Modèle multi-agents pour le filtrage collaboratif de l'information

Les systèmes de recommandation sont nés de la volonté de pallier le problème de surcharge d'information du web. Combinant des techniques de filtrage d'information, personnalisation, intelligence artificielle, réseaux sociaux et interaction personne-machine, les systèmes de recommandation fournissent à des utilisateurs des suggestions qui répondent à leurs besoins et préférences informationnelles. En effet, les systèmes de recommandation sont particulièrement sollicités dans les applications de commerce électronique. Cependant, ce type de système a été en grande partie confiné à une architecture centralisée. Récemment, l'architecture distribuée a connu une popularité croissante, comme en témoigne par exemple, les réseaux pair-à-pair (« peer-to-peer »), le calcul distribué (« Grid computing »), le web sémantique, etc., et s'impose peu à peu comme une alternative à l'approche client/serveur classique. L'hypothèse des chercheurs est que les systèmes de recommandation peuvent tirer profit d'une architecture distribuée. Dans cette thèse, nous étudions les défis que posent les systèmes de recommandation distribués et nous proposons une nouvelle architecture pair-à-pair, de filtrage collaboratif, basée sur la discrimination du voisinage. Nous étudions l'évolution de la performance, de la couverture et de la qualité des prédictions pour différentes techniques de recommandation. En outre, nous identifions la méthode de recommandation la plus efficace pour cette nouvelle architecture pair-à-pair. Bien que cette thèse se concentre essentiellement sur le domaine décentralisé de système de recommandation, nos contributions ne se limitent pas strictement à ce domaine de recherche. En effet, ces contributions touchent des problèmes de recherche dans plusieurs autres domaines de recherche (système multi-agents, gestions profils utilisateurs, réduction de la complexité computationnelle, collecte des préférences utilisateurs, PageRank, etc.). ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : Filtrage de l'information, Filtrage collaboratif, Système de recommandation, Système distribué, Agent social

Geochemistry and mineralogy of Paleocene-Eocene Oum El Khecheb phosphorites (Gafsa-Metlaoui Basin) Tunisia

International audienceUpper Paleocene to Lower Eocene phosphates of the Jebel Oum El Khecheb profile (Gafsa-Metlaoui Basin GMB, Tunisia) are mostly composed of brown phospharenites lithified or not, granular, with pellets, coproliths, bone clasts and fish teeth. The matrix in analyzed samples is reduced and constituted of various clay minerals (dominantly smectites, palygorskite, sepiolite and rare kaolinite), opal-CT and carbonates. Clinoptilolite is recognized along the series; very few amounts of quartz and feldspar grains are sparsely present. The phosphate mineral is a carbonate-fluorapatite (CFA) sulphated, sodic enriched in strontium and rare earth elements (REE). Statistical analyses of mineralogical and geochemical data highlight the associated elements in the main mineral lattice. In the CFA, the group is composed of CaO-P2O5-Na2O-SO3-F-Sr-Th-REE-U-Ba-Zr-Cr. In the silicates, the group contains SiO2-Al2O3-Fe2O3-MgO-K2O-Na2O-TiO2-Co-Cs-Rb-Sc-Ta-Sb-Mo-As and Cu. Sulphides are represented by Zn and Cd; iron oxides by mainly Fe and Ni. Dolomite and gypsum are present; SO3 is contained in the latter, but CFA may also have some of this radical in its structure. The Upper Continental Crust (UCC) normalized REE patterns present a variably negative Ce-anomaly, suggesting that the imprint of the marine genesis environment is very well-preserved in the main phosphatic units. Positive-Eu and negative-Sm anomalies were detected in some samples. The fractionation of light-REE to heavy-REE, represented by the La/Yb ratio of untreated values varies between 8.28 and 12.94, somewhat less than UCC value (13.64). Enrichment of HREE is due to phosphates. Both ratios reflect the marine genesis environment wherein the formation of the CFA took place in a confined and oxygen poor environment. Then the CFA removed the REE and the trace elements present from the interstitial waters, as was proposed for the various phosphatic basins of Tunisia

Accumulation of heavy metals in native plants growing near the phosphate treatment industry, Tunisia

International audienceThe enrichment of the phosphates by a humid process can be accompanied with rejects of mud poured in hydrographic network of the region of Gafsa-Metlaoui to Chott El Gharsa. These releases are rich in phosphate and trace elements; the average is around these values: P2O5: 10.96 %, Cd: 25.83 ppm, Zn: 260.91 ppm, Cr: 387.7 ppm, Cu: 16 ppm, Ni: 26 ppm and Sr: 717.15 ppm. Soils adjoining these rejects present total concentrations in Cd, Zn and Cr higher to the maximal contents tolerated in soils. A two step sequential extraction showed that heavy metals (Cd, Cr, Cu and Ni) were poorly labile (i.e. not soluble in diluted CaCl2), indicating that their leaching under natural conditions is probably very low. However, extraction with DTPA, CaCl2 and TEA generated significant amounts of metals (mainly Cd and Cu), suggesting that they were potentially mobilizable. However, zinc and strontium are more concentrated in the mobile phase, indicating that the Zn-Sr-solutions are possible in natural conditions. This study evaluates the potential for phytoremediation of 30 species collected from six localities, growing on a contaminated site of the study area. Plants and the associated soil samples were collected and analyzed for total metals concentrations. While total metals concentrations in soils vary from 1 to 36 ppm Cd, 1.6 to 2463 ppm Zn, 8.5 to 442 ppm Cr, 2.8 to 38.5 ppm Cu, 1 to 31.7 ppm Ni and 144.6 to 1310 ppm Sr, while those in the plants ranged from 0.11 to 82 ppm Cd, 0.5 to 567 ppm Zn, 0.04 to 174.7 ppm Cr, 0.95 to 51.73 ppm Cu, 0.17 to 6.9 ppm Ni and 6 ppm to 2858 ppm Sr. None of the plants were suitable for phytoextraction because no hyperaccumulator was identified. However, plants with a high bioconcentration factor (BCF, metal concentration ratio of plant roots to soil) and low translocation factor (TF, metal concentration ratio of plant leaves to roots) have the potential for phytostabilization. The plants most effective in the accumulation of metals in leaves are Malva aegyptiaca (TF=30.7) for Cd, Frankenia thymifolia (TF=8.55) for Zn, Peganum harmala (TF=29.14) for Cu and Citrulus sp (TF=10.42) for Sr. Anthemis stiparum was most suitable for phytostabilization of contaminated sites with Cd (BCF=23.51). Our study showed that native plant species growing on contaminated sites may have the potential for phytoremediation

Structural, stratigraphic, and petrological clues for a Cretaceous–Paleogene abortive rift in the southern Adria domain (southern Apennines, Italy)

This study provides new structural, stratigraphic, and geochemical data and a literature review of the Cretaceous–Paleogene stratigraphy, biostratigraphy, tectonics, and magmatism in the southern Apennines belt, Italy, with the aim to demostrate the occurrence of an Albian to Eocene abortive rifting stage in the southern Adria domain. During this time, the tectono‐stratigraphic evolution of the Adria domain is characterized by episodes of coeval uplift and drowning. Different sectors of the Apennine and Apulian platforms were so characterized by changes in the paleoenvironments, leading to different stratigraphic records (from shallow‐water to slope and basin), as well as the development of thick bauxitic levels. Contemporaneously, a large amount of calciclastic sediments supply from the emerging sectors was deposited in the basins surrounding the carbonate platforms (i.e., Ligurian and Lagonegro–Molise Basins). The Albian–Eocene interval was also characterized by the occurrence of anorogenic magmatism and synsedimentary extensional faulting that, along with the changed sedimentary facies distribution, points out for a crustal‐scale extensional tectonics. We suggest that such a tectonics is the result of a rifting episode, characterized by limited anorogenic magmatism, starting in the Albian and reaching its climax in the uppermost Cretaceous–Eocene times. In this scenario, the extensional tectonics recorded in the Adria domain was the product during an event of a single abortive rift system, which extended toward the south, from the southern margin of the Ligurian Ocean to the Hyblean (Sicily), Pelagian (Tunisia), and Sirte Basin Province Rift (Libya). KEYWORD