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    Improvements to the number field sieve for non-prime finite fields

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    We propose various strategies for improving the computation of discrete logarithms in non-prime fields of medium to large characteristic using the Number Field Sieve. This includes new methods for selecting the polynomials; the use of explicit automorphisms; explicit computations in the number fields; and prediction that some units have a zero virtual logarithm. On the theoretical side, we obtain a new complexity bound of Lpn(1/3,96/93)L_{p^n}(1/3,\sqrt[3]{96/9}) in the medium characteristic case. On the practical side, we computed discrete logarithms in Fp2F_{p^2} for a prime number pp with 8080 decimal digits.Nous décrivons plusieurs stratégies pour accélérer le calcul de logarithmes discrets dans un corps fini non premier de caractéristique moyenne ou grande à l'aide du crible algébrique. Parmi elles, de nouvelles méthodes de sélection polynomiale; l'utilisation explicite d'automorphismes; des calculs explicites dans les corps de nombres; et la prédiction de l'annulation des logarithmes virtuels d'unités bien choisies. D'un point de vue théorique, nous obtenons une complexité nouvelle en Lpn(1/3,96/93)L_{p^n}(1/3,\sqrt[3]{96/9}) dans le cas de la caractéristique moyenne. Du côté pratique, nous avons mené à bien le calcul de logarithmes discrets dans Fp2F_{p^2} avec pp premier de 8080 chiffres décimaux

    Utilisation d'OpenFlow et des modules Splite Data Plane de DELL pour traiter le DUID-MAC-spoofing des requêtes DHCPv6

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    National audienceIPv6 a longtemps été associé à un mécanisme d'auto-configuration sans-état (Router Advertisements). La mise à disposition tardive d'une implémentation de protocole d'attribution d'adresses IPv6, avec-état, par l'ISC, a été un frein à son déploiement sur un parc de postes clients " administrés ". Cependant, l'arrivée du protocole DHCPv6 ne se fait pas sans soulever de nouvelles difficultés opérationnelles. Dans les environnements à double pile IP, le fait que DHCPv4 et DHCPv6 n'utilisent pas les mêmes attributs pour identifier les objets IP (MAC dans le premier cas, DUID dans le second), complique la tâche des ASR (plusieurs variantes de DUID coexistent) et rend problématique la corrélation des adressages au sein des journaux d'activités réseau. Nous souhaitons pouvoir utiliser notre infrastructure DHCPv6 ISC préexistante, qui, ne supporte que les attributs DUID, tout en référençant nos postes clients par leurs adresses MAC, peu importe la version du protocole DHCP. Cette contrainte peut être résolue grâce au module Split Data Plane embarquant un Network Processor Octeon dans les commutateurs PowerConnect DELL. Ces modules sont pilotables via OpenFlow et permettent de supporter un large nombre de fonctionnalités réseau. Le Software Defined Network déporte le plan de contrôle du réseau sur un serveur externe ce qui permet de lever une part importante des contraintes des architectures actuelles basées sur les protocoles de contrôle. Le module Split Data Plane ouvre aussi le concept de fonctionnalité hardware modulaire et open source

    Developing low-cost graphene devices

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    In spite of numerous efforts for developing the applications of graphene, it remains difficult to put the remarkable physical properties of this material into devices. This is mainly due to the fact that large-area (industrial) graphene includes in its structure and on its surfaces a significant density of defects that make as many traps and scattering centres for charge carriers. The idea of the present work, contrary to diminishing the defect density, is to use the defects and the very large surface to volume ratio of that 2D material, to transform it into high sensitivity sensors. When defects are useful, low-temperature growth becomes the method that best satisfies both physical and financial demands. Here, we further decrease preparation costs by performing growth not only at low temperature directly on the final insulating substrate (glass), but also by printing the device contacts by ink-jet printing. Graphene layers actually develop at the interface between a metallic catalytic film and the insulating substrate during plasma-enhanced chemical vapour deposition (PE-CVD).1,2. Resistivity of the graphene foils was measured by the four-point methods using ink-jet printed electrods, and a resistivity as low as 820 ohms/sq were obtained. Moreover, the sensitivity of such graphene foils to water vapour was evaluated, with the prospect to use them in humidity sensors for civil engineering. In this presentation, we explain how graphite may precipitate at the interface in addition to the surface.2,3 Then we show examples of graphene obtained at temperatures in between 450 and 550°C, on glass (Fig.), fused silica, alumina and SiO2//Si. Transmission electron microscopy indicates that the structure is nanocrystalline. We finally show the humidity response of the fabricated device. Results seem to indicate that high-defect density, thin deposits are more sensitive to water vapour than thicker ones

    Optimal stock-out risk for a component mounted on several assembly lines in case of emergency supplies

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    This article focuses on the calculation of the optimal stock-out risk for a component, which is used by alternative modules mounted on several assembly lines. The studied context is a supply chain dedicated to the mass production of highly diversified products, which is common in the automotive industry. The Material Requirement Planning (MRP) approach is adapted for the monitoring of this chain; however, the distance between the production units leads to mix between production to stock and production to order for the component of interest. To prevent stock-out propagation along the downstream part of the supply chain, use of an emergency supply is triggered prior to its occurrence. The definition of the optimal safety stock and the associated optimal stock-out risk, are based on a monoperiod model that considers the cost of a safety stock and the costs incurred by the emergency supply (transportation and production). The analytical solutions that are dependent on these costs are illustrated in this study

    Compte rendus de la conférence "Journées Jeunes Chercheurs JJC2012"

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    National audienceFrom 2nd to 8th decembre 2012, the Journées Jeunes Chercheurs 2012 (Young Reseachers Meeting) in high energy physics were held in Munster (Alsace, France). During all this week, PhD students in various domains (particle and nuclear physics, cosmology and astrophysics) have presented their work in a pedagogic way to other membres. The meeting is organized by young permanent staff. Students were coming from French and Belgain laboratories working on fundamental research on the two infinites (from particles to stars and galaxies). Most of them were French but many were coming other european countries (Belgium, Italy, Spain, Ukraine) as well as the rest of the world (Asia, South America ...). Du 2 au 8 décembre 2012, la maison du Kleebach à Munster a accueilli les Journées Jeunes Chercheurs 2012 (JJC). Durant toute cette semaine, les étudiants en thèse dans des domaines variés (physique des particules et nucléaire, cosmologie et astrophysique) ont présenté leurs travaux de façon pédagogique dans une ambiance conviviale, le tout étant encadré par des jeunes chercheurs en poste. Les participants viennent des nombreux laboratoires de France et de Belgique travaillant sur la recherche fondamentale dans les deux infinis (les particules, les étoiles et galaxies). Ils sont originaires pour la plupart de France mais certains viennent d'autres pays Européens (Belgique, Italie, Espagne, Ukraine, etc) ou du reste du monde (Asie, Amérique du sud, etc)

    Two properties of two-velocity two-pressure models for two-phase flows

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    International audienceWe study a class of models of compressible two-phase flows. This class, which includes the Baer-Nunziato model, is based on the assumption that each phase is described by its own pressure, velocity and temperature and on the use of void fractions obtained from averaging process. These models are nonconservative and non-strictly hyperbolic. We prove that the mixture entropy is non-strictly convex and that the system admits a symmetric form

    Imaging flux distributions around superconductors: Geometrical susceptibility in the Meissner state

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    International audienceExperiment and analytical calculations show that the demagnetizing field of a superconductor is a sensitive probe of quantities otherwise difficult to measure, such as the sample-probe distance in flux-density imaging experiments, and the field of first flux penetration Hp. In particular, the ratio of the maximum field measured above the superconductor edge and the applied field can be determined unambiguously so as to define a linear "geometric" susceptibility. The evolution of this susceptibility with field depends on the regime of flux penetration, and can be used as a means to determine Hp and the effect of a parallel field component in magneto-optical imaging experiments

    Derivation of a homogenized two-temperature model from the heat equation

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    31 pages, no figureInternational audienceThis work studies the heat equation in a two-phase material with spherical inclusions. Under some appropriate scaling on the size, volume fraction and heat capacity of the inclusions, we derive a coupled system of partial differential equations governing the evolution of the temperature of each phase at a macroscopic level of description. The coupling terms describing the exchange of heat between the phases are obtained by using homogenization techniques originating from [D. Cioranescu, F. Murat: Collège de France Seminar vol. 2 (Paris 1979-1980) Res. Notes in Math. vol. 60, pp. 98-138. Pitman, Boston, London, 1982.

    Experimental observation of a traveling plasma grating formed by two crossing filaments in gases

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    International audienceThe spatial motion and effective duration of a traveling plasma grating formed by two interfering femtosecond laser filaments in gases is characterized by its Doppler effect imparted on a probe pulse. The shift velocity determined experimentally agrees with the theoretical calculations

    Etude STM in situ de la microstructure et de la croissance de films ultraminces d'alliages sur Au(111) synthétisés par co-dépôt

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    The fact that, in ultrathin films a large part of the atoms are located at the two interfaces substrate-film and film-environment, induces specific film microstructures and thus interesting new properties different from those of the bulk materials. We studied the microstructure of electrochemically co-deposited binary alloys, composed of an element B and of Ni, where B is either Pd, Au or Ag. We also studied the deposition of B over a predeposited Ni monolayer. In addition we devised a method to maintain the Au(111) surface un-reconstructed during film deposition limiting the alloying process with the substrate. On this un-reconstructed Au(111) surface, we studied the deposition of Ni monolayer which microstructure is well documented on the Au(111) reconstructed surface. We then studied the microstructure of the codeposited alloys as a function of their composition and deposition speed. Furthermore, the B elements (Ag, Au, Pd) allow us to study the influence of the mixing enthalpy on the alloys' microstructure. We showed using in situ scanning tunnelling microscopy that all cases lead to a phase separation but with a specific phase separation length for each case. B = {Ag, Au, Pd} deposition over a Ni monolayer showed that Ni atoms undergo place exchange with B atoms in the Ni layer to form an alloy. Alloys are made out of two phases, one rich or pure in B atoms and the other rich or pure in Ni. Nevertheless, the morphology of those alloys is different than what is obtained with codeposition.Les films ultraminces présentent une grande proportion d'atomes aux interfaces films-substrat et film-environnement. Grâce à cela, ces films possèdent des microstructures et donc des propriétés particulières différentes de celles des matériaux massifs. Nous avons étudié par microscopie à effet tunnel in situ d'une part la microstructure d'alliages binaires B - Ni où B = {Pd, Au, Ag} réalisés par co-dépôt électrochimique, et d'autre part la microstructure de films ultraminces où l'élément B est déposé sur une monocouche de nickel préalablement déposée sur Au(111). De plus, une méthode à été mise au point pour permettre de maintenir la surface de Au(111) non reconstruite pendant le les différents dépôts permettant de minimiser la formation d'alliage avec le substrat. Après avoir revisité le dépôt de Ni sur l'or non reconstruit, la microstructure des alliages co-déposés a été étudiée en fonction de leur composition et de la vitesse de dépôt. De plus, le choix des éléments B nous permet d'étudier l'influence de l'enthalpie de mélange du nickel avec Pd, Au et Ag sur la microstructure. Nous avons ainsi montré qu'il y a séparation de phase dans tous les cas mais à une échelle propre à chaque système. Le dépôt de B sur Ni a permis de mettre en évidence des processus de substitution entre B = {Pd, Au} et le nickel. Ces processus s'accompagnent d'une dissolution partielle du nickel extrudé de la couche. Les alliages formés sont composés d'un mélange de phase riches en nickel et riches en Pd ou Au. La morphologie de ces alliages est cependant très différente des alliages co-déposés

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