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38
Closure Properties of Constraints
 Journal of the ACM
, 1997
"... Many combinatorial search problems can be expressed as `constraint satisfaction problems', and this class of problems is known to be NPcomplete in general. In this paper we investigate the subclasses which arise from restricting the possible constraint types. We first show that any set of constrain ..."
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Cited by 133 (16 self)
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Many combinatorial search problems can be expressed as `constraint satisfaction problems', and this class of problems is known to be NPcomplete in general. In this paper we investigate the subclasses which arise from restricting the possible constraint types. We first show that any set of constraints which does not give rise to an NPcomplete class of problems must satisfy a certain type of algebraic closure condition. We then investigate all the different possible forms of this algebraic closure property, and establish which of these are sufficient to ensure tractability. As examples, we show that all known classes of tractable constraints over finite domains can be characterised by such an algebraic closure property. Finally, we describe a simple computational procedure which can be used to determine the closure properties of a given set of constraints. This procedure involves solving a particular constraint satisfaction problem, which we call an `indicator problem'. Keywords: Cons...
ConjunctiveQuery Containment and Constraint Satisfaction
 Journal of Computer and System Sciences
, 1998
"... Conjunctivequery containment is recognized as a fundamental problem in database query evaluation and optimization. At the same time, constraint satisfaction is recognized as a fundamental problem in artificial intelligence. What do conjunctivequery containment and constraint satisfaction have in c ..."
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Cited by 130 (13 self)
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Conjunctivequery containment is recognized as a fundamental problem in database query evaluation and optimization. At the same time, constraint satisfaction is recognized as a fundamental problem in artificial intelligence. What do conjunctivequery containment and constraint satisfaction have in common? Our main conceptual contribution in this paper is to point out that, despite their very different formulation, conjunctivequery containment and constraint satisfaction are essentially the same problem. The reason is that they can be recast as the following fundamental algebraic problem: given two finite relational structures A and B, is there a homomorphism h : A ! B? As formulated above, the homomorphism problem is uniform in the sense that both relational structures A and B are part of the input. By fixing the structure B, one obtains the following nonuniform problem: given a finite relational structure A, is there a homomorphism h : A ! B? In general, nonuniform tractability results do not uniformize. Thus, it is natural to ask: which tractable cases of nonuniform tractability results for constraint satisfaction and conjunctivequery containment do uniformize? Our main technical contribution in this paper is to show that several cases of tractable nonuniform constraint satisfaction problems do indeed uniformize. We exhibit three nonuniform tractability results that uniformize and, thus, give rise to polynomialtime solvable cases of constraint satisfaction and conjunctivequery containment.
Duality and polynomial testing of tree homomorphisms
 Trans. Amer. Math. Soc
, 1996
"... Abstract. Let H be a fixed digraph. We consider the Hcolouring problem, i.e., the problem of deciding which digraphs G admit a homomorphism to H. We are interested in a characterization in terms of the absence in G of certain treelike obstructions. Specifically, we say that H has tree duality if, ..."
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Cited by 53 (16 self)
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Abstract. Let H be a fixed digraph. We consider the Hcolouring problem, i.e., the problem of deciding which digraphs G admit a homomorphism to H. We are interested in a characterization in terms of the absence in G of certain treelike obstructions. Specifically, we say that H has tree duality if, for all digraphs G, G is not homomorphic to H if and only if there is an oriented tree which is homomorphic to G but not to H. Weprovethatif Hhas tree duality then the Hcolouring problem is polynomial. We also generalize tree duality to bounded treewidth duality and prove a similar result. We relate these duality concepts to the notion of the Xproperty studied by Gutjahr, Welzl, and Woeginger. We then focus on the case when H itself is an oriented tree. In fact, we are particularly interested in those trees that have exactly one vertex of degree three and all other vertices of degree one or two. Such trees are called triads. We have shown in a companion paper that there exist oriented triads H for which the Hcolouring problem is NPcomplete. We contrast these with several families of oriented triads H which have tree duality, or bounded treewidth duality, and hence polynomial Hcolouring problems. If P � = NP, then no oriented triad H with an NPcomplete Hcolouring problem can have bounded treewidth duality; however no proof of this is known, for any oriented triad H. We prove that none of the oriented triads H with NPcomplete Hcolouring problems given in the companion paper has tree duality. 1.
The Chromatic Number of Oriented Graphs
 J. Graph Theory
, 2001
"... . We introduce in this paper the notion of the chromatic number of an oriented graph G (that is of an antisymmetric directed graph) dened as the minimum order of an oriented graph H such that G admits a homomorphism to H . We study the chromatic number of oriented ktrees and of oriented graphs with ..."
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Cited by 48 (22 self)
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. We introduce in this paper the notion of the chromatic number of an oriented graph G (that is of an antisymmetric directed graph) dened as the minimum order of an oriented graph H such that G admits a homomorphism to H . We study the chromatic number of oriented ktrees and of oriented graphs with bounded degree. We show that there exist oriented ktrees with chromatic number at least 2 k+1 1 and that every oriented ktree has chromatic number at most (k + 1) 2 k . For 2trees and 3trees we decrease these upper bounds respectively to 7 and 16 and show that these new bounds are tight. As a particular case, we obtain that oriented outerplanar graphs have chromatic number at most 7 and that this bound is tight too. We then show that every oriented graph with maximum degree k has chromatic number at most (2k 1) 2 2k 2 . For oriented graphs with maximum degree 2 we decrease this bound to 5 and show that this new bound is tight. For oriented graphs with maximum degree 3 we decrease this bound to 16 and conjecture that there exists no such connected graph with chromatic number greater than 7. Keywords. Graph coloring, Graph homomorphism, Oriented coloring. 1
Constraints, Consistency, and Closure
 Artificial Intelligence
, 1998
"... Although the constraint satisfaction problem is NPcomplete in general, a number of constraint classes have been identified for which some fixed level of local consistency is sufficient to ensure global consistency. In this paper, we describe a simple algebraic property which characterises all possi ..."
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Cited by 46 (12 self)
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Although the constraint satisfaction problem is NPcomplete in general, a number of constraint classes have been identified for which some fixed level of local consistency is sufficient to ensure global consistency. In this paper, we describe a simple algebraic property which characterises all possible constraint types for which strong kconsistency is sufficient to ensure global consistency, for each k ? 2. We give a number of examples to illustrate the application of this result. 1 Introduction The constraint satisfaction problem provides a framework in which it is possible to express, in a natural way, many combinatorial problems encountered in artificial intelligence and elsewhere. The aim in a constraint satisfaction problem is to find an assignment of values to a given set of variables subject to constraints on the values which can be assigned simultaneously to certain specified subsets of variables. The constraint satisfaction problem is known to be an NPcomplete problem in ge...
The dichotomy of conjunctive queries on probabilistic structures
 In PODS
, 2007
"... We show that for every conjunctive query, the complexity of evaluating it on a probabilistic database is either PTIME or #Pcomplete, and we give an algorithm for deciding whether a given conjunctive query is PTIME or #Pcomplete. The dichotomy property is a fundamental result on query evaluation on ..."
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Cited by 46 (12 self)
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We show that for every conjunctive query, the complexity of evaluating it on a probabilistic database is either PTIME or #Pcomplete, and we give an algorithm for deciding whether a given conjunctive query is PTIME or #Pcomplete. The dichotomy property is a fundamental result on query evaluation on probabilistic databases and it gives a complete classification of the complexity of conjunctive queries. 1. PROBLEM STATEMENT Fix a relational vocabulary R1,..., Rk, denoted R. A tupleindependent probabilistic structure is a pair (A, p) where A = (A, R A 1,..., R A k) is first order structure and p is a function that associates to each tuple t in A a rational number p(t) ∈ [0, 1]. A probabilistic structure (A,p) induces a probability distribution on the set of substructures B of A by: p(B) = kY ( Y p(t) × i=1 t∈RB i Y t∈R A i −RB i (1 − p(t))) (1) where B ⊆ A, more precisely B = (A, R B 1,..., B B k) is s.t. R B i ⊆ R A i for i = 1, k. A conjunctive query, q, is a sentence of the form ∃¯x.(ϕ1 ∧... ∧ϕm), where each ϕi is a positive atomic predicate R(t), called a subgoal, and the tuple t consists of variables and/or constants. As usual, we drop the existential quantifiers and the ∧, writing q = ϕ1, ϕ2,..., ϕm. A conjunctive property is a property on structures defined by a conjunctive query q, and its probability on a probabilistic structure (A, p) is defined as: p(q) = X p(B) (2)
A Survey of Tractable Constraint Satisfaction Problems
, 1997
"... In this report we discuss constraint satisfaction problems. These are problems in which values must be assigned to a collection of variables, subject to specified constraints. We focus specifically on problems in which the domain of possible values for each variable is finite. The report surveys the ..."
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Cited by 41 (5 self)
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In this report we discuss constraint satisfaction problems. These are problems in which values must be assigned to a collection of variables, subject to specified constraints. We focus specifically on problems in which the domain of possible values for each variable is finite. The report surveys the various conditions that have been shown to be sufficient to ensure tractability in these problems. These are broken down into three categories: ffl Conditions on the overall structure; ffl Conditions on the nature of the constraints; ffl Conditions on bounded pieces of the problem. 1 Introduction A constraint satisfaction problem is a way of expressing simultaneous requirements for values of variables. The study of constraint satisfaction problems was initiated by Montanari in 1974 [34], when he used them as a way of describing certain combinatorial problems arising in imageprocessing. It was quickly realised that the same general framework was applicable to a much wider class of probl...
A Complete Classification of the Approximability of Maximization Problems Derived from Boolean Constraint Satisfaction
"... In this paper we study the approximability of boolean constraint satisfaction problems. A problem in this class consists of some collection of "constraints " (i.e., functions f: f0; 1g k! f0; 1g); an instance of a problem ..."
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Cited by 38 (6 self)
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In this paper we study the approximability of boolean constraint satisfaction problems. A problem in this class consists of some collection of "constraints " (i.e., functions f: f0; 1g k! f0; 1g); an instance of a problem
List Homomorphisms and Circular Arc Graphs
 Combinatorica
, 1999
"... List homomorphisms generalize list colourings in the following way: Given graphs G; H , and lists L(v) ` V (H); v 2 V (G), a list homomorphism of G to H with respect to the lists L is a mapping f : V (G) ! V (H) such that uv 2 E(G) implies f(u)f(v) 2 E(H), and f(v) 2 L(v) for all v 2 V (G). The l ..."
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Cited by 30 (6 self)
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List homomorphisms generalize list colourings in the following way: Given graphs G; H , and lists L(v) ` V (H); v 2 V (G), a list homomorphism of G to H with respect to the lists L is a mapping f : V (G) ! V (H) such that uv 2 E(G) implies f(u)f(v) 2 E(H), and f(v) 2 L(v) for all v 2 V (G). The list homomorphism problem for a fixed graph H asks whether or not an input graph G together with lists L(v) ` V (H), v 2 V (G), admits a list homomorphism with respect to L. The list homomorphism problem was introduced by Feder and Hell, who proved that for reflexive graphs H (that is, for graphs H in which every vertex has a loop), the problem is polynomial time solvable if H is an interval graph, and is NPcomplete otherwise. Here we consider graphs H without loops, and find that the problem is closely related to circular arc graphs. We show that the list homomorphism problem is polynomial time solvable if the complement of H is a circular arc graph of clique covering number two, and is NPcomplete otherwise. For the purposes of the proof we give a new characterization of circular arc graphs of clique covering number two, by the absence of a structure analogous to Gallai's asteroids. Both results point to a surprising similarity between interval graphs and the complements of circular arc graphs of clique covering number two. Key Words: Homomorphisms, listhomomorphisms, retractions, asteroidal triples, circular arc graphs, algorithms, complexity. 1
Definability and Descriptive Complexity on Databases of Bounded TreeWidth
 Database TheoryICDT'99, volume 1540 of Lecture Notes in Computer Science
, 1999
"... . We study the expressive power of various query languages on relational databases of bounded treewidth. Our first theorem says that fixedpoint logic with counting captures polynomial time on classes of databases of bounded treewidth. This result should be seen on the background of an importa ..."
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Cited by 26 (10 self)
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. We study the expressive power of various query languages on relational databases of bounded treewidth. Our first theorem says that fixedpoint logic with counting captures polynomial time on classes of databases of bounded treewidth. This result should be seen on the background of an important open question of Chandra and Harel [7] asking whether there is a query language capturing polynomial time on unordered databases. Our theorem is a further step in a larger project of extending the scope of databases on which polynomial time can be captured by reasonable query languages. We then prove a general definability theorem stating that each query on a class of databases of bounded treewidth which is definable in monadic secondorder logic is also definable in fixedpoint logic (or datalog). Furthermore, for each k 1 the class of databases of treewidth at most k is definable in fixedpoint logic. These results have some remarkable consequences concerning the definabilit...