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## Publications

Publications (68)

We study the complexity of the Distributed Constraint Satisfaction Problem (DCSP) on a synchronous, anonymous network from a theoretical standpoint. In this setting, variables and constraints are controlled by agents which communicate with each other by sending messages through fixed communication channels. Our results endorse the well-known fact f...

The fitting problem for conjunctive queries (CQs) is the problem to construct a CQ that fits a given set of labeled data examples. When a fitting CQ exists, it is in general not unique. This leads us to proposing natural refinements of the notion of a fitting CQ, such as most-general fitting CQ, most-specific fitting CQ, and unique fitting CQ. We g...

We study the power of the bounded-width consistency algorithm in the context of the fixed-template Promise Constraint Satisfaction Problem (PCSP). Our main technical finding is that the template of every PCSP that is solvable in bounded width satisfies a certain structural condition implying that its algebraic closure-properties include weak near u...

Given a pair of graphs $\textbf{A}$ and $\textbf{B}$, the problems of deciding whether there exists either a homomorphism or an isomorphism from $\textbf{A}$ to $\textbf{B}$ have received a lot of attention. While graph homomorphism is known to be NP-complete, the complexity of the graph isomorphism problem is not fully understood. A well-known com...

The Constraint Satisfaction Problem (CSP) and its counting counterpart appears under different guises in many areas of mathematics, computer science, and elsewhere. Its structural and algorithmic properties have been shown to play a crucial role in many of those applications. For instance, in the decision CSPs, structural properties of the relation...

We consider the feature-generation task wherein we are given a database with entities labeled as positive and negative examples, and we want to find feature queries that linearly separate the two sets of examples. We focus on conjunctive feature queries, and explore two problems: (a) deciding if separating feature queries exist (separability), and...

We answer the question which conjunctive queries are uniquely characterized by polynomially many positive and negative examples, and how to construct such examples efficiently. As a consequence, we obtain a new efficient exact learning algorithm for a class of conjunctive queries. At the core of our contributions lie two new polynomial-time algorit...

We study the complexity of the Distributed Constraint Satisfaction Problem (DCSP) on a synchronous, anonymous network from a theoretical standpoint. In this setting, variables and constraints are controlled by agents which communicate with each other by sending messages through fixed communication channels. Our results endorse the well-known fact f...

Ontology-mediated querying and querying in the presence of constraints are two key database problems where tuple-generating dependencies (TGDs) play a central role. In ontology-mediated querying, TGDs can formalize the ontology and thus derive additional facts from the given data, while in querying in the presence of constraints, they restrict the...

In this paper we study alternative characterizations of dismantlability properties of relational structures in terms of various connectedness and mixing notions. We relate these results with earlier work of Brightwell and Winkler, providing a generalization from the graph case to the general relational structure context. In addition, we develop pro...

We study the approximability of (Finite-)Valued Constraint Satisfaction Problems (VCSPs) with a fixed finite constraint language Γ consisting of finitary functions on a fixed finite domain. Ene et al. have shown that, under a mild technical condition, the basic LP relaxation is optimal for constant-factor approximation for VCSP(Γ) unless the Unique...

An instance of the Constraint Satisfaction Problem (CSP) is given by a family of constraints on overlapping sets of variables, and the goal is to assign values from a fixed domain to the variables so that all constraints are satisfied. In the optimization version, the goal is to maximize the number of satisfied constraints. An approximation algorit...

An instance of the Constraint Satisfaction Problem (CSP) is given by a family of constraints on overlapping sets of variables, and the goal is to assign values from a fixed domain to the variables so that all constraints are satisfied. In the optimization version, the goal is to maximize the number of satisfied constraints. An approximation algorit...

We study the complexity of constraint satisfaction problems for templates ? over the integers where the relations are first-order definable from the successor function. In the case that ? is locally finite (i.e., the Gaifman graph of ? has finite degree), we show that ? is homomorphically equivalent to a structure with one of two classes of polymor...

An algorithm for a constraint satisfaction problem is called robust if it outputs an assignment satisfying at least a (1 − f(ε))-fraction of constraints for each (1 − ε)-satisfiable instance (i.e., such that at most a ε-fraction of constraints needs to be removed to make the instance satisfiable), where f(ε) → 0 as ε → 0. We establish an algebraic...

The product homomorphism problem (PHP) takes as input a finite collection of
relational structures A1, ..., An and another relational structure B, all over
the same schema, and asks whether there is a homomorphism from the direct
product A1 x ... x An to B. This problem is clearly solvable in
non-deterministic exponential time. It follows from resu...

Model checking-deciding if a logical sentence holds on a structure-is a basic computational task that is well-known to be intractable in general. For first-order logic on finite structures, it is PSPACE-complete, and the natural evaluation algorithm exhibits exponential dependence on the formula. We study model checking on the quantified conjunctiv...

A schema mapping is a high-level specification of the relationship between a source schema and a target schema. Recently, a line of research has emerged that aims at deriving schema mappings automatically or semi-automatically with the help of data examples, that is, pairs consisting of a source instance and a target instance that depict, in some p...

The study of constraint satisfaction problems (CSPs) definable in various fragments of Datalog has recently gained considerable
importance.We consider CSPs that are definable in the smallest natural recursive fragment of Datalog—monadic linear Datalog
with at most one EDB (extensional database predicates) per rule, and also in the smallest nonlinea...

A natural and established way to restrict the constraint satisfaction problem
is to fix the relations that can be used to pose constraints; such a family of
relations is called a constraint language. In this article, we study arc
consistency, a heavily investigated inference method, and three extensions
thereof from the perspective of constraint la...

We study non-uniform constraint satisfaction problems definable in monadic Datalog stratified by the use of non-linearity. We show how such problems can be described in terms of homomorphism dualities involving trees of bounded pathwidth and in algebraic terms. For this, we introduce a new parameter for trees that closely approximates pathwidth and...

We study the complexity of constraint satisfaction problems for templates
Gamma that are first-order definable in (Z; succ), the integers with the
successor relation. Assuming a widely believed conjecture from finite domain
constraint satisfaction (we require the tractability conjecture by Bulatov,
Jeavons and Krokhin in the special case of transit...

The homomorphism problem for relational structures is an abstract way of formulating constraint satisfaction problems (CSP) and various problems in database theory. The decision version of the homomorphism problem received a lot of attention in literature; in particular, the way the graph-theoretical structure of the variables and constraints influ...

In this note, we show that every constraint satisfaction problem that has relational width 2 has also relational width 1. This is achieved by means of an obstruction-like characterization of relational width which we believe to be of independent interest.

We prove that the constraint languages invariant under a short sequence of Jónsson terms (containing at most three non-trivial
ternary terms) are tractable by showing that they have bounded width. This improves a previous result by Kiss and Valeriote
and presents some evidence that the Larose–Zádori conjecture holds in the congruence-distributive c...

On finite structures, there is a well-known connection between the expressive
power of Datalog, finite variable logics, the existential pebble game, and
bounded hypertree duality. We study this connection for infinite structures.
This has applications for constraint satisfaction with infinite templates. If
the template Gamma is omega-categorical, w...

The study of constraint satisfaction problems definable in various fragments of Datalog has recently gained considerable importance. We consider constraint satisfaction problems that are definable in the smallest natural recursive fragment of Datalog - monadic linear Datalog with at most one EDB per rule. We give combinatorial and algebraic charact...

Let B be a finite, core relational structure and let A be the algebra associated to B, i.e. whose terms are the operations on the universe of B that preserve the relations of B. We show that if A generates a so-called arithmetical variety then CSP(B), the constraint satisfaction problem associated to B, is solvable in Logspace; in fact notCSP(B) is...

The poset retraction problem for a poset P is whether a given poset Q containing P as a subposet admits a retraction onto P, that is, whether there is a homomorphism from Q onto P which fixes every element of P. We study this problem for finite series-parallel posets P. We present equivalent combinatorial, algebraic, and topological charaterisation...

We study certain constraint satisfaction problems which are the problems of deciding whether there exists a homomorphism from a given relational structure to a fixed structure with a majority polymorphism. We show that such a problem is equivalent to deciding whether the given structure admits a homomorphism from an obstruction belonging to a certa...

We provide a characterization of the resolution width introduced in the context of propositional proof complexity in terms of the existential pebble game introduced in the context of finite model theory. The characterization is tight and purely combinatorial. Our first application of this result is a surprising proof that the minimum space of refut...

Intersection-closed classes of concepts arise naturally in many contexts and have been intensively studied in computational learning theory. In this paper, we study intersection-closed classes that contain the concepts invariant under an operation satisfying a certain algebraic condition. We give a learning algorithm in the exact model with equival...

The k-consistency algorithm for constraint-satisfaction problems proceeds, roughly, by finding all partial solutions on at most
k variables and iteratively deleting those that cannot be extended to a partial solution by one more variable. It is known
that if the core of the structure encoding the scopes of the constraints has treewidth at most k, t...

The complexity class PP consists of all decision problems solvable by polynomial-time probabilistic Turing machines. It is well known that PP is a highly intractable complexity class and that PP-complete problems are in all likelihood harder than NP-complete problems. We investigate the existence of phase transitions for a family of PP-complete Boo...

The Counting Constraint Satisfaction Problem (#CSP) can be expressed as follows: given a set of variables, a set of values that can be taken by the variables, and a set of constraints specifying some restrictions on the values that can be taken simultaneously by some variables, determine the number of assignments of values to variables that satisfy...

Generalized majority-minority (GMM) operations are introduced as a common
generalization of near unanimity operations and Mal'tsev operations on finite
sets. We show that every instance of the constraint satisfaction problem (CSP),
where all constraint relations are invariant under a (fixed) GMM operation, is
solvable in polynomial time. This const...

A Mal'tsev operation is a ternary operation $\varphi$ that satisfies the identities $\varphi(x,y,y) = \varphi(y,y,x) = x$. Constraint satisfaction problems involving constraints invariant under a Mal'tsev operation constitute an important class of constraint satisfaction problems, which includes the affine satisfiability problem, subgroup and near...

We contribute to the algebraic study of the complexity of constraint satisfaction problems. We give a new sufficient condition
on a set of relations Γ over a domain S for the tractability of CSP(Γ): if S is a block-group (a particular class of semigroups) of exponent ω and Γ is a set of relations over S preserved by the operation defined by the pol...

The constraint satisfaction problem (CSP) and quantified constraint satisfaction problem (QCSP) are common frameworks for
the modelling of computational problems. Although they are intractable in general, a rich line of research has identified
restricted cases of these problems that are tractable in polynomial time. Remarkably, many tractable cases...

Let A be a finite set and let φ : A<sup>k</sup>→A with k≥3 be a k-ary operation on A. We say that φ is a generalized majority-minority (GMM) operation if for all a, b ∈ A we have that φ(x, y,...,y) = φ(y, x,..,y) =...=φ(y, y,..,x) = y for all x, y ∈ {a, b} or φ{x, y,..,y) = φ(y, y,..,x) = x for all x, y ∈ {a, b}. Near-unanimity and Mal'tsev operati...

The general intractability of the constraint satisfaction problem has motivated the study of restrictions on this problem that permit polynomial-time solvability. One major line of work has focused on structural restrictions, which arise from restricting the interaction among constraint scopes. In this paper, we engage in a mathematical investigati...

In this paper we systematically investigate the connections between logics
with a finite number of variables, structures of bounded pathwidth, and linear
Datalog Programs. We prove that, in the context of Constraint Satisfaction
Problems, all these concepts correspond to different mathematical embodiments
of a unique robust notion that we call boun...

In this paper we consider constraint satisfaction problems where the set of constraint relations is fixed. Feder and Vardi (1998) identified three families of constraint satisfaction problems containing all known polynomially solvable problems. We introduce a new class of problems called para-primal problems, incomparable with the families identifi...

For every class of relational structures C, let HOM(C,_) be the problem of deciding whether a structure A∈C has a homomorphism to a given arbitrary structure B. Grohe has proved that, under a certain complexity-theoretic assumption, HOM(C,_) is solvable in polynomial time if and only if the cores of all structures in C have bounded tree-width. We p...

In this paper we study the learning complexity of a vast class of quantifed formulas called Relatively Quantified Generalized Formulas. This class of formulas is parameterized by a set of predicates, called a basis. We give a complete classification theorem,
showing that every basis gives rise to quantified formulas that are either polynomially lea...

The constraint satisfaction problem (CSP) can be formu- lated as the problem of deciding, given a pair (A; B) of relational struc- tures, whether or not there is a homomorphism from A to B. Although the CSP is in general intractable, it may be restricted by requiring the "target structure" B to be xed; denote this restriction by CSP(B). In recent y...

A retraction from a structure P to its substructure Q is a homomorphism from P onto Q that is the identity on Q. We present an algebraic condition which completely characterises all posets and all reflexive graphs Q with the following property: the class of all posets or reflexive graphs, respectively, that admit a retraction onto Q is first-order...

We make use of the algebraic theory that has been used to study the complexity of constraint satisfaction problems, to investigate tractable quantified boolean formulas. We present a pair of results: the first is a new and simple algebraic proof of the tractability of quantified 2-satisfiability; the second is a purely algebraic characterization of...

Abstract A retraction from a structure P to its substructure Q is a homomorphism,from P onto Q that is the identity on Q. We present an algebraic condition which completely characterises all posets and all reflexive graphs Q such that the class of all posets or reflexive graphs, respectively, that admit a retraction onto Q is first-order definable....

The Counting Constraint Satisfaction Problem (#CSP) over a finite domain can be expressed as follows: given a first-order formula consisting of a conjunction of predicates, determine the number of satisfying assignments to the formula. #CSP can be parametrized by the set of allowed constraint predicates. In this paper we start a systematic study of...

In this paper we examine generalized satisfiability problems with limited variable occurrences. First, we show that 3 occurrences
per variable suffice to make these problems as hard as their unrestricted version. Then we focus on generalized satisfiability
problems with at most 2 occurrences per variable. It is known that some NP -complete general...

Abstract We provide a characterization of the resolution width introduced in the context of propositional proof complexity in terms of the existential pebble game introduced in the context of finite model theory. The characterization is tight and purely combinatorial. Our first application of this result is a surprising proof that the minimum,space...

We systematically investigate the connections between constraint sat- isfaction problems, structures of bounded treewidth, and d efinability in logics with a finite number of variables. We first show that constrain t satisfaction prob- lems on inputs of treewidth less than are definable using Datalog programs with at most variables; this provides a...

We study which constraint satisfaction problems (CSPs) are solvable in NL. In particular, we identify a general condition called bounded path duality, that explains all the families of CSPs previously known to be in NL. Bounded path duality captures the class of constraint satisfaction problems that can be solved by linear Datalog programs, i.e., D...

The study of phase transitions in algorithmic problems has revealed that usually the critical value of the constrainedness parameter at which the phase transition occurs coincides with the value at which the average cost of natural solvers for the problem peaks. In particular, this confluence of phase tran- sition and peak cost has been observed fo...

. In this paper we consider constraint satisfaction problems wherethe set of constraint relations is fixed. Feder and Vardi (1998) identified threefamilies of constraint satisfaction problems containing all known polynomiallysolvable problems. We introduce a new class of problems called para-primalproblems, incomparable with the families identified...

Boolean formulas are known not to be PAC-predictable even with membership queries under some cryptographic assumptions. In
this paper, we study the learning complexity of some subclasses of boolean formulas obtained by varying the basis of elementary
operations allowed as connectives. This broad family of classes includes, as a particular case, gen...

We consider the following classes of quantified formulas. Fix a set of basic relations called a basis. Take conjunctions of
these basic relations applied to variables and constants in arbitrary ways. Finally, quantify existentially or universally
some of the variables. We introduce some conditions on the basis that guarantee efficient learnability....

Local Consistency has proven to be an important notion inthe study of constraint satisfaction problems. We give an algebraic conditionthat characterizes all the constraint types for which generalizedarc-consistency is sufficient to ensure the existence of a solution. We givesome examples to illustrate the application of this result.

We consider the following classes of quantified formulas. Fix a set of basic relations called a basis. Take conjunctions of these basic relations applied to variables and constants in arbitrary ways. Finally, quantify existentially or universally some of the variables. We introduce some conditions on the basis that guarantee efficient learnability....