Faliszewski, Piotr


Elections with Few Voters: Candidate Control Can Be Easy

arXiv.org Artificial Intelligence

We study the computational complexity of candidate control in elections with few voters, that is, we consider the parameterized complexity of candidate control in elections with respect to the number of voters as a parameter. We consider both the standard scenario of adding and deleting candidates, where one asks whether a given candidate can become a winner (or, in the destructive case, can be precluded from winning) by adding or deleting few candidates, as well as a combinatorial scenario where adding/deleting a candidate automatically means adding or deleting a whole group of candidates. Considering several fundamental voting rules, our results show that the parameterized complexity of candidate control, with the number of voters as the parameter, is much more varied than in the setting with many voters.


What Do Multiwinner Voting Rules Do? An Experiment Over the Two-Dimensional Euclidean Domain

AAAI Conferences

We visualize aggregate outputs of popular multiwinner voting rules — SNTV, STV, Bloc, k-Borda, Monroe, Chamberlin–Courant, and PAV — for elections generated according to the two-dimensional Euclidean model. We consider three applications of multiwinner voting, namely, parliamentary elections, portfolio/movie selection, and shortlisting, and use our results to understand which of our rules seem to be best suited for each application. In particular, we show that STV (one of the few nontrivial rules used in real high-stake elections) exhibits excellent performance, whereas the Bloc rule (also often used in practice) performs poorly.


Complexity of Shift Bribery in Committee Elections

AAAI Conferences

We study the (parameterized) complexity of Shift Bribery for multiwinner voting rules. We focus on the SNTV, Bloc, k-Borda, and Chamberlin-Courant rules, as well as on approximate variants of the Chamberlin-Courant rule, since the original rule is NP-hard to compute. We show that Shift Bribery tends to be significantly harder in the multiwinner setting than in the single-winner one by showing settings where Shift Bribery is easy in the single-winner cases, but is hard (and hard to approximate) in the multiwinner ones. We show that the non-monotonicity of those rules which are based on approximation algorithms for the Chamberlin--Courant rule sometimes affects the complexity of Shift Bribery.


Multiwinner Analogues of the Plurality Rule: Axiomatic and Algorithmic Perspectives

AAAI Conferences

We characterize the class of committee scoring rules that satisfy the fixed-majority criterion. In some sense, the committee scoring rules in this class are multiwinner analogues of the single-winner Plurality rule, which is uniquely characterized as the only single-winner scoring rule that satisfies the simple majority criterion. We find that, for most of the rules in our new class, the complexity of winner determination is high (i.e., the problem of computing the winners is NP-hard), but we also show some examples of polynomial-time winner determination procedures, exact and approximate.


The Complexity of Manipulative Attacks in Nearly Single-Peaked Electorates (Extended Abstract)

AAAI Conferences

Many electoral control and manipulation problems — which we will refer to in general as manipulative actions problems — are NP-hard in the general case.  Many of these problems fall into polynomial time if the electorate is single-peaked, i.e., is  polarized along some axis/issue. However, real-world electorates are not truly single-peaked — for example, there may be some maverick voters — and to take this into account, we study the complexity of manipulative-action algorithms for  the case of nearly single-peaked electorates.


Elections with Few Voters: Candidate Control Can Be Easy

AAAI Conferences

We study the computational complexity of candidate control in elections with few voters (that is, we take the number of voters as a parameter). We consider both the standard scenario of adding and deleting candidates, where one asks if a given candidate can become a winner (or, in the destructive case, can be precluded from winning) by adding/deleting some candidates, and a combinatorial scenario where adding/deleting a candidate automatically means adding/deleting a whole group of candidates. Our results show that the parameterized complexity of candidate control (with the number of voters as the parameter) is much more varied than in the setting with many voters.


Fully Proportional Representation as Resource Allocation: Approximability Results

AAAI Conferences

We study the complexity of (approximate) winner determination under Monroe's and Chamberlin-Courant's multiwinner voting rules, where we focus on the total (dis)satisfaction of the voters (the utilitarian case) or the (dis)satisfaction of the worst-off voter (the egalitarian case). We show good approximation algorithms for the satisfaction-based utilitarian cases, and inapproximability results for the remaining settings.


Campaign Management under Approval-Driven Voting Rules

AAAI Conferences

Approval-like voting rules, such as Sincere-Strategy Preference-Based Approval voting (SP-AV), the Bucklin rule (an adaptive variant of k-Approval voting), and the Fallback rule (an adaptive variant of SP-AV) have many desirable properties: for example, they are easy to understand and encourage the candidates to choose electoral platforms that have a broad appeal. In this paper, we investigate both classic and parameterized computational complexity of electoral campaign management under such rules. We focus on two methods that can be used to promote a given candidate: asking voters to move this candidate upwards in their preference order or asking them to change the number of candidates they approve of. We show that finding an optimal campaign management strategy of the first type is easy for both Bucklin and Fallback. In contrast, the second method is computationally hard even if the degree to which we need to affect the votes is small. Nevertheless, we identify a large class of scenarios that admit a fixed-parameter tractable algorithm.


Coalitional Voting Manipulation: A Game-Theoretic Perspective

AAAI Conferences

Computational social choice literature has successfully studied the complexity of manipulation in variousvoting systems. However, the existing modelsof coalitional manipulation view the manipulatingcoalition as an exogenous input, ignoring thequestion of the coalition formation process. While such analysis is useful as a first approximation, a richer framework is required to model voting manipulationin the real world more accurately, and, inparticular, to explain how a manipulating coalitionarises and chooses its action. In this paper, we apply tools from cooperative game theory to developa model that considers the coalition formation processand determines which coalitions are likely toform and what actions they are likely to take. We explore the computational complexity of several standard coalitional game theory solution concepts in our setting, and study the relationship betweenour model and the classic coalitional manipulation problem as well as the now-standard bribery model.