Social dilemmas in an online social network: the structure and evolution of cooperation

We investigate two paradigms for studying the evolution of cooperation--Prisoner's Dilemma and Snowdrift game in an online friendship network obtained from a social networking site. We demonstrate that such social network has small-world property and degree distribution has a power-law tail. Besides, it has hierarchical organizations and exhibits disassortative mixing pattern. We study the evolutionary version of the two types of games on it. It is found that enhancement and sustainment of cooperative behaviors are attributable to the underlying network topological organization. It is also shown that cooperators can survive when confronted with the invasion of defectors throughout the entire ranges of parameters of both games. The evolution of cooperation on empirical networks is influenced by various network effects in a combined manner, compared with that on model networks. Our results can help understand the cooperative behaviors in human groups and society.Comment: 14 pages, 7 figure

Introducing Hierarchy in Energy Games

In this work we introduce hierarchy in wireless networks that can be modeled by a decentralized multiple access channel and for which energy-efficiency is the main performance index. In these networks users are free to choose their power control strategy to selfishly maximize their energy-efficiency. Specifically, we introduce hierarchy in two different ways: 1. Assuming single-user decoding at the receiver, we investigate a Stackelberg formulation of the game where one user is the leader whereas the other users are assumed to be able to react to the leader's decisions; 2. Assuming neither leader nor followers among the users, we introduce hierarchy by assuming successive interference cancellation at the receiver. It is shown that introducing a certain degree of hierarchy in non-cooperative power control games not only improves the individual energy efficiency of all the users but can also be a way of insuring the existence of a non-saturated equilibrium and reaching a desired trade-off between the global network performance at the equilibrium and the requested amount of signaling. In this respect, the way of measuring the global performance of an energy-efficient network is shown to be a critical issue.Comment: Accepted for publication in IEEE Trans. on Wireless Communication

Aggregation in Game Theoretical Situations

The thesis deals with the class of Aggregative Games, namely strategic form games where each payoff function depends on the corresponding player's strategy and on some aggregation among strategies of all involved players. The first part of the thesis is devoted to the multi-leader multi-follower equilibrium concept for the class of aggregative games: the considered game presents aymmetry between two groups of players, acting noncooperatively within the group and one group is the leader in a leader-follower hierarchical model. Moreover, as it happens in concrete situations, the model is affected by uncertainty and the game is considered in a stochastic context. Assuming an exogenous uncertainty affecting the aggregator, the multi-leader multi-follower equilibrium model is presented and existence results for the stochastic resulting game are obtained in the smooth case of nice aggregative games, where payoff functions are continuous and concave in own strategies, as well as in the general case of aggregative games with strategic substitutes. These results apply to the global emission game and the teamwork project game. Then, an investment in Common-Pool Resources is studied: the situation of many agents interested in a common-pool resource, like water resource, is modeled as an aggregative game and existence results of Nash equilibria are obtained with or without convexity-like assumptions. In the special case of quadratic return functions, the game is also considered under uncertainty i.e. when the possibility of a natural disaster with a given probability may occur. In the second part of the thesis, in line with the literature on additively separable aggregative games, a class of non cooperative games, called Social Purpose Games, is introduced. In this class of games the payoff of each player depends separately on his own strategy and on a function of the strategy profile, the aggregation function, which is the same for all players, weighted by an individual benefit parameter which enlightens the asymmetry between agents toward the social part of the benefit. The two parts of the payoff function represent respectively the individual and the social benefits. For the class of social purpose games it has been showed that they have a potential, providing also a comparison between the Nash equilibrium strategies and the social optimum strategies, namely when all the players agree in maximizing the aggregate profit. For social purpose games we study the existence of the so called coalition leadership equilibrium: it is a multi-leader multi-follower model where a cooperative behaviour is assumed between players of the leading group and they decide to maximize the aggregation of their payoffs. The rest of the players act noncooperatively. This kind of equilibrium presents a mixture of cooperative and noncooperative behaviour, situation that often occurs in many applicative examples. The weights affecting the aggregation function allow to derive explicit conditions under which the leading coalition is stable. An application to a water resource game is illustrated

Decentralized Cooperative Planning for Automated Vehicles with Hierarchical Monte Carlo Tree Search

Today's automated vehicles lack the ability to cooperate implicitly with others. This work presents a Monte Carlo Tree Search (MCTS) based approach for decentralized cooperative planning using macro-actions for automated vehicles in heterogeneous environments. Based on cooperative modeling of other agents and Decoupled-UCT (a variant of MCTS), the algorithm evaluates the state-action-values of each agent in a cooperative and decentralized manner, explicitly modeling the interdependence of actions between traffic participants. Macro-actions allow for temporal extension over multiple time steps and increase the effective search depth requiring fewer iterations to plan over longer horizons. Without predefined policies for macro-actions, the algorithm simultaneously learns policies over and within macro-actions. The proposed method is evaluated under several conflict scenarios, showing that the algorithm can achieve effective cooperative planning with learned macro-actions in heterogeneous environments

Hierarchy and Competition in CSCW applications: Model and case study

CSCW applications need to adapt themselves to the functional and organizational structures of people that use them. However they do not usually support division in groups with a certain hierarchical structure among them. In this paper, we propose and study a theoretical model of groupware appliations that reflects those hierarchical interactions. The proposed model is also intended to evaluate the effects in performance derived from competitive and collaborative relationships among the components of a hierarchy of groups. In order to demonstrate the above ideas, a groupware game, called Alymod, was designed and implemented using a modified version of a well-known CSCW Toolkit, namely Groupkit. Groupkit was modified in order to support group interactions in the same CSCW application. In Alymod, participants compete or collaborate within a hierarchical structure to achieve a common goal (completing gaps in a text, finishing numerical series, resolving University course examinations, etc.).Publicad

Spatial SINR Games of Base Station Placement and Mobile Association

We study the question of determining locations of base stations that may belong to the same or to competing service providers. We take into account the impact of these decisions on the behavior of intelligent mobile terminals who can connect to the base station that offers the best utility. The signal to interference and noise ratio is used as the quantity that determines the association. We first study the SINR association-game: we determine the cells corresponding to each base stations, i.e., the locations at which mobile terminals prefer to connect to a given base station than to others. We make some surprising observations: (i) displacing a base station a little in one direction may result in a displacement of the boundary of the corresponding cell to the opposite direction; (ii) A cell corresponding to a BS may be the union of disconnected sub-cells. We then study the hierarchical equilibrium in the combined BS location and mobile association problem: we determine where to locate the BSs so as to maximize the revenues obtained at the induced SINR mobile association game. We consider the cases of single frequency band and two frequency bands of operation. Finally, we also consider hierarchical equilibria in two frequency systems with successive interference cancellation