1,534 research outputs found
Fermionic field theory for directed percolation in (1+1) dimensions
We formulate directed percolation in (1+1) dimensions in the language of a
reaction-diffusion process with exclusion taking place in one space dimension.
We map the master equation that describes the dynamics of the system onto a
quantum spin chain problem. From there we build an interacting fermionic field
theory of a new type. We study the resulting theory using renormalization group
techniques. This yields numerical estimates for the critical exponents and
provides a new alternative analytic systematic procedure to study
low-dimensional directed percolation.Comment: 20 pages, 2 figure
Rolling contact fatigue of railways wheels: Influence of steel grade and sliding conditions
AbstractThe aim of this work is to develop a numerical approach which is able to compare the different steel grades influence on rolling contact fatigue of railways wheels according to practical conditions. The main stages are the identification of the material behavior, the determination of the stress-strain fields and the application of a fatigue criterion. Two steels usually used for the manufacturing of wheels have been studied, R9T and 50CrMo4. Their influence has been numerically studied. Results show that the threshold of elastic and plastic shakedown differs depending on the steel grades and consequently the risk of damage can be affected. This methodology allows a classification of the material grades face the risk on rolling contact fatigue
Construction participative dâun modĂšle rĂ©gional pour lâĂ©valuation de la demande en eau agricole: un exemple dans le bassin de la DrĂŽme
International audienceLa " gestion intĂ©grĂ©e des ressources en eau " tente de rendre compte du fonctionnement dâ" hydro-systĂšmes " qui rassemblent un ensemble de ressources et dâusages de lâeau en interaction au sein dâun territoire, qui lui-mĂȘme recouvre diverses unitĂ©s administratives ou politiques. Pour mettre en place cette gestion intĂ©grĂ©e, certains pays ont optĂ© pour une dĂ©centralisation : la gestion de lâeau est alors confiĂ©e Ă des organisations de bassin au sein desquels siĂšgent des porte-parole des diffĂ©rents intĂ©rĂȘts en jeu. Ces organisations soulignent le peu de support pour favoriser la concertation. Les outils disponibles mettent lâaccent sur les processus biophysiques, mais proposent une reprĂ©sentation trĂšs simplifiĂ©e des usages notamment agricoles. Ces usages sont non seulement liĂ©s aux disponibilitĂ©s en eau et Ă ses conditions dâaccĂšs, mais aussi au contexte technologique, Ă©conomique, et institutionnel dans lequel les agriculteurs Ă©voluent. Pour favoriser la concertation, nous proposons de construire avec les acteurs locaux une reprĂ©sentation du systĂšme dans lequel ils agissent, touchant Ă la fois aux aspects techniques, et Ă©conomiques. Lâobjectif est de reprĂ©senter les interactions entre ressource et usages (notamment agricoles), et dâĂ©valuer les consĂ©quences de scĂ©narios imaginĂ©s Ă lâĂ©chelle du territoire. Ce modĂšle rĂ©gional comprend donc (i) une caractĂ©risation hydrologique simplifiĂ©e qui permet dâĂ©valuer en entrĂ©e les disponibilitĂ©s initiales de la ressource et de mesurer les impacts des prĂ©lĂšvements, (ii) un modĂšle bio-physique PILOT qui permet dâĂ©valuer les consommations dâeau, et les rendements selon les pratiques agricoles et les conditions agroclimatiques, et (iii) un modĂšle technico-Ă©conomique qui rend compte des choix techniques des agriculteurs et de leurs consĂ©quences en termes Ă©conomiques. Le modĂšle technicoĂ©conomique des activitĂ©s agricoles Ă lâĂ©chelle rĂ©gionale sâappuie sur une typologie des exploitations et des ateliers de productions agricoles. Dans ce modĂšle, (i) les activitĂ©s agricoles Ă lâĂ©chelle rĂ©gionale sont la somme pondĂ©rĂ©e des activitĂ©s des exploitations types (les pondĂ©rations correspondent aux effectifs dâexploitations), et (ii) les activitĂ©s dâune exploitation type sont la somme pondĂ©rĂ©e dâateliers de production types (les pondĂ©rations correspondent aux dimensions des ateliers). Cette structure permet dâagrĂ©ger Ă lâĂ©chelle rĂ©gionale les impacts des activitĂ©s et de dĂ©sagrĂ©ger Ă lâĂ©chelle des exploitations et de leurs activitĂ©s les consĂ©quences de mesures globales. La construction participative du modĂšle rĂ©gional sâappuie sur la mise en place dĂšs le dĂ©marrage de lâĂ©tude dâun " groupe de pilotage ", composĂ© des reprĂ©sentants des acteurs locaux. Ce groupe de pilotage participe Ă la modĂ©lisation en fournissant les donnĂ©es nĂ©cessaires et en validant chaque Ă©tape de la construction. Le modĂšle, construit et validĂ© par Colloque - 13th IWRA World water congress, 1-4 september 2008, Montpellier tous les acteurs, peut alors contribuer Ă aider les dĂ©cideurs locaux ou rĂ©gionaux Ă Ă©laborer des scĂ©narios raisonnĂ©s pour dĂ©finir des stratĂ©gies de gestion intĂ©grĂ©e et durable de lâeau. Nous avons mis en oeuvre cette dĂ©marche dans le bassin de la DrĂŽme dans le cadre du programme APPEAU financĂ© par lâAgence Nationale de la Recherche (programme " Agriculture et DĂ©veloppement Durable "), et en nous appuyant sur les travaux du programme MIPAIS financĂ© par lâUnion EuropĂ©enne (programme Interreg III Medoc). / The "integrated management of water resources" tries to convey the functioning of the "hydro-systems" which brings together a group of resources and uses of water. This group of resources and uses interacts within a territory, which itself includes various administrative or political units. In order to develop this integrated management, some countries have chosen decentralization: water management is then given to basin institutions within which representatives of the various key interests sit. These institutions emphasize the poor support they get to encourage the dialogue. Available tools emphasize biophysical process but provide a very simplified representation of uses, and particularly of farm uses. These uses are not only linked to water availabilities and to its conditions of access, but also to the technological, economic, and institutional background in which the farmers operate. With the local stakeholders, we suggest to build a model of the system in which they act. At the same time, this model includes the technical, economic, and environmental sides. Therefore, this regional model includes (i) a hydrological model, which enables to assess as input the initial resource availabilities and to measure the impacts of withdrawals, (ii) a biophysical model which enables to assess crop water needs, yields and environmental impacts according to agricultural practices and agro-climatic conditions, and (iii) a technical and economic model which conveys the technical choices of farmers and theirs economic consequences. The construction of the hydrological and biophysical models can be limited to the parameterization of pre-existing models. The technical and economic model of farm activities at the regional scale relies on a typology of farms and of farm production units; its structure has been designed to enable an aggregation and a desegregation of the results between various scales, which provides a detailed analysis of the studied scenarios. The participative construction of the regional model relies on the creation, from the start of the study, of a âsteering groupâ compound of the representatives of local stakeholders. This steering group takes part in the modelling by supplying the necessary data and by validating each stage of the model construction. The model, build and validated by all the stakeholders, can then contribute to help the local or regional decision-makers to develop reasoned scenarios in order to define strategies of integrated and sustainable water management. We have implemented this approach in the DrĂŽme basin within the framework of the APPEAU project funded by the "Agriculture et DĂ©veloppement Durable" program of the French National Research Agency, and by relying on the works of the MIPAIS project funded by the Programme Interreg III Medoc of the European Union. This paper presents the first results obtained
Dzyaloshinsky-Moriya Anisotropy in the Spin-1/2 Kagom\'e Compound ZnCu(OH)Cl
We report the determination of the Dzyaloshinsky-Moriya interaction, the
dominant magnetic anisotropy term in the \kagome spin-1/2 compound {\herbert}.
Based on the analysis of the high-temperature electron spin resonance (ESR)
spectra, we find its main component K to be perpendicular to the
\kagome planes. Through the temperature dependent ESR line-width we observe a
building up of nearest-neighbor spin-spin correlations below 150 K.Comment: 4 pages, 3 figures, minor modification
A balanced complex chromosomal rearrangement (BCCR) in a family with reproductive failure
Balanced complex chromosomal rearrangements are very rare events in the human population. Translocations involving three or more chromosomes frequently lead to a severe reproductive impairment secondary to meiotic disturbance in males and to chromosomal imbalance in gametes of females. We report a new familial case of complex chromosome anomaly involving chromosomes 13, 14 and 22. Cytogenetic investigations showed a complex chromosomal chromosome rearrangement involving: (i) a Robertsonian translocation between chromosomes 13 and 14; and (ii) a reciprocal translocation between the long arms of chromosome 14 and the long arm of chromosome 22. The aetiology of the translocation was characterized by conventional fluorescence inâsitu hybridization (FISH) studies and routine Râ and Gâbanding (RTBG and GBTG) combined with α and ÎČ satellite centromeric FISH probes. Predicted configuration of the hexavalent at pachytene stage of meiosis was used to consider the modes of segregation; only two configurations resulted in a normal or balanced gamete karyotype. Reproductive management and genetic counselling are discusse
Statistical-Mechanical Measure of Stochastic Spiking Coherence in A Population of Inhibitory Subthreshold Neurons
By varying the noise intensity, we study stochastic spiking coherence (i.e.,
collective coherence between noise-induced neural spikings) in an inhibitory
population of subthreshold neurons (which cannot fire spontaneously without
noise). This stochastic spiking coherence may be well visualized in the raster
plot of neural spikes. For a coherent case, partially-occupied "stripes"
(composed of spikes and indicating collective coherence) are formed in the
raster plot. This partial occupation occurs due to "stochastic spike skipping"
which is well shown in the multi-peaked interspike interval histogram. The main
purpose of our work is to quantitatively measure the degree of stochastic
spiking coherence seen in the raster plot. We introduce a new spike-based
coherence measure by considering the occupation pattern and the pacing
pattern of spikes in the stripes. In particular, the pacing degree between
spikes is determined in a statistical-mechanical way by quantifying the average
contribution of (microscopic) individual spikes to the (macroscopic)
ensemble-averaged global potential. This "statistical-mechanical" measure
is in contrast to the conventional measures such as the "thermodynamic" order
parameter (which concerns the time-averaged fluctuations of the macroscopic
global potential), the "microscopic" correlation-based measure (based on the
cross-correlation between the microscopic individual potentials), and the
measures of precise spike timing (based on the peri-stimulus time histogram).
In terms of , we quantitatively characterize the stochastic spiking
coherence, and find that reflects the degree of collective spiking
coherence seen in the raster plot very well. Hence, the
"statistical-mechanical" spike-based measure may be used usefully to
quantify the degree of stochastic spiking coherence in a statistical-mechanical
way.Comment: 16 pages, 5 figures, to appear in the J. Comput. Neurosc
Invasive alien species in the food chain : advancing risk assessment models to address climate change, economics and uncertainty
Economic globalization depends on the movement of people and goods between countries. As these exchanges increase, so does the potential for translocation of harmful pests, weeds, and pathogens capable of impacting our crops, livestock and natural resources (Hulme 2009), with concomitant impacts on global food security (Cook et al. 2011)
Finite-size and correlation-induced effects in Mean-field Dynamics
The brain's activity is characterized by the interaction of a very large
number of neurons that are strongly affected by noise. However, signals often
arise at macroscopic scales integrating the effect of many neurons into a
reliable pattern of activity. In order to study such large neuronal assemblies,
one is often led to derive mean-field limits summarizing the effect of the
interaction of a large number of neurons into an effective signal. Classical
mean-field approaches consider the evolution of a deterministic variable, the
mean activity, thus neglecting the stochastic nature of neural behavior. In
this article, we build upon two recent approaches that include correlations and
higher order moments in mean-field equations, and study how these stochastic
effects influence the solutions of the mean-field equations, both in the limit
of an infinite number of neurons and for large yet finite networks. We
introduce a new model, the infinite model, which arises from both equations by
a rescaling of the variables and, which is invertible for finite-size networks,
and hence, provides equivalent equations to those previously derived models.
The study of this model allows us to understand qualitative behavior of such
large-scale networks. We show that, though the solutions of the deterministic
mean-field equation constitute uncorrelated solutions of the new mean-field
equations, the stability properties of limit cycles are modified by the
presence of correlations, and additional non-trivial behaviors including
periodic orbits appear when there were none in the mean field. The origin of
all these behaviors is then explored in finite-size networks where interesting
mesoscopic scale effects appear. This study leads us to show that the
infinite-size system appears as a singular limit of the network equations, and
for any finite network, the system will differ from the infinite system
Un outil de simulation adaptĂ© Ă la gestion des ressources en eau du territoire dans le cadre dâune dĂ©marche participative
National audienceThis paper presents a simulation tool adapted to water resources management on a territorial scale, as part of a participatory process. This tool combines the PILOTE-TER crop model and the Olympe technico-economical simulator. PILOTE-TER links the crop model Pilote developed by Irstea with a database managed through a graphical interface. It makes possible to use the Pilote model to evaluate the water demand and the crop yield at the scale of the field, the farm and the territory. The parameters of the model come from the results of experiments performed by Irstea. PILOTE-TER is linked to the technico-economic simulator Olympe (from INRA) to produce economical indicators. These tools are used for a territorial representation of agriculture build with local stakeholders, allowing the comparison of several scenarios (based on economical, policy or climate changes).Cet article prĂ©sente un outil de simulation adaptĂ© Ă la gestion de ressources en eau Ă lâĂ©chelle dâun territoire dans le cadre de dĂ©marches participatives. Cet outil associe un modĂšle de culture, Pilote Ter, et le simulateur technico-Ă©conomique Olympe. Pilote Ter intĂšgre le modĂšle de culture Pilote dĂ©veloppĂ© par Irstea et un systĂšme de base de donnĂ©es gĂ©rĂ© Ă partir dâune interface graphique. Il permet une utilisation territorialisĂ©e du modĂšle de culture Pilote pour Ă©valuer la demande en eau agricole et les rendements des cultures Ă lâĂ©chelle de la parcelle, de lâexploitation agricole et du territoire. Le paramĂ©trage du modĂšle sâappuie sur les rĂ©sultats des expĂ©rimentations menĂ©es par Irstea. Pilote Ter est couplĂ© au simulateur technico-Ă©conomique Olympe (INRA) pour produire des indicateurs Ă©conomiques. Ces deux logiciels sont le support dâune reprĂ©sentation de lâagriculture du territoire co-construite avec les acteurs locaux permettant lâĂ©tude de scĂ©narios Ă©conomiques, rĂ©glementaires ou climatiques
Rationale and study design of the Prospective comparison of Angiotensin Receptor neprilysin inhibitor with Angiotensin receptor blocker MEasuring arterial sTiffness in the eldERly (PARAMETER) study.
Hypertension in elderly people is characterised by elevated systolic blood pressure (SBP) and increased pulse pressure (PP), which indicate large artery ageing and stiffness. LCZ696, a first-in-class angiotensin receptor neprilysin inhibitor (ARNI), is being developed to treat hypertension and heart failure. The Prospective comparison of Angiotensin Receptor neprilysin inhibitor with Angiotensin receptor blocker MEasuring arterial sTiffness in the eldERly (PARAMETER) study will assess the efficacy of LCZ696 versus olmesartan on aortic stiffness and central aortic haemodynamics
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