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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)

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)

- 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)

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)

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)

- Christian Bessiere, Remi Coletta, +5 authors Toby Walsh
- IJCAI
- 2013

We learn constraint networks by asking the user partial queries. That is, we ask the user to classify assignments to subsets of the variables as positive or negative. We provide an algorithm that, given a negative example, focuses onto a constraint of the target network in a number of queries logarithmic in the size of the example. We give information… (More)

- Jessica Davies, George Katsirelos, Nina Narodytska, Toby Walsh, Lirong Xia
- Artif. Intell.
- 2014

We investigate manipulation of the Borda voting rule, as well as two elimination style voting rules, Nanson's and Baldwin's voting rules, which are based on Borda voting. We argue that these rules have a number of desirable computational properties. For unweighted Borda voting, we prove that it is NP-hard for a coalition of two manipulators to compute a… (More)