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1、Scale-Free Networks Provide a Unifying Framework for the Emergence of CooperationPRL :F. C. Santosand and J. M. Pachecopublished 26 August 2005 邵中尉PD(prisoners dilemma )PD(prisoners dilemma ) Two men are arrested, but the police do not possess enough information for a conviction. Following the separ

2、ation of the two men, the police offer both a similar dealif one testifies against his partner (defects/betrays), and the other remains silent (cooperates/assists), the betrayer goes free and the one that remains silent receives the full one-year sentence. If both remain silent, both are sentenced t

3、o only one month in jail for a minor charge. If each rats out the other, each receives a three-month sentence. Each prisoner must choose either to betray or remain silent; the decision of each is kept quiet. What should they do? PD(prisoners dilemma )In PD ,two players simultaneously decide whether

4、to cooperate of defect: R: mutual cooperation P: mutual defection T: a defector exploiting a cooperator S: the exploited cooperators TRPS SG (snowdrift game )SG (snowdrift game ) Imagine two drivers on their way home that are caught in a blizzard and trapped on either side of a snowdrift. Each drive

5、r has the option to get out and start shovelling or to remain in the cozy warmth of the car. If both start shovelling each has the benefit of getting home while sharing the labour costs. However, if only one shovels both drivers still get home but the lazy bum avoids the labour costs. Nevertheless a

6、nd despite the shovelling, the benefit of getting home outweighs the awkward prospects of waiting for spring to melt the obstacle.SG (snowdrift game ) In SG ,the order of P and S is exchanged : R: mutual cooperation P: mutual defection T: a defector exploiting a cooperator S: the exploited cooperato

7、rs TRSPIn this paper For PD : T=b1, R=1, P=S=0 For SG: T= 1,R= -1/2,S= -1,P=0, r=1/(2 -1) Simulation rules(1) In each generation, all pairs of individuals x and y ,directly connected ,engage in a single round of a given game ,their accumulated payoffs being stored as Px and Py respectively.(2) Whene

8、ver PyPx ,the chosen neighbor takes over site x with probability given by (Py-Px)/(Dk),where k is the largest between kx and ky. For PD,D=T-S, For SG,D=T-PSimulation level The population : N=10000 individuals Simulation time : 10000 generations Equilibrium frequencies : averaging over 1000 generatio

9、nsResult1Z: the average connectivityConclusion Cooperation is dramatically enhanced on SF NOCs for both games , dominating over the entire ranges of b and r.Result2Conclusion It shows the same qualitative behavior as a function of the game parameters , for both games.Result3Conclusion Results show t

10、hat whenever age correlations are suppressed , cooperation is inhibited and no longer dominates for large values of b and r.SUM UP Scale-free networks of contacts lead to unprecedented values for the equilibrium frequencies of cooperators , such that cooperation becomes not only competitive but ofte

11、n the predominant for cooperation to dominate , irrespective of whether individuals engage in the PD or SG , feature which subsist down to small community sizes.references1 Genetic and Cultural Evolution of Cooperation, edited byPeter Hammerstein (MIT, Cambridge, MA, 2003).2 J. Maynard Smith, Evolution and the Theory of Games(Cambridge University Press, Cambridge, England, 1982).3 H. Gintis, Game Theory Evolving(Princeton University,Princeton, NJ, 2000).4 H. Milinsky, J. H. Lu

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