Publications

 

CTN Newsletter n.16 - September 2011

Index

 


Editorial

by Agnieszka Rusinowska, Centre d'Economie de la Sorbonne

Dear All,

The next CTN workshop (CTN17) will be held in Paris, on February 3-4, 2012. CTN17 will be aimed at providing an avenue for presenting and discussing new contributions on network, coalition and matching theory, with particular focus on financial networks. This theme, being closely related to current economic problems, and consequently containing topics of importance, has not been emphasized in the previous CTN workshops. The format of CTN17 is going to be similar to the CTN16 workshop format in 2011, i.e., we will have a two-day workshop with one session at a time (no parallel sessions), which will allow participants to follow all presentations of the meeting, and speakers to have time to introduce and present  their recent work in detail. The guest speakers will be Franklin ALLEN (University of Pennsylvania), Rajiv VOHRA (Brown University), and Yves ZENOU (Stockholm University). 
   
We would like to draw your attention to the fact that the year 2012 is going to be very rich in scientific meetings that can be of interest for the CTN members: the PET (Public Economic Theory) conference on June 12-14 in Taipei (http://www.apet.org/), the SAET (Society for the Advancement of Economic Theory) conference from June 30 to July 3 in Queensland (http://www.saet.illinois.edu/future-events.html), SING8 (Spain Italy Netherlands Meeting on Game Theory) on July 16-18 in Budapest (http://sing8.iehas.hu/), Game Theory Society World Congress on July 22-26 in Istanbul (http://games2012.bilgi.edu.tr/), and the Meeting of Society for Social Choice and Welfare on August 17 – 20 in New Delhi (http://www.isid.ac.in/~sscw2012/Welcome.html). CTN17 in Paris is then going to be a pleasant and inspiring start of this impressive series of meetings.

Should you have any query about CTN17, please contact Agnieszka Rusinowska at the Centre d’Economie de la Sorbonne (agnieszka.rusinowska@univ-paris1.fr). The CTN17 website, which will allow you to follow updates of the workshop, and the workshop e-mail address (ctn2012@univ-paris1.fr) will be available very soon.
We very much look forward to seeing you in Paris during CTN17!

Agnieszka Rusinowska

 

 


CTN Announcements

SAVE THE DATE - 17th Coalition Theory Network Workshop
Paris, France, 3-4 February 2012

The Centre d'Economie de la Sorbonne will be organizing the 17th Coalition Theory Network Workshop in Paris, France, on 3-4 February 2012.
The guest speakers will be

The scientific coordinators of the event will be Francis Bloch, Hubert Kempf, Agnieszka Rusinowska, Emily Tanimura.

Call information soon!



Workshop on Cooperation, Matching and Collective Goods
San Luis, Argentina, 24-25 November 2011
http://www.workshop2011.unsl.edu.ar/

The Instituto de Matemática Aplicada San Luis (IMASL) of Universidad Nacional de San Luis and Consejo Nacional de Ciencia y Tecnología (CONICET), in conjunction with the Association for Public Economic Theory (APET), will be organizing the Workshop "Cooperation, Matching and Collective Goods". The keynote speakers will be:




Article: Majority rules and stability in games with spillovers

Sergio Currarini, FEEM and University of Venice
Marco Marini, University of Urbino "Carlo Bo"

Introduction

Most social situations are such that agents' actions aect the welfare of some or all other agents in the system. This is the case, for instance, of public goods production, global pollution control, imperfectly competitive markets, auctions, and so on. In these cases, what agents or coalitions of agents expect to attain in absence of global cooperation depends on how the remaining agents are expected to behave in such occurrency. If one defines a stable social outcome as a globally agreed norm of behaviour that no agent or coalition would improve upon by its sole forces, the analysis of stability crucially hinges upon the expected behaviour in case of disagreement. This problem is well known and extensively studied in the theory of coalitional games with externalities (Ray and Vohra 1998, Bloch 1996 and 2003 Yi 1997 and 2003, Maskin 2003, Ray 2007, Hafalir 2007), where the expected reaction in case of coalitional objections have been modeled in various ways. In particular, core allocations (that is, social outcomes that arenot objected by single players or coalitions) fail to exist even when the game possess strong aggregative properties (such as a notion of convexity extended to account for externalities, see Hafalir 2007), both when the players in complementary coalitions are assumed to stick together (delta core or core with merging expectations) as well as when they organize optimally to maximize their own payoffs (rational expectations core).
One way of recovering stability in cooperative situations is to limit the objecting (or blocking) power to a subset of all coalitions that may form. For games without interaction across coalitions, the existence of core stable allocations is guaranteed in all problems where agents can be completely ordered and the blocking power is restricted only to connected coalitions in the order (see Greenberg and Weber and further work by Demange). In this note we consider a class of symmetric and supermodular games that generate externalities across coalitions, and show that if the blocking power is restricted to majority coalitions, the core is nonempty for all expectations on outside players' behaviour. This class of games has the property that coalitions with small size achieve per capita payoffs that exceed those of larger coalitions - a condition already employed in earlier works on coalition formation (see Yi, 2003). We also show that if majorities can extract resources from minorities, then stability requires that blocking power is limited to special majorities, and that the threshold size of these special majorities increases with their extraction power.

Read the full article



Scientific report of the SING VII Congress held in Paris, France on 18-20 July 2011

by Agnieszka Rusinowska, Centre d’Economie de la Sorbonne, CNRS – Université Paris 1

The Seventh Spain-Italy-Netherlands Meeting on Game Theory (SING7, http://ces.univ-paris1.fr/SING_7.htm) took place at TELECOM ParisTech from July 18th to 20th, 2011. The conference was jointly organized by University of Paris I, Paris School of Economics, and TELECOM ParisTech. The local organizing committee (chaired by Michel Grabisch) was formed in majority by members of Centre d’Economie de la Sorbonne.

SING7 was the seventh in the series of Spain-Italy-Netherlands Meetings on Game Theory, and the first one organized in France. As suggested by the name of this series of conferences, the general aim of the SING meetings is to provide an avenue for presenting and discussing new results in Game Theory. The scientific program included 5 parallel tracks of sessions (in total 50 parallel sessions) with more than 170 contributions and 5 plenary lectures. One could remark that among different works in cooperative and noncooperative games, the CTN topics of interest were strongly represented, since about 20% of the presentations concerned network and coalition theory.

A particular feature of the SING7 conference was its international character. The organizers were very happy to host more than 200 participants from 29 different countries (Algeria, Austria, Belgium, Canada, Chile, Czech Republic, Denmark, Finland, France, Germany, Hungary, Ireland, Israel, Italy, Japan, Kazakhstan, Luxembourg, Mexico, Moldova, New Zealand, Poland, Russia, Spain, Switzerland, Taiwan, The Netherlands, Turkey, United Kingdom, USA).

The invited speakers of the SING7 conference were: Matthew Jackson (Stanford University), Annick Laruelle (University of Basque Country), Hervé Moulin (Rice University), Hans Peters (University of Maastricht), and Peter Sudhölter (University of Southern Denmark).

The lecture of Matthew Jackson was based on his joint work with Tomas Rodriguez-Barraquer and Xu Tan on network patterns of favor exchange. In this work, the authors focus on situations where the interaction between any two individuals is insufficiently frequent to provide incentives to exchange favors over time, but where the social pressure of the possible loss of multiple relationships can sustain exchange. They characterize the equilibrium network patterns of exchange that are robust in a sense that deleted relationships only result in a local loss of favor exchange. Such robustness necessitates "social quilts'', i.e., networks that are tree-like unions of completely connected sub-networks.

Annick Laruelle delivered a presentation on artificial distinction, real discrimination,which is a joint work with Elena Inarra. The authors introduce a new variation of the hawk-dove game suggested by an experiment that studies the behavior of a group of domestic fowls when a subgroup has been marked. They consider a population formed by two types of individuals that fail to recognize their own type but do recognize the other type. In this game two evolutionarily stable strategies are found. In each of them, individuals from one type are always attacked more, whatever proportion of the population they represent. The theoretical results are consistent with the conclusions drawn from experimental work in which marked fowls received more pecks than their unmarked counterparts.

Hervé Moulin presented joint work with Jay Sethuraman on the bipartite rationing problem. In the bipartite rationing problem, a set of agents share a single resource available in different "types", each agent has a claim over only a subset of the resource-types, and these claims overlap in an arbitrary fashion. The goal is to divide fairly the various types of resources between the claimants, when resources are in short supply. The authors find that most parametric rationing methods cannot be consistently extended, and come close to characterize the subset of those that can. The latter reduce essentially to the loss calibrated rationing methods, a new family of methods containing the proportional method, and the uniform gains and uniform losses as limit points.

The lecture by Hans Peters concerned estate division problems. The problem is approached by considering a noncooperative claim game, where the estate is represented by a real interval, and each player can claim each arbitrary subinterval, a nonnegative integer number of times. Next, each interval is divided among those players that claim it at least once, in proportion to the numbers of claims. The author characterizes the set of all divisions of the estate that can occur in a Nash equilibrium of the claim game. Some variations of this model are considered, e.g., the claim game associated with the multiple estate division problem and non-homogenous preferences, and an infinite continuous version of this model, where every point in a given interval corresponds to an estate.

Peter Sudhölter delivered a talk on core stability, vital exact extendability, and applications. A coalition is vital if there is some core element such that none of the proper subcoalitions is effective for this element. It is exact if it is effective for some core element. The author shows that if all coalitions that are vital and exact are extendable, then the game has a stable core. Though vital exact extendability is still just sufficient for core stability in general, it is a necessary condition for the foregoing classes of games. Moreover, the condition is necessary and sufficient for core stability of a matching game. The author also presents classes of TU games, e.g., assignment games, whose modiclus, a relative of the prenucleolus, belongs to the core provided that the core is stable.

Numerous interesting papers on networks and coalitions have been presented during the SING7 conference. In particular, there were series of invited sessions that concerned network theory. One of them was the session on Dynamic Network Games organized by Myrna Wooders. We had the pleasure to listen to 3 papers in this session. The first one, Myopic or farsighted? An experiment on network formation, was presented by Ana Mauleon (joint work with Marco Mantovani, Georg Kirchsteiger and Vincent Vannetelbosch). The authors design a simple network formation experiment to test stability theories. Their results provide support for farsighted stability and strongly reject the idea of myopic behavior. Christophe Bravard gave a talk on confirming information flows in networks (joint work with P. Billand, J. Kamphorst and S. Sarangi). The authors consider situations where an agent has concerns about the validity of information that she acquires from another agent. They characterize the set of strict Nash networks and the set of strict efficient networks. Vincent Vannetelbosch presented a paper on bargaining and delay in trading networks (joint work with Mikel Bedayo and Ana Mauleon), in which they study a model in which pairs of players (indirectly) connected in a network are randomly matched to bargain through a chain of intermediaries. The authors analyze the question of how the network architecture affects the outcomes and dynamics of bargaining when agents have private information about their bargaining strength.

Furthermore, I would like to mention another (4-paper) session on Social Networks, in which Dunia López-Pintado presented her joint work with Matthew Jackson on diffusion and segregation. The authors study how behavior spreads among agents in an interaction network. Individuals in the population are distinguished by their types which together with segregated interaction patterns induce heterogeneous preferences with respect to the new behavior. Conditions that determine whether or not behavior can spread and become persistent in the society are identified. The paper on forming opinions in social networks: the effects of strategic interaction presented by Berno Büchel, is his joint work with Tim Hellmann and Stefan Klössner. In this paper, a model of opinion formation based on repeated averaging (DeGroot model) is studied. In contrast to the literature, the authors assume that opinions of other individuals cannot be observed, i.e., they introduce the possibility that agents are dishonest about their true opinion. Assuming that the agents play Nash equilibrium in every period, leads to a substantive extension of the classic DeGroot model. A talk on group connectivity and cooperation was delivered by José Vila (joint work with Angel Sánchez and Amparo Urbano). The authors introduce the concept of high-dimensional connectivity to analyze group connectivity among agents in a network. The key idea is to consider that groups of agents can be linked beyond pair-wise interactions. They associate an abstract simplicial complex to a given network. Joost Vandenbossche presented a paper on paths to stability in the partnership problem. He studies a decentralized matching process for the stable partnership problem, a one-sided many-to-many matching problem. This problem is approached by means of a choice function which is assumed to satisfy substitutability and the increasing property (law of aggregate demand).

Finally, let me also report on my own invited session on Models of Influence and Network Theory (parts I and II, 6 papers). Sylvain Béal presented his joint work with Eric Rémila and Philippe Solal on the number of blocks required to access the coalition structure core. The authors find the number of steps required to switch from a payoff configuration out of the coalition structure core to a payoff configuration in the coalition structure core. This number considerably improves the upper bound found so far in the literature. Another talk was delivered by Alexandre Skoda on convexity of games induced by the strength of weighted communication graphs (joint work with Michel Grabisch). In this paper, the authors study cooperative games associated with a communication structure which takes into account a level of communication between players. They compute the so called strength of a graph and use the corresponding partition to determine a particular coalition structure. Jean-François Caulier presented his joint work with Michel Grabisch and Agnieszka Rusinowska on allocation rules for dynamic random network formation processes. The authors examine the dynamic formation of networks whose evolution across time periods is stochastic. Time-series of networks are studied that describe processes of network formation, where several players or links may appear or disappear at any period. A paper by René van den Brink and Youngsub Chun concerned the balanced consistency and balanced cost reduction for sequencing problems. In a sequencing problem, a group of agents must be served in a facility which can handle only one agent at a time. The authors investigate the implication of two axioms related to the effect of leaving agents: balanced consistency, and balanced cost reduction. A paper presented by Stéphane Robin on a model of ingratiation: an experimental study ishis joint work with Agnieszka Rusinowska and Marie-Claire Villeval. The authors test experimentally a model of ingratiation, in which a decider has to choose one candidate for a promotion, and the reported opinion of a candidate may be different from his preliminarily expressed opinion. I myself presented a paper on a dynamic model of influence based on aggregation functions (joint work with Michel Grabisch). In this work, we consider the stochastic influence mechanism and investigate a model of influence in which each agent aggregates the current opinion of the other agents to update his own opinion.

The organizers of SING7 are quite satisfied with the conference and sincerely hope that the participants could benefit significantly from this meeting. We are all looking forward to the next SING8 conference, which will be organized next year (July 16-18, 2012) in Budapest.




Visit the CTN Members' seminars web pages

CES: http://centredeconomiesorbonne.univ-paris1.fr/bandeau-haut/seminaires/
CORE: http://www.uclouvain.be/en-43617.html
FEEM: http://www.feem.it/getpage.aspx?id=82&sez=Events&padre=21&tab=1
GREQAM: http://www.greqam.fr/spip.php?rubrique13&lang=fr
Maastricht: http://www.fdewb.unimaas.nl/meteor-seminar-et
MOVE: http://www.movebarcelona.eu/index.php/events/upcoming-events
Vanderbilt: http://www.vanderbilt.edu/econ/seminars-and-research/index.html
Warwick: http://www2.warwick.ac.uk/fac/soc/economics/forums/

 

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The CTN Newsletter is prepared with the contribution of all the CTN Partner Institutions. Please send comments and questions to: silvia.bertolin@feem.it.
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