4,073 research outputs found
The stability of a graph partition: A dynamics-based framework for community detection
Recent years have seen a surge of interest in the analysis of complex
networks, facilitated by the availability of relational data and the
increasingly powerful computational resources that can be employed for their
analysis. Naturally, the study of real-world systems leads to highly complex
networks and a current challenge is to extract intelligible, simplified
descriptions from the network in terms of relevant subgraphs, which can provide
insight into the structure and function of the overall system.
Sparked by seminal work by Newman and Girvan, an interesting line of research
has been devoted to investigating modular community structure in networks,
revitalising the classic problem of graph partitioning.
However, modular or community structure in networks has notoriously evaded
rigorous definition. The most accepted notion of community is perhaps that of a
group of elements which exhibit a stronger level of interaction within
themselves than with the elements outside the community. This concept has
resulted in a plethora of computational methods and heuristics for community
detection. Nevertheless a firm theoretical understanding of most of these
methods, in terms of how they operate and what they are supposed to detect, is
still lacking to date.
Here, we will develop a dynamical perspective towards community detection
enabling us to define a measure named the stability of a graph partition. It
will be shown that a number of previously ad-hoc defined heuristics for
community detection can be seen as particular cases of our method providing us
with a dynamic reinterpretation of those measures. Our dynamics-based approach
thus serves as a unifying framework to gain a deeper understanding of different
aspects and problems associated with community detection and allows us to
propose new dynamically-inspired criteria for community structure.Comment: 3 figures; published as book chapte
Thermocapillary valve for droplet production and sorting
Droplets are natural candidates for use as microfluidic reactors, if active
control of their formation and transport can be achieved. We show here that
localized heating from a laser can block the motion of a water-oil interface,
acting as a microfluidic valve for two-phase flows. A theoretical model is
developed to explain the forces acting on a drop due to thermocapillary flow,
predicting a scaling law which favors miniaturization. Finally, we show how the
laser forcing can be applied to sorting drops, thus demonstrating how it may be
integrated in complex droplet microfluidic systems.Comment: Five pages, four figure
Cells in Space
Discussions and presentations addressed three aspects of cell research in space: the suitability of the cell as a subject in microgravity experiments, the requirements for generic flight hardware to support cell research, and the potential for collaboration between academia, industry, and government to develop these studies in space. Synopses are given for the presentations and follow-on discussions at the conference and papers are presented from which the presentations were based. An Executive Summary outlines the recommendations and conclusions generated at the conference
A Turbulent Origin for Flocculent Spiral Structure in Galaxies: II. Observations and Models of M33
Fourier transform power spectra of azimuthal scans of the optical structure
of M33 are evaluated for B, V, and R passbands and fit to fractal models of
continuum emission with superposed star formation. Power spectra are also
determined for Halpha. The best models have intrinsic power spectra with 1D
slopes of around -0.7pm0.7, significantly shallower than the Kolmogorov
spectrum (slope =-1.7) but steeper than pure noise (slope=0). A fit to the
power spectrum of the flocculent galaxy NGC 5055 gives a steeper slope of
around -1.5pm0.2, which could be from turbulence. Both cases model the optical
light as a superposition of continuous and point-like stellar sources that
follow an underlying fractal pattern. Foreground bright stars are clipped in
the images, but they are so prominent in M33 that even their residual affects
the power spectrum, making it shallower than what is intrinsic to the galaxy. A
model consisting of random foreground stars added to the best model of NGC 5055
fits the observed power spectrum of M33 as well as the shallower intrinsic
power spectrum that was made without foreground stars. Thus the optical
structure in M33 could result from turbulence too.Comment: accepted by ApJ, 13 pages, 10 figure
Simulation of mitochondrial metabolism using multi-agents system
Metabolic pathways describe chains of enzymatic reactions. Their modelling is
a key point to understand living systems. An enzymatic reaction is an
interaction between one or several metabolites (substrates) and an enzyme
(simple protein or enzymatic complex build of several subunits). In our
Mitochondria in Silico Project, MitoScop, we study the metabolism of the
mitochondria, an intra-cellular organelle. Many ordinary differential equation
models are available in the literature. They well fit experimental results on
flux values inside the metabolic pathways, but many parameters are dicult
to transcribe with such models: localization of enzymes, rules about the
reactions scheduler, etc Moreover, a model of a significant part of
mitochondrial metabolism could become very complex and contain more than 50
equations. In this context, the multi-agents systems appear as an alternative
to model the metabolic pathways. Firstly, we have looked after membrane design.
The mitochondria is a particular case because the inner mitochondrial space, ie
matricial space, is delimited by two membranes: the inner and the outer one. In
addition to matricial enzymes, other enzymes are located inside the membranes
or in the inter-membrane space. Analysis of mitochondrial metabolism must take
into account this kind of architecture
Reduction of the model noise in non-linear reconstruction via an efficient calculation of the incident field: application to a 434 MHz Scanner
Microwave tomography has been drastically boosted by the development of efficient reconstruction algorithms based on an iterative solution of the corresponding non-linear inverse problem. The accuracy of the electric field radiated by the antennas of a microwave scanner, inside the target area, has been shown to play a significant role on the overall image quality. Taking into account the antenna environment is of prime importance, especially when operating at low frequency. For instance, the wall of a 60 cm diameter whole-body microwave scanner cannot be neglected at 434 MHz, even when using the immersion technique consisting of putting the target in water. Indeed, at such a frequency, the attenuation introduced by water is not sufficient to avoid multiple reflections on the scanner boundary walls. Consequently, the method of calculating the incident field constitutes a key factor in iteratively solving non-linear inverse problems. The selected technique must accommodate high accuracy while maintaining acceptable calculation complexity. In this paper, three distinct techniques are analysed. They are based on the use of i) free-space and ii) non free-space Green's function, and iii) a FDTD approach. All these techniques have been firstly investigated for their 2D version, being used in 2D reconstruction algorithms. However, the scattered field data are collected in a 3D scanner. For assessing the validity of the previous 2D techniques, their results have been compared to both experimentally and 3D-FDTD results.Peer ReviewedPostprint (published version
Gravity Data Interpretation in the Northern Edge of the Congo Craton, South-Cameroon
Gravity data in the southern Cameroon are interpreted to better understand the organization of underlying structuresthroughout the northern edge of the Congo craton. The Bouguer anomaly maps of the region are characterized by an elongated trending trending negative gravity anomaly which correspond to a collapsed structure associated with a granitic intrusion beneath the cente center of the region r of the region of the region and limited by fault systems. �e applied 3�D gravity modelling and inversion in order to obtain the 3D density structure of the area. Our result demonstrated that observed gravity anomalies in the region are associated to tectonic structures in the subsurface. The resulting model agrees with the hypothesis of the existence of a major continental collision zone between the Congo Craton and the Pan�African belt. The presence of deep granulites structures in the northern part of the region expresses a continental collision
Interpretação gravimétrica na Borda Norte do Cráton do Congo, Sul de Camarões
Gravity data in the southern Cameroon are interpreted to better understand the organization of underlying structures throughout the northern edge of the Congo craton. The Bouguer anomaly maps of the region are characterized by an elongated SW-NE trending negative gravity anomaly which correspond to a collapsed structure associated with a granitic intrusion beneath the center of the region and limited by fault systems. We applied 3-D gravity modelling and inversion in order to obtain the 3-D density structure of the area. Our result demonstrated that observed gravity anomalies in the region are associated to tectonic structures in the subsurface. The resulting model agrees with the hypothesis of the existence of a major continental collision zone between the Congo Craton and the Pan-African belt. The presence of deep granulites structures in the northern part of the region expresses a continental collision.Um conjunto de dados gravimétricos, provenientes do sul de Camarões, foram interpretados para o melhor entendimento das estruturas em sub superfície na borda norte do Cráton do Congo. Os mapas de anomalia Bouguer desta região foram caracterizados por uma anomalia gravimétrica negativa de direção SW-NE, que corresponde a uma estrutura de colapso associada com uma intrusão granítica, abaixo do centro desta região, e que está limitada por um sistema de falhas. Foram utilizados métodos de modelagem gravimétrica 3 D e inversão, para se obter uma estrutura densa 3-D desta área. Os resultados demonstraram que as anomalias gravimétricas observadas na região estão associadas com estruturas tectônicas em sub superfície. O modelo resultado está em consonância com a hipótese de existência de uma zona de colisão continental principal entre o Cráton do Congo e o Cinturão Pan-Africano. A presença de estruturas granulíticas profundas na borda norte desta área indica uma colisão continental
Microwave imaging techniques for biomedical applications
Microwaves have been considered for medical applications involving the detection of organ movements and changes in tissue water content. More particularly cardiopulmonary interrogation via microwaves has resulted in various sensors monitoring ventricular volume change or movement, arterial wall motion, respiratory movements, pulmonary oedema, etc. In all these applications, microwave sensors perform local measurements and need to be displaced for obtaining an image reproducing the spatial variations of a given quantity. Recently, advances in the area of inverse scattering theory and microwave technology have made possible the development of microwave imaging and tomographic instruments. This paper provides a review of such equipment developed at Suplec and UPC Barcelona, within the frame of successive French-Spanish PICASSO cooperation programs. It reports the most significant results and gives some perspectives for future developments. Firstly, a brief historical survey is given. Then, both technological and numerical aspects are considered. The results of preliminary pre-clinical assessments and in-lab experiments allow to illustrate the capabilities of the existing equipment, as well as its difficulty in dealing with clinical situations. Finally, some remarks on the expected development of microwave imaging techniques for biomedical applications are given.Peer ReviewedPostprint (published version
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