Analysing degeneracies in networks spectra

Many real-world networks exhibit a high degeneracy at few eigenvalues. We show that a simple transformation of the network's adjacency matrix provides an understanding of the origins of occurrence of high multiplicities in the networks spectra. We find that the eigenvectors associated with the degenerate eigenvalues shed light on the structures contributing to the degeneracy. Since these degeneracies are rarely observed in model graphs, we present results for various cancer networks. This approach gives an opportunity to search for structures contributing to degeneracy which might have an important role in a network.Comment: 5 pages, 3 figures and Supplementary Materia

Antibody-mediated cross-linking of gut bacteria hinders the spread of antibiotic resistance

The body is home to a diverse microbiota, mainly in the gut. Resistant bacteria are selected for by antibiotic treatments, and once resistance becomes widespread in a population of hosts, antibiotics become useless. Here, we develop a multiscale model of the interaction between antibiotic use and resistance spread in a host population, focusing on an important aspect of within-host immunity. Antibodies secreted in the gut enchain bacteria upon division, yielding clonal clusters of bacteria. We demonstrate that immunity-driven bacteria clustering can hinder the spread of a novel resistant bacterial strain in a host population. We quantify this effect both in the case where resistance pre-exists and in the case where acquiring a new resistance mutation is necessary for the bacteria to spread. We further show that the reduction of spread by clustering can be countered when immune hosts are silent carriers, and are less likely to get treated, and/or have more contacts. We demonstrate the robustness of our findings to including stochastic within-host bacterial growth, a fitness cost of resistance, and its compensation. Our results highlight the importance of interactions between immunity and the spread of antibiotic resistance, and argue in the favor of vaccine-based strategies to combat antibiotic resistance.Comment: 49 pages, 11 figure

Partial decoupling between strain and polarization in mono-oriented Pb(Zr0.2Ti0.8)O3 thin film

The structural evolution of epitaxial mono-oriented (i.e. with the c-axis perpendicular to the interface) ferroelectric Pb(Zr0.2,Ti0.8)O3 thin film has been investigated, using high resolution, temperature dependent, X-ray diffraction. The full set of lattice parameters was obtained, it allowed to estimate the variation of the polarization as a function of temperature, underlying the difference between the polarization-induced tetragonality and the elastic one. The temperature evolution of the misfit strain has been calculated and found to be in good agreement with the theoretical temperature-misfit strain phase diagramComment: 11 pages, 3 figure

Discussion on various model of multiple scattering in acoustics and elastic heterogeneous media

International audienceThe propagation of acoustic or elastic wave through an heterogeneous domain is investigated in 2D. For heterogeneous medium composed of discrete scattered embedded in a homogeneous matrix, effective medium theories consists in modeling the coherent part of the wave obtained form equivalent propagation along various distribution of scatterer. The main objective is to determine attenuation and dispersion of the wave. In the case of half-space or slab, transmission and reflection coefficient can be obtained. Here various model are presented and compared in the classical case: half-space heterogeneous domain probed by incident plane wave. A synthetic formalism is the occasion to generalized this problem to any bounded domain. It is the occasion to highlight some particular features of this full 2D-problem (where translational invariance along one direction is broken). In particular a new synthetic formulation of the QCA is proposed. Asymptotic calculation will be presented for low-frequency regime or dilute media

Solving the stochastic dynamics of population growth

Population growth is a fundamental process in ecology and evolution. The population size dynamics during growth are often described by deterministic equations derived from kinetic models. Here, we simulate several population growth models and compare the size averaged over many stochastic realizations with the deterministic predictions. We show that these deterministic equations are generically bad predictors of the average stochastic population dynamics. Specifically, deterministic predictions overestimate the simulated population sizes, especially those of populations starting with a small number of individuals. Describing population growth as a stochastic birth process, we prove that the discrepancy between deterministic predictions and simulated data is due to unclosed-moment dynamics. In other words, the deterministic approach does not consider the variability of birth times, which is particularly important with small population sizes. We show that some moment-closure approximations describe the growth dynamics better than the deterministic prediction. However, they do not reduce the error satisfactorily and only apply to some population growth models. We explicitly solve the stochastic growth dynamics, and our solution applies to any population growth model. We show that our solution exactly quantifies the dynamics of a community composed of different strains and correctly predicts the fixation probability of a strain in a serial dilution experiment. Our work sets the foundations for a more faithful modeling of community and population dynamics. It will allow the development of new tools for a more accurate analysis of experimental and empirical results, including the inference of important growth parameters

UN USAGE CITOYEN DU MÉTIER D'HISTORIEN : REPENSER LA TRANSITION ÉNERGÉTIQUE SANS LA COP21

Nous avons pour ambition de croiser nos connaissances d’historiens des sciences et des techniques avec nos engagements de citoyens pour prendre position dans le débat public sur la transition énergétique. L’apport se situe à deux niveaux. Au niveau des discours, l’analyse épistémologique montre qu’une véritable « transition énergétique » ne peut être conçue comme une redistribution de pourcentages entre différentes sources d’énergie mais implique des transformations sociales et économiques profondes pour s’éloigner du dogme croissance=bonheur. Au niveau des structures sociales, l’évolution des « systèmes énergétiques fossiles » montre la mise en place de la démesure de la civilisation industrielle. Une sobriété énergétique, une relocalisation des activités et une prise en compte des critiques alternatives (Luddites, romantiques, néo-malthusiens, décroissants, zadistes) permettraient de réduire drastiquement les besoins énergétiques des sociétés humaines. Ainsi, pourraient être résolus les problèmes environnementaux, reconfigurés les rapports sociaux et redéfinis les comportements culturels

Etude multi-échelle des déterminants des patrons de structuration et de dynamique spatiale de populations de coléoptères carabiques dans les agroécosystèmes

The aim of this study was to determine (i) factors which structure populations of dominant carabid beetles over agricultural landscapes and (ii) their spatial and temporal scales of influence, in order to infer population processes involved in species maintain, in response to agroecosystems’ spatiotemporal heterogeneity. Surveys were conducted in three agricultural areas of western France contrasted in their degree of agriculturalintensification, using samplings based on pitfall and emergence traps and realized at several periods in the year. One of the main novelties of this work is to have characterized agroecosystems at different scales, both (i) spatial: the patch of habitat, landscape, region; and (ii) temporal, considering both the current environment and that of the previous year. Moreover, we essentially characterized the influence of habitat dynamics ondistribution and abundance dynamics and dispersal of carabid populations.Our study yielded three major findings. (1) The spatial dynamics of studied carabid species is strongly influenced by habitat types and their temporal instability at different spatial scales. Indeed, we have emphasized the importance of temporal heterogeneity of agricultural landscapes on the spatial dynamics of carabid populations using the agricultural matrix. (2) The dispersal and inter-field movements are an important condition to the success of these species in agricultural landscapes and seem to be mainly induced by crop rotations. (3) Due to large population sizes and an important effective dispersion, spatial and genetic structure of the populations remains low across agricultural landscapes.Dans ce travail de thèse nous avons cherché à déterminer (i) les facteurs structurants des populations des carabiques dominants dans les paysages agricoles et (ii) leur(s) échelle(s) spatiale(s) et temporelle(s) d’influence, afin d’inférer des processus individuels et populationnels impliqués dans le maintien des espèces, en réponse à l’hétérogénéité spatiotemporelle des agroécosystèmes. Notre étude a été réalisée dans trois régions agricoles de l’ouest de la France, contrastées selon leur degré d’intensification agricole, en utilisant des échantillonnages basés sur des pots-pièges et des tentes à émergence et réalisés à plusieurs périodes de l’année.Une des principales originalités de ce travail est d’avoir caractérisé les agroécosystèmes à différentes échelles, tant (i) spatiales : le patch d’habitat, le paysage, la région ; que (ii) temporelles, en considérant à la fois l’environnement présent et celui de l’année précédente. De plus, nous avons essentiellement caractérisé l’influence de la dynamique des habitats sur les dynamiques de distribution et d’abondance et la dispersion despopulations de carabiques Notre étude a abouti à trois résultats majeurs. (1) La dynamique spatiale des espèces de carabiques étudiées est fortement influencée par les types d’habitats et leur instabilité temporelle à différentes échelles spatiales. Nous avons en effet mis en avant l’importance de l’hétérogénéité temporelle des paysages agricoles sur la dynamique spatiale des populations carabiques utilisant la matrice agricole. (2) La dispersion et les mouvements entre parcelles sont une condition importante du succès de ces espèces dans les paysagesagricoles et semblent être principalement induits par les rotations de cultures. (3) Du fait de grandes tailles de populations et d’une dispersion efficace importante, la structuration spatiale et génétique des populations reste faible à l’échelle des paysages agricoles

Efficient shape reconstruction of non-circular tubes using broadband acoustic measurements

International audienceWe propose an algorithm for reconstructing the boundaries of a non-circular cylindrical tube from broadband acoustic measurements. This algorithm is based on the minimization of a cost function, which is the average over frequency of the absolute difference between the estimated and the measured scattered field. The estimated field is computed efficiently (very fast) using ICBA, an analytic method that provides an approximate solution of the forward problem. Numerical results show that our algorithm is robust and provides an accurate reconstruction without any explicit regularization