Structural Control in Weighted Voting Games

Inspired by the study of control scenarios in elections and complementing manipulation and bribery settings in cooperative games with transferable utility, we introduce the notion of structural control in weighted voting games. We model two types of influence, adding players to and deleting players from a game, with goals such as increasing a given player\u27s Shapley-Shubik or probabilistic Penrose-Banzhaf index in relation to the original game. We study the computational complexity of the problems of whether such structural changes can achieve the desired effect

Equilibrium refinement through negotiation in binary voting

We study voting games on binary issues, where voters might hold an objective over some issues at stake, while willing to strike deals on the remaining ones, and can influence one another’s voting decision before the vote takes place. We analyse voters’ rational behaviour in the resulting two-phase game, showing under what conditions undesirable equilibria can be removed as an effect of the prevote phase

Complexity Results for Manipulation, Bribery and Control of the Kemeny Judgment Aggregation Procedure

We study the computational complexity of several scenarios of strategic behavior for the Kemeny procedure in the setting of judgment aggregation. In particular, we investigate (1) manipulation, where an individual aims to achieve a better group outcome by reporting an insincere individual opinion, (2) bribery, where an external agent aims to achieve an outcome with certain properties by bribing a number of individuals, and (3) control (by adding or deleting issues), where an external agent aims to achieve an outcome with certain properties by influencing the set of issues in the judgment aggregation situation. We show that determining whether these types of strategic behavior are possible (and if so, computing a policy for successful strategic behavior) is complete for the second level of the Polynomial Hierarchy. That is, we show that these problems are $\Sigma^p_2$-complete

Egalitarian judgment aggregation

Egalitarian considerations play a central role in many areas of social choice theory. Applications of egalitarian principles range from ensuring everyone gets an equal share of a cake when deciding how to divide it, to guaranteeing balance with respect to gender or ethnicity in committee elections. Yet, the egalitarian approach has received little attention in judgment aggregation—a powerful framework for aggregating logically interconnected issues. We make the first steps towards filling that gap. We introduce axioms capturing two classical interpretations of egalitarianism in judgment aggregation and situate these within the context of existing axioms in the pertinent framework of belief merging. We then explore the relationship between these axioms and several notions of strategyproofness from social choice theory at large. Finally, a novel egalitarian judgment aggregation rule stems from our analysis; we present complexity results concerning both outcome determination and strategic manipulation for that rule.publishedVersio

Iterative Voting, Control and Sentiment Analysis

In multi-agent systems agents often need to take a collective decision based on the preferences of individuals. A voting rule is used to decide which decision to take, mapping the agents' preferences over the possible candidate decisions into a winning decision for the collection of agents. In these kind of scenarios acting strategically can be seen in two opposite way. On one hand it may be desirable that agents do not have any incentive to act strategically. That is, to misreport their preferences in order to influence the result of the voting rule in their favor or acting on the structure of the election to change the outcome. On the other hand manipulation can be used to improve the quality of the outcome by enlarging the consensus of the winner. These two different scenarios are studied in this thesis. The first one by modeling and describing a natural form of control named ``replacement control'' and characterizing for several voting rules its computational complexity. The second scenario is studied in the form of iterative voting frameworks where individuals are allowed to change their preferences to change the outcome of the election. Computational social choice techniques can be used in very different scenarios. This work reports a first attempt to introduce the use of voting procedures in the field of sentiment analysis. In this area computer scientists extract the opinion of the community about a specific item. This opinion is extracted aggregating the opinion expressed by each individual which leaves a text in a blog or social network about the given item. We studied and proposed a new aggregation method which can improve performances of sentiment analysis, this new technique is a new variance of a well-known voting rule called Borda