Measuring the effect of node aggregation on community detection

Many times the nodes of a complex network, whether deliberately or not, are aggregated for technical, ethical, legal limitations or privacy reasons. A common example is the geographic position: one may uncover communities in a network of places, or of individuals identified with their typical geographical position, and then aggregate these places into larger entities, such as municipalities, thus obtaining another network. The communities found in the networks obtained at various levels of aggregation may exhibit various degrees of similarity, from full alignment to perfect independence. This is akin to the problem of ecological and atomic fallacies in statistics, or to the Modified Areal Unit Problem in geography. We identify the class of community detection algorithms most suitable to cope with node aggregation, and develop an index for aggregability, capturing to which extent the aggregation preserves the community structure. We illustrate its relevance on real-world examples (mobile phone and Twitter reply-to networks). Our main message is that any node-partitioning analysis performed on aggregated networks should be interpreted with caution, as the outcome may be strongly influenced by the level of the aggregation.Comment: 12 pages, 5 figure

Anomalous diffusion mediated by atom deposition into a porous substrate

Constant flux atom deposition into a porous medium is shown to generate a dense overlayer and a diffusion profile. Scaling analysis shows that the overlayer acts as a dynamic control for atomic diffusion in the porous substrate. This is modeled by generalizing the porous diffusion equation with a time-dependent diffusion coefficient equivalent to a nonlinear rescaling of timeComment: 4 page

SPAWN: An Iterative, Potentials-Based, Dynamic Scheduling and Partitioning Tool

International audienceMany applications of physics modeling use regular meshes on which computations of highly variable cost can occur. Distributing the underlying cells over manycore architec-tures is a critical load balancing step that should increase the period until another step is required. Graph partitioning tools are known to be very effective for such problems, but they exhibit scalability problems as the number of cores and the number of cells increases. We introduce a dynamic task scheduling approach inspired by physical particles interactions. Our method allows cores to virtually move over a 2D/3D mesh of tasks and uses a Voronoi domain decomposition to balance workload among cores. Displacements of cores are the result of force computations using a carefully chosen pair potential. We evaluate our method against graph partitioning tools and existing task schedulers with a representative physical application, and demonstrate the relevance of our approach

A weak lensing study of the Coma cluster

Due to observational constraints, dark matter determinations in nearby clusters based on weak lensing are still extremely rare, in spite of their importance for the determination of cluster properties independent of other methods. We present a weak lensing study of the Coma cluster (redshift 0.024) based on deep images obtained at the CFHT. After obtaining photometric redshifts for the galaxies in our field based on deep images in the u (1x1 deg2), and in the B, V, R and I bands (42'x52'), allowing us to eliminate foreground galaxies, we apply weak lensing calculations on shape measurements performed in the u image. We derive a map of the mass distribution in Coma, as well as the radial shear profile, and the mass and concentration parameter at various radii. We obtain M_200c = 5.1+4.3-2.1 x10^14 Msun and c_200c=5.0+3.2-2.5, in good agreement with previous measurements. With deep wide field images it is now possible to analyze nearby clusters with weak lensing techniques, thus opening a broad new field of investigation

Continuum and line modelling of discs around young stars. I. 300000 disc models for Herschel/GASPS

We have combined the thermo-chemical disc code ProDiMo with the Monte Carlo radiative transfer code MCFOST to calculate a grid of ~300000 circumstellar disc models, systematically varying 11 stellar, disc and dust parameters including the total disc mass, several disc shape parameters and the dust-to-gas ratio. For each model, dust continuum and line radiative transfer calculations are carried out for 29 far IR, sub-mm and mm lines of [OI], [CII], 12CO and o/p-H2O under 5 inclinations. The grid allows to study the influence of the input parameters on the observables, to make statistical predictions for different types of circumstellar discs, and to find systematic trends and correlations between the parameters, the continuum fluxes, and the line fluxes. The model grid, comprising the calculated disc temperatures and chemical structures, the computed SEDs, line fluxes and profiles, will be used in particular for the data interpretation of the Herschel open time key programme GASPS. The calculated line fluxes show a strong dependence on the assumed UV excess of the central star, and on the disc flaring. The fraction of models predicting [OI] and [CII] fine-structure lines fluxes above Herschel/PACS and Spica/SAFARI detection limits are calculated as function of disc mass. The possibility of deriving the disc gas mass from line observations is discussed.Comment: accepted by MNRAS. 5 pages, 4 figures, 3 table

Synthèse et réticulation de polymères énergétiques azoturés

RÉSUMÉ: Cette thèse présente d’une part le développement détaillé de polyesters énergétiques azoturés et d’outils servant à mettre en forme par réticulation les polymères énergétiques azoturés, et d’autre part, le développement sommaire de quelques polymères énergétiques additionnels obtenus par cycloaddition et des polyélectrolytes de poly(5-vinyltétrazole). Le développement de nouveaux polymères énergétiques s’est effectué par plusieurs mécanismes, à partir de précurseurs commerciaux avec une approche dirigée pour satisfaire des contraintes de sécurité et d’industrialisation fixées à la base du projet. La voie de synthèse privilégiée dans cette thèse détaille la polymérisation par transestérification en masse de diols halogénés avec le diphényl carbonate par un catalyseur neutre, une réaction développée pour éviter l’utilisation du phosgène dans la préparation de polycarbonates halogénés. Ces polymères ont été azoturés dans un solvant polaire en présence d’un catalyseur pour obtenir des polycarbonates azoturés ciblés pour leurs propriétés énergétiques avantageuses. Les polycarbonates obtenus, le poly(2,2’-bisazidométhyl-1,3-propanediyl carbonate) et le poly(3-azido-1,2-propanediyl carbonate) sont des pré-polymères liquides dont les propriétés peuvent être comparées à celles du polyazoture de glycidyle, le seul autre polymère énergétique azoturé produit commercialement----------ABSTRACT: This thesis presents the detailed development of energetic azido polyesters and of tools useful in the preparation of cross-linked azido polymers, as well as a summary development of additional energetic polymers obtained from the cycloaddition polymerization between diazides and dialkynes, and polyelectrolytes of poly(5-vinyltetrazole).The development of new energetic polymers was achieved through various mechanisms, starting from commercial precursors with an approach that satisfies the security and economic constraints imposed from the start of the project. The main synthesis route presented details the polymerization by bulk transesterification of halogenated diols with diphenyl carbonate in the presence of a neutral catalyst, a process developed to avoid altogether the use of phosgene in the preparation of halogenated polycarbonates. These polymers can then be substituted by azides in a polar solvent in the presence of a catalyst to afford azido polycarbonates with considerable energetic properties. The polycarbonates obtained are poly(2,2’-bisazidomethyl-1,3-propanediyl carbonate) and poly(3-azido-1,2-propanediyl carbonate), liquid pre-polymers with properties comparable to glycidyl azide polymers, the only other azido polymer available commercially