            |-
            !align="center"|''B''
            |align="center"|{{fontcolor|red|8}}, {{fontcolor|blue|6}}
            |align="center"|{{fontcolor|red|6}}, {{fontcolor|blue|4}}
        |}
        {|border="1" cellpadding="2" cellspacing="0" style="margin:0.5em; border:1px #aaa solid; border-collapse:collapse; display:inline-table;"
            |
            !align="center"|''l''
            !align="center"|''r''
            |-
            !align="center"|''L''
            |align="center"|{{fontcolor|green|2}}, {{fontcolor|blue|9}}
            |align="center"|{{fontcolor|green|3}}, {{fontcolor|blue|3}}
            |-
            !align="center"|''R''
            |align="center"|{{fontcolor|green|2}}, {{fontcolor|blue|4}}
            |align="center"|{{fontcolor|green|1}}, {{fontcolor|blue|5}}
        |}
    </div>
If strategy profile (B,R,l) was chosen, player I's utility would be 9+8=17, player II's utility would be 1+2=3, and player III's utility would be 6+4=10.
|}
In a [[polymatrix game]] (also known as a ''multimatrix game''), there is a utility matrix for every pair of players ''(i,j)'', denoting a component of player i's utility. Player i's final utility is the sum of all such components. The number of utilities values required to represent such a game is <math>O(n^2*s^2)</math>.
 
Polymatrix games always have at least one mixed Nash equilibrium.<ref name="Howson1972"/> The problem of finding a Nash equilibrium in a polymatrix game is PPAD-complete.<ref name="Daskalakis2006"/> Finding a correlated equilibrium of a polymatrix game can be done in polynomial time.<ref name="Papadimitriou2008"/>
{{Clear}}
 
===Circuit games===
{|align="right" border="1" cellpadding="4" cellspacing="0" width="230px" style="margin: 1em 1em 1em 1em; background: #f9f9f9; border: 1px #aaa solid; border-collapse: collapse; font-size: 70%;"
|colspan="5"|Let us now equate the players' various strategies with the Boolean values "0" and "1", and let X stand for player I's choice, Y for player II's choice and Z for player III's choice. Let us assign each player a circuit:
<div style="margin-left:10px;">
Player I: X ∧ (Y ∨ Z)<br/>
Player II: X ⨁ Y ⨁ Z''<br/> <!--xor'd-->
Player III: X ∨ Y
</div>
These describe the utility table below.
|-
|
!align="center"|''0'', ''0''
!align="center"|''0'', ''1''
!align="center"|''1'', ''0''
!align="center"|''1'', ''1''
|-
!align="center"|''0''
|align="center"|{{fontcolor|red|0}}, {{fontcolor|green|0}}, {{fontcolor|blue|0}}
|align="center"|{{fontcolor|red|0}}, {{fontcolor|green|1}}, {{fontcolor|blue|0}}
|align="center"|{{fontcolor|red|0}}, {{fontcolor|green|1}}, {{fontcolor|blue|1}}
|align="center"|{{fontcolor|red|0}}, {{fontcolor|green|0}}, {{fontcolor|blue|1}}
|-
!align="center"|''1''
|align="center"|{{fontcolor|red|0}}, {{fontcolor|green|1}}, {{fontcolor|blue|1}}
|align="center"|{{fontcolor|red|1}}, {{fontcolor|green|0}}, {{fontcolor|blue|1}}
|align="center"|{{fontcolor|red|1}}, {{fontcolor|green|0}}, {{fontcolor|blue|1}}
|align="center"|{{fontcolor|red|1}}, {{fontcolor|green|1}}, {{fontcolor|blue|1}}
|}
The most flexible of way of representing a succinct game is by representing each player by a polynomial-time bounded [[Turing machine]], which takes as its input the actions of all players and outputs the player's utility. Such a Turing machine is equivalent to a [[Boolean circuit]]<!--https://www.cs.princeton.edu/~chazelle/pubs/UnboundedHardwEquivDetTuring.pdf ?-->, and it is this representation, known as [[circuit game]]s, that we will consider.
 
Computing the value of a 2-player [[zero-sum]] circuit game is an [[EXPTIME|EXP]]-complete problem,<ref name="Feigenbaum1995"/> and approximating the value of such a game up to a multiplicative factor is known to be in [[PSPACE]].<ref name="Fortnow2005"/> Determining whether a pure Nash equilibrium exists is a <math>\Sigma_2^{\rm P}</math>-complete problem (see [[Polynomial hierarchy]]).<ref name="Schoenebeck2006"/>
{{Clear}}
 
===Other representations===
Many other types of succinct game exist (many having to do with allocation of resources). Examples include [[congestion game]]s, [[network congestion game]]s, [[scheduling game]]s, [[local effect game]]s, [[facility location game]]s, [[action-graph game]]s, [[hypergraphical game]]s and more.
 
==Summary of complexities of finding equilibria==
Below is a table of some known complexity results for finding certain classes of equilibria in several game representations. "NE" stands for "Nash equilibrium", and "CE" for "correlated equilibrium". ''n'' is the number of players and ''s'' is the number of strategies each player faces (we're assuming all players face the same number of strategies). In graphical games, ''d'' is the maximum indegree of the game graph. For references, see main article text.
{|class="wikitable" style="width:100%;"
!Representation        !! Size (''O(...)'')                    !! Pure NE                              !! Mixed NE        !! CE              !! Optimal CE
|-
|Normal form game      || <math>ns^n</math>                    || Linear                              || PPAD-complete    || P                || P
|-
|Graphical game        || <math>ns^{d+1}</math>                || NP-complete                          || PPAD-complete    || P                || NP-hard
|-
|Symmetric game        || <math>s\tbinom{n+s-1}{s-1}</math>    || NP-complete                          || PPAD-complete    || P                || P
|-
|Anonymous game        || <math>sn\tbinom{n+s-1}{s-1}</math>  || NP-hard                              ||                  || P                || P
|-
|Polymatrix game        || <math>n^2*s^2</math>                ||                                      || PPAD-complete    || P                || NP-hard
|-
|Circuit game          ||                                      || <math>\Sigma_2^{\rm P}</math>-complete                          ||                  ||                  ||
|-
|Congestion game        ||                                      || [[PLS (complexity)|PLS-complete]]    ||                  || P                || NP-hard
|}
 
<!--I strongly suggest editing this table in an external editor with line wrapping disabled.-->
 
==Notes==
{{Reflist|30em|refs=
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        | title        = Algorithmic Game Theory
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        | year = 2006
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    <ref name="Howson1972">
        {{Cite journal
        | issn = 0025-1909
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    <ref name="Cheng2004">
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}}
 
==External links==
* [http://agtb.wordpress.com/2009/11/19/the-computational-complexity-of-pure-nash/ Algorithmic Game Theory: The Computational Complexity of Pure Nash]
 
{{Game theory}}
 
[[Category:Game theory]]

Latest revision as of 01:28, 24 August 2014

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