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Jordi Levy

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28 papers
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28

SAT Conference 2025 Conference Paper

An Algebraic Approach to MaxCSP

  • Ilario Bonacina
  • Jordi Levy

Recently, there have been some attempts to base SAT and MaxSAT solvers on calculi beyond Resolution, even trying to solve SAT using MaxSAT proof systems. One of these directions was to perform MaxSAT sound inferences using polynomials over finite fields, extending the proof system Polynomial Calculus, which is known to be more powerful than Resolution. In this work, we extend the use of the Polynomial Calculus for optimization, showing its completeness over many-valued variables. This allows using a more direct and efficient encoding of CSP problems (e. g. , k-Coloring) and solving the maximization version of the problem on such encoding (e. g. , Max-k-Coloring).

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JAIR Journal 2024 Journal Article

Weighted, Circular and Semi-Algebraic Proofs

  • Ilario Bonacina
  • Maria Luisa Bonet
  • Jordi Levy

In recent years there has been an increasing interest in studying proof systems stronger than Resolution, with the aim of building more efficient SAT solvers based on them. In defining these proof systems, we try to find a balance between the power of the proof system (the size of the proofs required to refute a formula) and the difficulty of finding the proofs. In this paper we consider the proof systems circular Resolution, Sherali-Adams, Nullstellensatz and Weighted Resolution and we study their relative power from a theoretical perspective. We prove that circular Resolution, Sherali-Adams and Weighted Resolution are polynomially equivalent proof systems. We also prove that Nullstellensatz is polynomially equivalent to a restricted version of Weighted Resolution. The equivalences carry on also for versions of the systems where the coefficients/weights are expressed in unary. The practical interest in these systems comes from the fact that they admit efficient algorithms to find proofs in case these have small width/degree.

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IJCAI Conference 2024 Conference Paper

Weighted, Circular and Semi-Algebraic Proofs (Abstract Reprint)

  • Ilario Bonacina
  • Maria Luisa Bonet
  • Jordi Levy

In recent years there has been an increasing interest in studying proof systems stronger than Resolution, with the aim of building more efficient SAT solvers based on them. In defining these proof systems, we try to find a balance between the power of the proof system (the size of the proofs required to refute a formula) and the difficulty of finding the proofs. In this paper we consider the proof systems circular Resolution, Sherali-Adams, Nullstellensatz and Weighted Resolution and we study their relative power from a theoretical perspective. We prove that circular Resolution, Sherali-Adams and Weighted Resolution are polynomially equivalent proof systems. We also prove that Nullstellensatz is polynomially equivalent to a restricted version of Weighted Resolution. The equivalences carry on also for versions of the systems where the coefficients/weights are expressed in unary. The practical interest in these systems comes from the fact that they admit efficient algorithms to find proofs in case these have small width/degree.

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SAT Conference 2023 Conference Paper

Polynomial Calculus for MaxSAT

  • Ilario Bonacina
  • Maria Luisa Bonet
  • Jordi Levy

MaxSAT is the problem of finding an assignment satisfying the maximum number of clauses in a CNF formula. We consider a natural generalization of this problem to generic sets of polynomials and propose a weighted version of Polynomial Calculus to address this problem. Weighted Polynomial Calculus is a natural generalization of MaxSAT-Resolution and weighted Resolution that manipulates polynomials with coefficients in a finite field and either weights in ℕ or ℤ. We show the soundness and completeness of these systems via an algorithmic procedure. Weighted Polynomial Calculus, with weights in ℕ and coefficients in 𝔽₂, is able to prove efficiently that Tseitin formulas on a connected graph are minimally unsatisfiable. Using weights in ℤ, it also proves efficiently that the Pigeonhole Principle is minimally unsatisfiable.

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AAMAS Conference 2021 Conference Paper

A Heuristic Algorithm for Multi-Agent Vehicle Routing with Automated Negotiation

  • Dave de Jonge
  • Filippo Bistaffa
  • Jordi Levy

We investigate a problem that lies at the intersection of three research areas, namely Automated Negotiation, Vehicle Routing, and Multi-Objective Optimization. Specifically, we investigate the scenario that multiple competing logistics companies aim to cooperate by delivering truck loads for one another, in order to improve efficiency and reduce the distance they drive. In order to do so, these companies need to find ways to exchange their truck loads such that each of them individually benefits. We present a new heuristic algorithm that, given one set of orders to deliver for each company, tries to find the set of all order-exchanges that are Pareto-optimal and individually rational. Furthermore, we present experiments based on real-world test data from two major logistics companies, which show that our algorithm is able to find hundreds of solutions in a matter of minutes.

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IJCAI Conference 2021 Conference Paper

Reducing SAT to Max2SAT

  • Carlos Ansótegui
  • Jordi Levy

In the literature, we find reductions from 3SAT to Max2SAT. These reductions are based on the usage of a gadget, i. e. , a combinatorial structure that allows translating constraints of one problem to constraints of another. Unfortunately, the generation of these gadgets lacks an intuitive or efficient method. In this paper, we provide an efficient and constructive method for Reducing SAT to Max2SAT and show empirical results of how MaxSAT solvers are more efficient than SAT solvers solving the translation of hard formulas for Resolution.

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SODA Conference 2021 Conference Paper

The Impact of Heterogeneity and Geometry on the Proof Complexity of Random Satisfiability

  • Thomas Bläsius
  • Tobias Friedrich 0001
  • Andreas Göbel 0001
  • Jordi Levy
  • Ralf Rothenberger

Satisfiability is considered the canonical NP-complete problem and is used as a starting point for hardness reductions in theory, while in practice heuristic SAT solving algorithms can solve large-scale industrial SAT instances very efficiently. This disparity between theory and practice is believed to be a result of inherent properties of industrial SAT instances that make them tractable. Two characteristic properties seem to be prevalent in the majority of real-world SAT instances, heterogeneous degree distribution and locality. To understand the impact of these two properties on SAT, we study the proof complexity of random k -SAT models that allow to control heterogeneity and locality. Our findings show that heterogeneity alone does not make SAT easy as heterogeneous random k -SAT instances have superpolynomial resolution size. This implies intractability of these instances for modern SAT-solvers. On the other hand, modeling locality with an underlying geometry leads to small unsatisfiable subformulas, which can be found within polynomial time. A key ingredient for the result on geometric random k -SAT can be found in the complexity of higher-order Voronoi diagrams. As an additional technical contribution, we show an upper bound on the number of non-empty Voronoi regions, that holds for points with random positions in a very general setting. In particular, it covers arbitrary p-norms, higher dimensions, and weights affecting the area of influence of each point multiplicatively. Our bound is linear in the total weight. This is in stark contrast to quadratic lower bounds for the worst case.

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SAT Conference 2020 Conference Paper

Equivalence Between Systems Stronger Than Resolution

  • Maria Luisa Bonet
  • Jordi Levy

Abstract In recent years there has been an increasing interest in studying proof systems stronger than Resolution, with the aim of building more efficient SAT solvers based on them. In defining these proof systems, we try to find a balance between the power of the proof system (the size of the proofs required to refute a formula) and the difficulty of finding the proofs. Among those proof systems we can mention Circular Resolution, MaxSAT Resolution with Extensions and MaxSAT Resolution with the Dual-Rail encoding. In this paper we study the relative power of those proof systems from a theoretical perspective. We prove that Circular Resolution and MaxSAT Resolution with extension are polynomially equivalent proof systems. This result is generalized to arbitrary sets of inference rules with proof constructions based on circular graphs or based on weighted clauses. We also prove that when we restrict the Split rule (that both systems use) to bounded size clauses, these two restricted systems are also equivalent. Finally, we show the relationship between these two restricted systems and Dual-Rail MaxSAT Resolution.

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JAIR Journal 2019 Journal Article

Community Structure in Industrial SAT Instances

  • Carlos Ansótegui
  • Maria Luisa Bonet
  • Jesús Giráldez-Cru
  • Jordi Levy
  • Laurent Simon

Modern SAT solvers have experienced a remarkable progress on solving industrial instances. It is believed that most of these successful techniques exploit the underlying structure of industrial instances. Recently, there have been some attempts to analyze the structure of industrial SAT instances in terms of complex networks, with the aim of explaining the success of SAT solving techniques, and possibly improving them. In this paper, we study the community structure, or modularity, of industrial SAT instances. In a graph with clear community structure, or high modularity, we can find a partition of its nodes into communities such that most edges connect variables of the same community. Representing SAT instances as graphs, we show that most application benchmarks are characterized by a high modularity. On the contrary, random SAT instances are closer to the classical Erdös-Rényi random graph model, where no structure can be observed. We also analyze how this structure evolves by the effects of the execution of a CDCL SAT solver, and observe that new clauses learned by the solver during the search contribute to destroy the original structure of the formula. Motivated by this observation, we finally present an application that exploits the community structure to detect relevant learned clauses, and we show that detecting these clauses results in an improvement on the performance of the SAT solver. Empirically, we observe that this improves the performance of several SAT solvers on industrial SAT formulas, especially on satisfiable instances.

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IJCAI Conference 2017 Conference Paper

Locality in Random SAT Instances

  • Jesús Giráldez-Cru
  • Jordi Levy

Despite the success of CDCL SAT solvers solving industrial problems, there are still many open questions to explain such success. In this context, the generation of random SAT instances having computational properties more similar to real-world problems becomes crucial. Such generators are possibly the best tool to analyze families of instances and solvers behaviors on them. In this paper, we present a random SAT instances generator based on the notion of locality. We show that this is a decisive dimension of attractiveness among the variables of a formula, and how CDCL SAT solvers take advantage of it. To the best of our knowledge, this is the first random SAT model that generates both scale-free structure and community structure at once.

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LOPSTR Conference 2016 Conference Paper

Nominal Unification of Higher Order Expressions with Recursive Let

  • Manfred Schmidt-Schauß
  • Temur Kutsia
  • Jordi Levy
  • Mateu Villaret

Abstract A sound and complete algorithm for nominal unification of higher-order expressions with a recursive let is described, and shown to run in non-deterministic polynomial time. We also explore specializations like nominal letrec-matching for plain expressions and for DAGs and determine their complexity.

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IJCAI Conference 2015 Conference Paper

A Modularity-Based Random SAT Instances Generator

  • Jes
  • uacute; s Gir
  • aacute; ldez-Cru
  • Jordi Levy

Nowadays, many industrial SAT instances can be solved efficiently by modern SAT solvers. However, the number of real-world instances is finite. Therefore, the process of development and test of SAT solving techniques can benefit of new models of random formulas that capture more realistically the features of real-world problems. In many works, the structure of industrial instances has been analyzed representing them as graphs and studying some of their properties, like modularity. In this paper, we use modularity, or community structure, to define a new model of pseudoindustrial random SAT instances, called Community Attachment. We prove that the phase transition point, if exists, is independent on the modularity. We evaluate the adequacy of this model to real industrial problems in terms of SAT solvers performance, and show that modern solvers do actually exploit this community structure.

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SAT Conference 2015 Conference Paper

Using Community Structure to Detect Relevant Learnt Clauses

  • Carlos Ansótegui
  • Jesús Giráldez-Cru
  • Jordi Levy
  • Laurent Simon 0001

Abstract Nowadays, Conflict-Driven Clause Learning (CDCL) techniques are one of the key components of modern SAT solvers specialized in industrial instances. Last years, one of the focuses has been put on strategies to select which learnt clauses are removed during the search. Originally, one need for removing clauses was motivated by the finiteness of memory. Recently, it has been shown that more aggressive clause deletion policies may improve solvers performance, even when memory is sufficient. Also, the utility of learnt clauses has been related to the modular structure of industrial SAT instances. In this paper, we show that augmenting SAT instances with learnt clauses does not always make them easier for the SAT solver. In fact, it makes worse the solver performance in many cases. However, we identify a set of highly useful learnt clauses, and we show that augmenting SAT instances with this set of clauses contributes to improve the solver performance in many cases, especially in satisfiable formulas. These clauses are related to the community structure of the formula, and they can be computed in a fast preprocessing step. This would suggest that the community structure may play an important role in clause deletion policies.

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SAT Conference 2012 Conference Paper

The Community Structure of SAT Formulas

  • Carlos Ansótegui
  • Jesús Giráldez-Cru
  • Jordi Levy

Abstract The research community on complex networks has developed techniques of analysis and algorithms that can be used by the SAT community to improve our knowledge about the structure of industrial SAT instances. It is often argued that modern SAT solvers are able to exploit this hidden structure, without a precise definition of this notion. In this paper, we show that most industrial SAT instances have a high modularity that is not present in random instances. We also show that successful techniques, like learning, (indirectly) take into account this community structure. Our experimental study reveal that most learnt clauses are local on one of those modules or communities.

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AAAI Conference 2010 Conference Paper

A New Algorithm for Weighted Partial MaxSAT

  • Carlos Ansotegui
  • Maria Luisa Bonet
  • Jordi Levy

We present and implement a Weighted Partial MaxSAT solver based on successive calls to a SAT solver. We prove the correctness of our algorithm and compare our solver with other Weighted Partial MaxSAT solvers.

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IJCAI Conference 2009 Conference Paper

  • Carlos Ansótegui
  • Maria Luisa Bonet
  • Jordi Levy

We focus on the random generation of SAT instances that have computational properties that are similar to real-world instances. It is known that industrial instances, even with a great number of variables, can be solved by a clever solver in a reasonable amount of time. This is not possible, in general, with classical randomly generated instances. We provide different generation models of SAT instances, extending the uniform and regular 3-CNF models. They are based on the use of non-uniform probability distributions to select variables. Our last model also uses a mechanism to produce clauses of different lengths as in industrial instances. We show the existence of the phase transition phenomena for our models and we study the hardness of the generated instances as a function of the parameters of the probability distributions. We prove that, with these parameters we can adjust the difficulty of the problems in the phase transition point. We measure hardness in terms of the performance of different solvers. We show how these models will allow us to generate random instances similar to industrial instances, of interest for testing purposes.

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SAT Conference 2009 Conference Paper

Solving (Weighted) Partial MaxSAT through Satisfiability Testing

  • Carlos Ansótegui
  • Maria Luisa Bonet
  • Jordi Levy

Abstract Recently, Fu and Malik described an unweighted Partial MaxSAT solver based on successive calls to a SAT solver. At the k th iteration the SAT solver tries to certify that there exist an assignment that satisfies all but k clauses. Later Marques-Silva and Planes implemented and extended these ideas. In this paper we present and implement two Partial MaxSAT solvers and the weighted variant of one of them. Both are based on Fu and Malik ideas. We prove the correctness of our algorithm and compare our solver with other (Weighted) MaxSAT and (Weighted) Partial MaxSAT solvers.

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AAAI Conference 2008 Conference Paper

Measuring the Hardness of SAT Instances

  • Carlos Ansótegui
  • Jordi Levy

The search of a precise measure of what hardness of SAT instances means for state-of-the-art solvers is a relevant research question. Among others, the space complexity of treelike resolution (also called hardness), the minimal size of strong backdoors and of cycle-cutsets, and the treewidth can be used for this purpose. We propose the use of the tree-like space complexity as a solid candidate to be the best measure for solvers based on DPLL. To support this thesis we provide a comparison with the other mentioned measures. We also conduct an experimental investigation to show how the proposed measure characterizes the hardness of random and industrial instances.

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IJCAI Conference 2007 Conference Paper

  • Carlos Ansotegui
  • Maria Luisa Bonet
  • Jordi Levy
  • Felip Manya

We define a translation from Weighted CSP to signed Max-SAT, and a complete resolution-style calculus for solving signed Max-SAT. Based on these results, we then describe an original exact algorithm for solving Weighted CSP. Finally, we define several derived rules and prove that they enforce the main soft arc consistency defined in the literature when applied to Weighted CSP instances.

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AAAI Conference 2007 Conference Paper

Inference Rules for High-Order Consistency in Weighted CSP

  • Carlos Ansotegui
  • Jordi Levy

Recently defined resolution calculi for Max-SAT and signed Max-SAT have provided a logical characterization of the solving techniques applied by Max-SAT and WCSP solvers. In this paper we first define a new resolution rule, called signed Max-SAT parallel resolution, and prove that it is sound and complete for signed Max-SAT. Second, we define a restriction and a generalization of the previous rule called, respectively, signed Max-SAT i-consistency resolution and signed Max-SAT (i, j)-consistency resolution. These rules have the following property: if a WCSP signed encoding is closed under signed Max-SAT i-consistency, then the WCSP is i-consistent, and if it is closed under signed Max-SAT (i, j)-consistency, then the WCSP is (i, j)-consistent. A new and practical insight derived from the definition of these new rules is that algorithms for enforcing high order consistency should incorporate an efficient and effective component for detecting minimal unsatisfiable cores. Finally, we describe an algorithm that applies directional soft consistency with the previous rules.

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SAT Conference 2007 Conference Paper

Mapping CSP into Many-Valued SAT

  • Carlos Ansótegui
  • Maria Luisa Bonet
  • Jordi Levy
  • Felip Manyà

Abstract We first define a mapping from CSP to many-valued SAT which allows to solve CSP instances with many-valued SAT solvers. Second, we define a new many-valued resolution rule and prove that it is refutation complete for many-valued CNF formulas and, moreover, enforces CSP ( i, j )-consistency when applied to a many-valued SAT encoding of a CSP. Instances of our rule enforce well-known local consistency properties such as arc consistency and path consistency.

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SAT Conference 2006 Conference Paper

A Complete Calculus for Max-SAT

  • Maria Luisa Bonet
  • Jordi Levy
  • Felip Manyà

Abstract Max-SAT is the problem of finding an assignment minimizing the number of unsatisfied clauses of a given CNF formula. We propose a resolution-like calculus for Max-SAT and prove its soundness and completeness. We also prove the completeness of some refinements of this calculus. From the completeness proof we derive an exact algorithm for Max-SAT and a time upper bound.

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