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- Jessica Davies, George Katsirelos, Nina Narodytska, Toby Walsh
- AAAI
- 2011

We prove that it is NP-hard for a coalition of two manipu-lators to compute how to manipulate the Borda voting rule. This resolves one of the last open problems in the computational complexity of manipulating common voting rules. Because of this NP-hardness, we treat computing a manipulation as an approximation problem where we try to minimize the number of… (More)

- Nina Narodytska, Fahiem Bacchus
- AAAI
- 2014

Core-guided approaches to solving MAXSAT have proved to be effective on industrial problems. These approaches solve a MAXSAT formula by building a sequence of SAT formulas, where in each formula a greater weight of soft clauses can be relaxed. The soft clauses are relaxed via the addition of blocking variables, and the total weight of soft clauses that can… (More)

We show that some common and important global constraints like ALL-DIFFERENT and GCC can be decomposed into simple arithmetic constraints on which we achieve bound or range consistency, and in some cases even greater pruning. These de-compositions can be easily added to new solvers. They also provide other constraints with access to the state of the… (More)

- Nina Narodytska, Toby Walsh
- ECAI
- 2014

In many real world elections, agents are not required to rank all candidates. We study three of the most common methods used to modify voting rules to deal with such partial votes. These methods modify scoring rules (like the Borda count), elimination style rules (like single transferable vote) and rules based on the tournament graph (like Copeland)… (More)

The SEQUENCE constraint is useful in modelling car sequencing, ros-tering, scheduling and related problems. We introduce half a dozen new encod-ings of the SEQUENCE constraint, some of which do not hinder propagation. We prove that, down a branch of a search tree, domain consistency can be enforced on the SEQUENCE constraint in just O(n 2 log n) time. This… (More)

- Christian Bessiere, George Katsirelos, Nina Narodytska, Toby Walsh
- IJCAI
- 2009

We show that tools from circuit complexity can be used to study decompositions of global constraints. In particular, we study decompositions of global constraints into conjunctive normal form with the property that unit propagation on the decomposition enforces the same level of consistency as a specialized propagation algorithm. We prove that a constraint… (More)

We propose new filtering algorithms for the SEQUENCE constraint and some extensions of the SEQUENCE constraint based on network flows. We enforce domain consistency on the SEQUENCE constraint in O(n 2) time down a branch of the search tree. This improves upon the best existing domain consistency algorithm by a factor of O(log n). The flows used in these… (More)

We study propagation algorithms for the conjunction of two ALLDIFFERENT constraints. Solutions of an ALLDIFFERENT constraint can be seen as perfect matchings on the variable/value bipartite graph. Therefore, we investigate the problem of finding simultaneous bipartite match-ings. We present an extension of the famous Hall theorem which characterizes when… (More)

We revisit the SEQBIN constraint [1]. This meta-constraint subsumes a number of important global constraints like CHANGE [2], SMOOTH [3] and INCREASINGNVALUE [4]. We show that the previously proposed filtering algorithm for SEQBIN has two drawbacks even under strong restrictions: it does not detect bounds disentailment and it is not idempotent. We identify… (More)

- Andrea Loreggia, Nina Narodytska, Francesca Rossi, Kristen Brent Venable, Toby Walsh
- MPREF@AAAI
- 2014

We consider elections where the chair may attempt to influence the result by replacing candidates with the intention to make a specific candidate lose (destructive control). We call this form of control " replacement control " and we study its computational complexity. We focus in particular on Plural-ity and Veto, for which we prove that destructive… (More)