692 research outputs found
Optimal Probabilistic Ring Exploration by Asynchronous Oblivious Robots
We consider a team of identical, oblivious, asynchronous mobile robots
that are able to sense (\emph{i.e.}, view) their environment, yet are unable to
communicate, and evolve on a constrained path. Previous results in this weak
scenario show that initial symmetry yields high lower bounds when problems are
to be solved by \emph{deterministic} robots. In this paper, we initiate
research on probabilistic bounds and solutions in this context, and focus on
the \emph{exploration} problem of anonymous unoriented rings of any size. It is
known that robots are necessary and sufficient to solve the
problem with deterministic robots, provided that and are coprime.
By contrast, we show that \emph{four} identical probabilistic robots are
necessary and sufficient to solve the same problem, also removing the coprime
constraint. Our positive results are constructive
Mercury sources and bioavailability in lakes located in the mining district of Chibougamau, eastern Canada
The presence of Hg in the fish of the lakes from the CuâZnâAu mining region of Chibougamau (Canada)
represents a serious source of concern for the health of local sports and subsistence fishers. This study focuses on identifying the origin of Hg present in the sediments of lakes with mine wastes stored in tailing facilities located on their shores. In addition to C/N ratios and determination of total Hg, Methyl Hg and other metals, a series of lignin biomarkers were used to contrast the history of the mining contamination with the nature and the intensity of terrigenous organic matter (TOM) inputs from the watersheds to the sampled lakes. It appears that sediments located nearest to mine tailings are as expected most enriched in total Hg and other metals (Cu, As, Al, Fe, Pb). Nevertheless, the presence of only small amounts of refractory TOM in these contaminated sediments could explain why only a very small fraction of Hg is found as Methyl Hg. In sediments with little or no impact by mining activities, a relationship was observed between logging activities in the lake watershed and increased TOM derived from inorganic gymnosperms soils horizons and increased Hg transport to the lakes. Nevertheless, it appears that the additional TOM transported to logged lakes is refractory enough not to promote high levels of Methyl Hg. The highest fractions of Methyl Hg relative to total Hg in lake sediments of the studied area were thus observed in relatively pristine environments where least degraded TOM is brought from the watersheds
Reconstitution de la dynamique du paysage alluvial de trois secteurs fonctionnels de la riviĂšre allier (1946-2000), Massif central, France
LâĂ©cocomplexe de la plaine dâinondation de la riviĂšre Allier a Ă©tĂ© Ă©tudiĂ© dans trois secteurs fonctionnels prĂ©sentant une dynamique gĂ©omorphologique contrastĂ©e : le secteur Ă mĂ©andres de Joze-Maringues oĂč lâAllier est corsetĂ© par des enrochements et touchĂ© par les extractions de granulats, le secteur Ă mĂ©andres mobiles de la RĂ©serve Naturelle du Val dâAllier et le secteur Ă bras multiples situĂ© entre les villages de Villeneuve-sur-Allier et du Veurdre. LâĂ©tude se fonde sur six Ă huit missions de photographies aĂ©riennes prises entre 1946 et 2000 pour dĂ©celer la dynamique du paysage alluvial. Lâoccupation du sol, la largeur de la bande active et le taux dâĂ©rosion des berges ont Ă©tĂ© estimĂ©s par photo-interprĂ©tation et leur Ă©volution spatiotemporelle a Ă©tĂ© mise en Ă©vidence par des traitements statistiques et cartographiques. Les rĂ©sultats indiquent un accroissement des surfaces vouĂ©es Ă lâagriculture Ă partir des annĂ©es 1950 et une progression du recouvrement de la vĂ©gĂ©tation arborĂ©e en lien avec la modification des pratiques culturales et la rĂ©troaction de la bande active. Dans les trois secteurs, le pourcentage de la vĂ©gĂ©tation arborĂ©e est passĂ© de 10 Ă 17 % en 50 ans. Depuis la seconde moitiĂ© du 20e siĂšcle, la modification des pratiques agro-pastorales semble ĂȘtre un agent prĂ©pondĂ©rant pour expliquer la vĂ©gĂ©talisation progressive de la plaine alluviale de la riviĂšre Allier. LâĂ©rosion des berges ne sâest pas attĂ©nuĂ©e durant cette pĂ©riode. Elle Ă©tait plus active pendant la dĂ©cennie 1970-1980 en raison des crues plus frĂ©quentes et plus intenses.The landscape mosaics of the Allier river floodplain has been studied in three functional reaches with contrasted geomorphic activities : the Joze-Maringues meander reach where the river is constrained by embankments and affected by gravel mining, the shifting meander reach of the Val dâAllier Natural Reserve, and the anabranching reach between the village of Villeneuve-sur-Allier and Le Veurdre. This study is based on six to eight aerial photography, series taken between 1946 and 2000, to underline the dynamics of the alluvial landscape. Land cover types, active channel width and river bank erosion rate have been assessed from the photo interpretation, and their change in space and time has been statistically analysed and mapped. The results point to an increase of the cultivated areas from the 1950âs, and an extension of the tree-cover in relation with a change of agricultural practices and the decrease of the channel activity. In the three reaches, the percentage of the tree-cover has moved from 10 to 17% in 50 years. From the second half of the 20th century, the change in the agro-pastoral practices seems to be a major factor explaining the increase of the floodplain tree and bush cover. Bank erosion has not diminished during this period. It was more active in the 1970-1980 decade owing to more severe and frequent floods
Snap-Stabilizing Committee Coordination
International audienceIn the committee coordination problem, a committee consists of a set of professors and committee meetingsare synchronized, so that each professor participates in at most one committee meeting at a time. Inthis paper, we propose two snap-stabilizing distributed algorithms for the committee coordination. Snapstabilizationis a versatile property which requires a distributed algorithm to efficiently tolerate transientfaults. Indeed, after a finite number of such faults, a snap-stabilizing algorithm immediately operates correctly,without any external intervention. We design snap-stabilizing committee coordination algorithmsenriched with some desirable properties related to concurrency, (weak) fairness, and a stronger synchronizationmechanism called 2-Phase Discussion. In our setting, all processes are identical and each processhas a unique identifier. The existing work in the literature has shown that (1) in general, fairness cannotbe achieved in committee coordination, and (2) it becomes feasible if each professor waits for meetingsinfinitely often. Nevertheless, we show that even under this latter assumption, it is impossible to implementa fair solution that allows maximal concurrency. Hence, we propose two orthogonal snap-stabilizingalgorithms, each satisfying 2-phase discussion, and either maximal concurrency or fairness. The algorithmthat implements fairness requires that every professor waits for meetings infinitely often. Moreover, forthis algorithm, we introduce and evaluate a new efficiency criterion called the degree of fair concurrency.This criterion shows that even if it does not satisfy maximal concurrency, our snap-stabilizing fair algorithmstill allows a high level of concurrency
Source et devenir du mercure dans les lacs de la région miniÚre de Chibougamau
Les communautĂ©s autochtones et non autochtones de la rĂ©gion de Chibougamau s'inquiĂštent de la prĂ©sence de mines sur le bassin versant de lacs frĂ©quemment utilisĂ©s pour la pĂȘche sportive et de subsistance. Cette Ă©tude concerne quatre lacs dont trois sont impactĂ©s par le lessivage de parcs Ă rĂ©sidus issus de l'extraction industrielle de cuivre et de zinc. Cette recherche caractĂ©rise la pollution au mercure due Ă la biodisponibilitĂ© de cet Ă©lĂ©ment et les risques de contamination des poissons. L'objectif est de diffĂ©rencier l'origine du Hg sĂ©dimentaire, entre le lessivage du bassin versant et celui des parcs Ă rĂ©sidus, puis d'Ă©valuer le potentiel de mĂ©thylation du Hg par rapport aux particularitĂ©s biogĂ©ochimiques de ces milieux. Cette recherche utilise des indicateurs de gĂ©ochimie inorganique (concentrations en mĂ©taux et mĂ©taux lourds, diffractomĂ©trie aux rayons X, granulomĂ©trie) de maniĂšre Ă caractĂ©riser l'intensitĂ©, l'extension gĂ©ographique et l'Ă©volution chronologique de la contamination des sĂ©diments. Ces donnĂ©es sont complĂ©tĂ©es par des indicateurs organiques tels que le rapport (C/N)a et les biomarqueurs de la lignine pour caractĂ©riser le transfert de matiĂšre organique terrigĂšne (MOT) depuis les bassins versants. Les diffĂ©rences observĂ©es dans la nature et l'origine de la matiĂšre organique terrigĂšne permettent d'expliquer certaines variations des teneurs en mercure. Par ailleurs la caractĂ©risation molĂ©culaire de la MOT permet d'Ă©valuer son Ă©tat de dĂ©gradation. Ces nouvelles donnĂ©es sont mises en relation avec les teneurs en mĂ©thyle mercure afin d'expliquer dans une certaine mesure le potentiel de mĂ©thylation des milieux caractĂ©risĂ©s. Il apparaĂźt ainsi que des milieux situĂ©s Ă proximitĂ© des mines ont subi une contamination importante en Hg. Pourtant le potentiel de mĂ©thylation de ces milieux est moins Ă©levĂ© que celui des sĂ©diments situĂ©s plus en aval, ayant reçu des apports en Hg moins importants. De la mĂȘme façon des sĂ©diments situĂ©s en dehors de toute influence miniĂšre ont des teneurs en Hg plus Ă©levĂ©es que certains milieux proches des mines. Ces diffĂ©rences s'expliquent notamment par la quantitĂ© et la qualitĂ© de la MOT arrivant dans le systĂšme aquatique qui agit sur le mĂ©tabolisme des bactĂ©ries mĂ©thylatrices
Optimal Torus Exploration by Oblivious Robots
International audienceWe consider autonomous robots that are endowed with motion actuators and visibility sensors. The robots we consider are weak, i.e., they are anonymous, uniform, unable to explicitly communicate, and oblivious (they do not remember any of their past actions). In this paper, we propose an optimal (w.r.t. the number of robots) solution for the terminating exploration of a torus-shaped network by a team of such robots. In more details, we first show that it is impossible to explore a simple torus of arbitrary size with (strictly) less than four robots, even if the algorithm is probabilistic. If the algorithm is required to be deterministic, four robots are also insufficient. This negative result implies that the only way to obtain an optimal algorithm (w.r.t. the number of robots participating to the algorithm) is to make use of probabilities. Then, we propose a probabilistic algorithm that uses four robots to explore all simple tori of size , where . Hence, in such tori, four robots are necessary and sufficient to solve the (probabilistic) terminating exploration. As a torus can be seen as a 2-dimensional ring, our result shows, perhaps surprisingly, that increasing the number of possible symmetries in the network (due to increasing dimensions) does not come at an extra cost w.r.t. the number of robots that are necessary to solve the problem
Optimal torus exploration by oblivious robots
International audienceWe deal with a team of autonomous robots that are endowed with motion actuators and visibility sensors. Those robots are weak and evolve in a discrete environment. By weak, we mean that they are anonymous, uniform, unable to explicitly communicate, and oblivious. We first show that it is impossible to solve the terminating exploration of a simple torus of arbitrary size with less than 4 or 5 such robots, respectively depending on whether the algorithm is probabilistic or deterministic. Next, we propose in the SSYNC model a probabilistic solution for the terminating exploration of torus-shaped networks of size âĂL, where 7â€ââ€L, by a team of 4 such weak robots. So, this algorithm is optimal w.r.t. the number of robots
Environment-aware non-rigid registration in surgery using physics-based simulation
International audienceThis paper presents a system for capturing the deformations of soft objects undergoing elastic deformations and contacts with their environment, using image and point cloud data provided by an RGB-D sensor. We improve upon previous works by integrating environment constraints in the frame-by-frame registration process. The approach combines a physics-based elastic model of the considered objects, computed in real-time using an optimized Finite Element Method (FEM), which is driven by surface constraints on the objects. Additional forces, such as gravity are added. A case study in open surgery on the liver is here described. Yet in this case a major improvement in the accuracy of the registration is provided by the integration of anatomical shape constraints, which are naturally hidden from the RGB-D camera, and that we account for through a registration with the pre-operative CT data. With a comparative study, we demonstrate the relevance of our method in a real world application mimicking an open surgery scenario where the liver has to be tracked to provide an augmented reality view
100ââkHz Yb-fiber laser pumped 3ââÎŒm optical parametric amplifier for probing solid-state systems in the strong field regime
We report on a laser source operating at 100 kHz repetition rate and delivering 8 ÎŒJ few-cycle mid-IR pulses at 3 ÎŒm. The system is based on optical parametric amplification pumped by a high repetition rate Yb-doped femtosecond fiber-chirped amplifier. This high-intensity ultrafast system is a promising tool for strong-field experiments (up to 50 GV/m and 186 T) in low ionization potential atomic and molecular systems, or solid-state physics with coincidence measurements. As a proof of principle, up to the sixth harmonic has been generated in a 1 mm zinc selenide sample
Réactivité de polyphénols du vin sous conditions oxydantes (hémisynthÚse des mongolicaïnes, et d'adduits entre polyphénols et thiols odorants)
Le vin est un milieu complexe qui Ă©volue tout au long des Ă©tapes devinification. Pour mieux apprĂ©hender ses qualitĂ©s et ses dĂ©fauts, de nombreuses Ă©quipesde recherche s intĂ©ressent Ă la comprĂ©hension de la chimie du vin. Dans ce contexte, lesujet de ce mĂ©moire concerne l Ă©tude de l Ă©volution chimique de certaines molĂ©culespolyphĂ©noliques du vin sous conditions oxydantes et/ou acides, afin d isoler et decaractĂ©riser de nouveaux composĂ©s susceptibles de se former dans le vin. Deux aspectssont examinĂ©s. Le premier concerne l oxydation de deux flavano ellagitannins, lesacutissimines A et B, formĂ©es Ă partir d un monomĂšre de tannins condensĂ©s, lacatĂ©chine, et d un ellagitannin C glucosidique, la vescalagine, extraite du bois de chĂȘnepar le vin lors de l Ă©levage en barrique. Cette Ă©tude a permis d isoler les mongolicaĂŻnes Aet B et deux analogues du camelliatannin G et de mettre en Ă©vidence leur formation parun mĂ©canisme d autoxydation. Le deuxiĂšme aspect concerne l Ă©valuation desconsĂ©quences de la prĂ©sence de certains polyphĂ©nols dans le vin sur les composĂ©sthiolĂ©s odorants. Leur comportement et leur rĂ©activitĂ© chimiques sont dĂ©crits dans desmilieux diffĂ©rents, avec l hĂ©misynthĂšse de thio ellagitannins sous conditions acides, et laformation d adduits thio catĂ©chols et thio pyrogallols sous conditions oxydantes,transformations chimiques pouvant occasionner la perte des odeurs et arĂŽmes du vindus aux composĂ©s thiolĂ©s odorants.The wine is a complex medium that evolves throughout the different stages ofthe wine making process. To understand both the qualities and defects of wine,numerous research team worldwide investigate the chemistry of wine. In this context,the subject of this thesis concerns the study of the chemical evolution of some winepolyphenolic molecules under oxidizing and/or acidic conditions in the aim of isolatingand characterizing new compounds likely formed in wine. Two aspects are examined.The first one is the study of the oxidation of two flavano ellagitannins, acutissimins Aand B, formed from a monomer of condensed tannins, catechin, and a C glucosidicellagitannin, vescalagin, extracted from oak wood by the wine solution during its agingin barrels. This study led to the isolation of mongolicains A and B and two analogues ofcamelliatannin G, and revealed their formation according to an autoxydationmechanism. The second aspect of this work concerns the consequences of the presencein wine of some polyphenols on wine odorous thiols. Their chemical behavior andreactivity are described in different media, with the hemisynthesis of thio ellagitanninsunder acidic conditions, and the formation of thio catechol and thio pyrogallol adductsunder oxidizing conditions, chemical transformations that could explain the loss ofodors and aromas due to wine odorous thiols.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF
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