A Simple Model to Generate Hard Satisfiable Instances

In this paper, we try to further demonstrate that the models of random CSP instances proposed by [Xu and Li, 2000; 2003] are of theoretical and practical interest. Indeed, these models, called RB and RD, present several nice features. First, it is quite easy to generate random instances of any arity since no particular structure has to be integrated, or property enforced, in such instances. Then, the existence of an asymptotic phase transition can be guaranteed while applying a limited restriction on domain size and on constraint tightness. In that case, a threshold point can be precisely located and all instances have the guarantee to be hard at the threshold, i.e., to have an exponential tree-resolution complexity. Next, a formal analysis shows that it is possible to generate forced satisfiable instances whose hardness is similar to unforced satisfiable ones. This analysis is supported by some representative results taken from an intensive experimentation that we have carried out, using complete and incomplete search methods.Comment: Proc. of 19th IJCAI, pp.337-342, Edinburgh, Scotland, 2005. For more information, please click http://www.nlsde.buaa.edu.cn/~kexu/papers/ijcai05-abstract.ht

Une étude des supports résiduels pour la consistance d'arc

Pour un algorithme établissant la consistance d'arc (AC), un support résiduel, ou résidu, est un support qui a été trouvé et enregistré lors d'une exécution de la procédure qui détermine si une valeur est supportée par une contrainte. Le point important est qu'un résidu n'offre pas la garantie de représenter un minorant du plus petit support courant de la valeur en question. Dans cet article, nous étudions l'impact théorique d'exploiter des résidus au niveau de l'algorithme élémentaire AC3. Tout d'abord, nous prouvons que AC3r(m) (i.e. AC3 exploitant des résidus) est optimal pour une dureté de contrainte faible ou élevée. Ensuite, nous montrons que MAC2001 présente, par rapport à MAC3r(m), un sur-coût en O(μed) par branche de l'arbre binaire construit par MAC, avec μ représentant le nombre de réfutations de la branche, e le nombre de contraintes et d la taille du plus grand domaine. L'une des conséquences est que, MAC3r(m) admet une complexité temporelle (dans le pire des cas) meilleure que MAC2001 pour une branche impliquant μ réfutations lorsque μ > d2 ou lorsque μ > d et que la dureté de chaque contrainte est soit faible soit élevée. Nos résultats expérimentaux montrent clairement que le fait d'exploiter des résidus permet d'améliorer l'efficacité des algorithmes MAC et SAC embarquant des algorithmes AC à gros grain

STR et compression de contraintes tables

National audienceOver the recent years, many ltering algorithms have been developed for table constraints. STR2, one of the most e cient algorithms, is based on the technique of simple tabular reduction, meaning that it maintains dynamically the list of supports in each constraint table during inference and search. However, for some speci c problems, the approach that consists in representing in a compact way tables by means of multi-valued deci- sion diagrams (MDD) overcomes STR2. In this paper, we study the possibility of combining simple tabular reduction with a compression form of tables based on the detection of recurrent patterns in tuples

Inférence de supports pour les algorithmes de filtrage générique

http://www710.univ-lyon1.fr/~csolnonDans cet article, nous proposons une analyse statique des différentes contraintes d'un réseau afin d'identifier certaines propriétés (ou caractéristiques) générales. L'utilisation de ces propriétés rend possible une inférence de supports qui permet de réduire le nombre de tests de consistance. En effet, l'exploitation de certaines propriétés identifiées lors d'une phase de pré-traitement peut apporter une amélioration substantielle des algorithmes de recherche qui maintiennent une forme de consistance locale telle que la consistance d'arc. Les résultats d'expérimentations menées sur de nombreuses classes d'instances démontrent l'intérêt de cette approche

The Changing Landscape for Stroke\ua0Prevention in AF: Findings From the GLORIA-AF Registry Phase 2

Background GLORIA-AF (Global Registry on Long-Term Oral Antithrombotic Treatment in Patients with Atrial Fibrillation) is a prospective, global registry program describing antithrombotic treatment patterns in patients with newly diagnosed nonvalvular atrial fibrillation at risk of stroke. Phase 2 began when dabigatran, the first non\u2013vitamin K antagonist oral anticoagulant (NOAC), became available. Objectives This study sought to describe phase 2 baseline data and compare these with the pre-NOAC era collected during phase 1. Methods During phase 2, 15,641 consenting patients were enrolled (November 2011 to December 2014); 15,092 were eligible. This pre-specified cross-sectional analysis describes eligible patients\u2019 baseline characteristics. Atrial fibrillation disease characteristics, medical outcomes, and concomitant diseases and medications were collected. Data were analyzed using descriptive statistics. Results Of the total patients, 45.5% were female; median age was 71 (interquartile range: 64, 78) years. Patients were from Europe (47.1%), North America (22.5%), Asia (20.3%), Latin America (6.0%), and the Middle East/Africa (4.0%). Most had high stroke risk (CHA2DS2-VASc [Congestive heart failure, Hypertension, Age  6575 years, Diabetes mellitus, previous Stroke, Vascular disease, Age 65 to 74 years, Sex category] score  652; 86.1%); 13.9% had moderate risk (CHA2DS2-VASc = 1). Overall, 79.9% received oral anticoagulants, of whom 47.6% received NOAC and 32.3% vitamin K antagonists (VKA); 12.1% received antiplatelet agents; 7.8% received no antithrombotic treatment. For comparison, the proportion of phase 1 patients (of N = 1,063 all eligible) prescribed VKA was 32.8%, acetylsalicylic acid 41.7%, and no therapy 20.2%. In Europe in phase 2, treatment with NOAC was more common than VKA (52.3% and 37.8%, respectively); 6.0% of patients received antiplatelet treatment; and 3.8% received no antithrombotic treatment. In North America, 52.1%, 26.2%, and 14.0% of patients received NOAC, VKA, and antiplatelet drugs, respectively; 7.5% received no antithrombotic treatment. NOAC use was less common in Asia (27.7%), where 27.5% of patients received VKA, 25.0% antiplatelet drugs, and 19.8% no antithrombotic treatment. Conclusions The baseline data from GLORIA-AF phase 2 demonstrate that in newly diagnosed nonvalvular atrial fibrillation patients, NOAC have been highly adopted into practice, becoming more frequently prescribed than VKA in Europe and North America. Worldwide, however, a large proportion of patients remain undertreated, particularly in Asia and North America. (Global Registry on Long-Term Oral Antithrombotic Treatment in Patients With Atrial Fibrillation [GLORIA-AF]; NCT01468701

Extraction de noyaux insatisfiables minimaux de réseaux de contraintes

Nous nous intéressons au problème de l'extraction de noyaux insatisfiables minimaux (MUCs) de réseaux de contraintes. Ce problème a un intérêt pratique dans de nombreux domaines d'application tels que la configuration, la planification, le diagnostique, etc. En effet, identifier un ou plusieurs MUCs indépendants, i.e. des MUCs qui ne partagent aucune contrainte, permet d'isoler différentes sources d'inconsistance et de corriger un système incohérent. Dans cet article, nous proposons une approche originale pour extraire un MUC d'un réseau de contraintes. Cette approche comporte deux étapes. La première consiste à exécuter plusieurs fois un algorithme de recherche complète, en utilisant la pondération de contraintes, de manière à circonscrire une partie inconsistante du réseau. La seconde consiste à identifier, en utilisant un processus dichotomique, les contraintes de transition appartenant à un MUC. Nous montrons l'intérêt de cette approche en avançant des arguments théoriques et pratiques

Des cas d'utilisation en UML à la gestion de rôles dans un système d'information

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Contrôle statistique du processus de propagation de contraintes

National audienceIn this paper, we investigate constraint propagation, a mechanism that is run at each basic step of a backtrack search algorithm such as the popular MAC. From a statistical analysis of some relevant features concerning propagation on a large set of CSP instances, we show that it is possible to make reasonable predictions about the capability of constraint propagation to detect inconsistency. Using this observation in order to control propagation effort, we show its practical effectiveness

A study of residual supports in arc consistency

Abstract. In an Arc Consistency (AC) algorithm, a residual support, or residue, is a support that has been stored during a previous execution of the procedure which determines if a value is supported by a constraint. The point is that a residue is not guaranteed to represent a lower bound of the smallest current support of a value. In this paper, we study the theoretical impact of exploiting residues with respect to the basic algorithm AC3. First, we prove that AC3r (AC3 with uni-directional residues) and AC3rm (AC3 with multi-directional residues) are optimal for low and high constraint tightness. Second, we show that when AC has to be maintained during a backtracking search (the well-known MAC algorithm), MAC2001 presents, with respect to MAC3r and MAC3rm, an overhead in O(µed) per branch of the binary tree built by MAC, where µ denotes the number of refutations of the branch, e the number of constraints and d the greatest domain size of the constraint network. One consequence is that, MAC3r and MAC3rm admit a better worst-case time complexity than MAC2001 for a branch involving µ refutations when either µ> d 2 or µ> d and the tightness of any constraint is either low or high. Our experimental results clearly show that exploiting residues allows enhancing MAC and SAC algorithms.

Une plate-forme UML-XML pour la gestion des rôles s'un système d'information

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