Une nouvelle sémantique pour la programmation logique capturant la sémantique des modèles stables : la sémantique des extensions

National audienceAnswer set programming is a well studied framework in logic programming. Many research works had been done in order to de ne a semantics for logic programs. Most of these semantics are iterated xed point semantics. The main idea is the canonical model approach which is a declarative semantics for logic programs that can be de ned by selecting for each program one of its canonical models. The notion of canonical models of a logic program is what it is called the stable models. The stable models of a logic program are in a certain sense the minimal Herbrand models of its "reduct" programs. Here we introduce a new semantics for logic programs that is di erent from the known xed point semantics. In our approach, logic programs are expressed as CNF formulas (sets of clauses) of a propositional logic for which we de ne a notion of extension. We prove in this semantics, that each consistent CNF formula admits at least an extension and for each given stable model of a logic program there exists an extension of its corresponding CNF formula which logically entails it. On the other hand, we show that some of the extensions do not entail any stable model, in this case, we de ne a simple descrimination condition which allows to recognize such extensions. These extensions could be very important, but are not captured by the stable models semantics. Our approach, extends the stable model semantics in this sense. Following the new semantics, we give a full characterization of the stable models of a logic program by means of the extensions of its CNF encoding verifying a simple condition, and provide a procedure which can be used to compute such extensions from which we deduce the stable models of the given logic program.La programmation par ensembles r éponses (Answer Set Programming) est un cadre bien étudi é en programmation logique. Plusieurs travaux ont été faits pour d éfinir une s émantique pour les programmes logiques. La plupart de ces s émantiques sont en fait des s émantiques de point fi xe. L'id ée principale est le calcul de mod èles canoniques du programme logique consid ér é, appel és mod èles stables. Les mod èles stables sont dans un certain sens des mod èles minimaux des programmes r éduits. Nous introduisons une nouvelle s emantique pour les programmes logiques, à partir d'une notion d'extension d'une formule propositionnelle classique. Ces extensions peuvent être calcul és de mani ère it érative. Un programme logique est alors cod é par un ensemble de clauses de la logique propositionnelle. On prouve que chaque formule consistante admet au moins une extension et que, pour chaque mod èle stable d'un programme logique, il existe une extension de son codage qui l'implique logiquement. Certaines des extensions ne correspondent pas à un mod èle stable mais sont int eréssantes. Nous donnons une condition discriminante simple qui permet de reconnaitre de telles extensions. En fin, nous d écrivons un algorithme qui calcule les extensions de la formule CNF codant le programme logique. De cet ensemble d'extension on peut extraire les mod èles stables du programme logique initial

Finiteness properties for Pisot SS-adic tilings

International audienceIn this paper, we will first formulate and prove some equivalent sufficient conditions to obtain the tiling property for a Pisot unimodular substitution. We will then apply these condition to the more general framework of adic systems, to extend to this more general (and non algebraic) case results already known for the substitutive case

Des ensembles Horn strong backdoor aux ensembles ordonnés strong backdoor

L'identification et l'exploitation de structures cachées dans un problème est reconnue comme étant un moyen fondamental pour contrecarrer l'explosion combinatoire de sa résolution. Récemment, une structure particulière appelée (strong) backdoor a été identifiée pour le problème de satisfaisabilité de formule CNF (SAT). Certaines connexions entre les ensembles strong backdoor et la difficulté intrinsèque des problèmes SAT ont été mises en évidence, permettant une meilleure approximation de la borne de complexité en temps dans le pire des cas. On peut calculer des ensembles strong backdoor pour chaque classe polynomiale. Dans [Parisetals06], une méthode d'approximation d'ensembles strong backdoor pour la classe des formules de Horn a été proposée. Cette approximation est réalisée en deux étapes. Dans un premier temps, on calcule le meilleur Horn renommage du point de vue du nombre de clauses de Horn de la CNF de départ. Ensuite on extrait un ensemble Horn strong backdoor de la partie non Horn de la formule renommée. Dans cet article, nous proposons de calculer des ensembles Horn strong backdoor en utilisant le même procédé mais en minimisant le nombre de littéraux positifs dans la partie non Horn de la formule renommée au lieu du nombre de clauses. Puis nous étendons cette méthode à la classe des formules ordonnée [benoist99] qui est une extension de la classe des formules de Horn. Cette méthode nous garantit l'obtention d'ensembles Ordonné strong backdoor de taille plus petite ou égale à ceux des ensembles Horn strong backdoor (jamais plus grande). Les résultats expérimentaux montrent que ces nouvelles méthodes permettent de réduire la taille des ensembles strong backdoor sur certaines instances et que leur exploitation permet également d'améliorer les performances des solveurs SAT

Dipole Moments of ff-Bonded Complexes

This paper deals with the information which can be obtained from dielectric measurements about the structure of H-bonded complexes in the liquid phase. In the first part the basic equations used in the determination of the dipole moments in the liquid phase are discussed. For pure polar liquid Onsager\u27s equations lead to values of the moments which may differ from those of the gas phase. According to Kirkwood these deviations are due to preferential orientation effects between the molecules. In pure liquids these deviations are only very important in the case of the formation of H-bonds. The interpretation of experimental dipole data for self associated compounds such as alcohols, carboxylic acids, amides, amines, anilines and pyridines is presented. A method used for the experimental determination of dipole moments for one-one hydrogen bonded complexes is discussed. μab depends not only on the moments of the separate partners μa and μb but also on the angles -&a and Db which these moments form with the direction of the hydrogen bond. Furthermore, flab .... also depends on the dipole increment, /),.μ, originated by the displacements of electrons and nuclei brought about by the formation of the. ...b ond. /),.μ in turn, will depend on the f),.pKa, the difference between the pK. of the conjugated acid of the proton acceptor and that of the acid. Sigmoidal curves are obtained which can be interpreted as resulting from a tautomerism between »normal« and »proton transfer« hydrogen bonds. The dependence of /),.μ on the enthalpy of bond formation, - /),.Hh, also gives a sigmoidal curve which is approximately the same for all H-bonds of a given kind (0-H ... 0, 0-H ... N etc.) in a given solvent. This dependence can be used for the calculation of /),.μ. Using this value with the experimental moments μab• μa and /lb it is then possible to deduce angular parameters for a given H-bonded complex. A few examples are discussed

Fractal representation of the attractive lamination of an automorphism of the free group

N°RR 05066 (2005)International audienceIn this paper, we extend to automorphisms of free groups some results and constructions that classically hold for morphisms of the free monoid, i.e., so-called substitutions. A geometric representation of the attractive lamination of a class of automorphisms of the free group (irreducible with irreducible powers ({\it iwip}) automorphisms) is given in the case where the dilation coefficient of the automorphism is a unit Pisot number. The shift map associated with the attractive symbolic lamination is, in this case, proved to be measure-theoretically isomorphic to a domain exchange on a self-similar Euclidean compact set. This set is called the central tile of the automorphism, and is inspired by Rauzy fractals associated with Pisot primitive substitutions. The central tile admits some specific symmetries, and is conjectured under the Pisot hypothesis to be a fundamental domain for a toral translation

Approximation d'ensembles horn strong backdoor par recherche locale

Dans ce papier, nous proposons une nouvelle approche pour calculer un strong backdoor pour des formules mises sous forme normale conjonctive (CNF). Elle est basée sur une utilisation originale d'une méthode de recherche locale qui fournit un renommage maximisant la sous-formule horn-renommable d'une CNF donnée. Plus précisément, à chaque étape, on choisit de renommer la variable qui fait le plus diminuer le nombre de clauses non-horn. S'il ne reste plus de clauses strictement positives (ou strictement négatives) ou de clauses non-horn dans la formule, notre méthode répond au problème de satisfaisabilité de la formule originale; sinon, on utilise la plus petite sous-formule qui ne soit pas de horn pour en extraire un ensemble de variables (strong backdoor) tel qu'une fois ces variables instanciées, le reste du problème appartient à une classe polynômiale. Les premiers résultats expérimentaux montrent que notre approche est prometteuse sur un grand nombre d'instances SAT

Metabolic Profiles Associated With Metformin Efficacy in Cancer

Metformin is one of the most commonly prescribed medications for the treatment of type 2 diabetes. Numerous reports have suggested potential anti-cancerous and cancer preventive properties of metformin, although these findings vary depending on the intrinsic properties of the tumor, as well as the systemic physiology of patients. These intriguing studies have led to a renewed interest in metformin use in the oncology setting, and fueled research to unveil its elusive mode of action. It is now appreciated that metformin inhibits complex I of the electron transport chain in mitochondria, causing bioenergetic stress in cancer cells, and rendering them dependent on glycolysis for ATP production. Understanding the mode of action of metformin and the consequences of its use on cancer cell bioenergetics permits the identification of cancer types most susceptible to metformin action. Such knowledge may also shed light on the varying results to metformin usage that have been observed in clinical trials. In this review, we discuss metabolic profiles of cancer cells that are associated with metformin sensitivity, and rationalize combinatorial treatment options. We use the concept of bioenergetic flexibility, which has recently emerged in the field of cancer cell metabolism, to further understand metabolic rearrangements that occur upon metformin treatment. Finally, we advance the notion that metabolic fitness of cancer cells increases during progression to metastatic disease and the emergence of therapeutic resistance. As a result, sophisticated combinatorial approaches that prevent metabolic compensatory mechanisms will be required to effectively manage metastatic disease

The Linear Multiplet and Quantum 4-D String Effective Actions

In 4-D heterotic superstrings, the dilaton and antisymmetric tensor fields belong to a linear N=1 supersymmetric multiplet L. We study the lagrangian describing the coupling of one linear multiplet to chiral and gauge multiplets in global and local supersymmetry, with particular emphasis on string tree-level and loop-corrected effective actions. This theory is dual to an equivalent one with chiral multiplets only. But the formulation with a linear multiplet appears to have decisive advantages beyond string tree-level since, in particular, is the string loop-counting parameter and the duality transformation is in general not exactly solvable beyond tree-level. This formulation allows us to easily deduce some powerful non-renormalization theorems in the effective theory and to obtain explicitly some loop corrections to the string effective supergravity for simple compactifications. Finally, we discuss the issue of supersymmetry breaking by gaugino condensation using this formalism.Comment: 55p, latex, NEIP-93-007, IEM-FT-83/9