6,869 research outputs found
The anatomy of urban social networks and its implications in the searchability problem
The appearance of large geolocated communication datasets has recently
increased our understanding of how social networks relate to their physical
space. However, many recurrently reported properties, such as the spatial
clustering of network communities, have not yet been systematically tested at
different scales. In this work we analyze the social network structure of over
25 million phone users from three countries at three different scales: country,
provinces and cities. We consistently find that this last urban scenario
presents significant differences to common knowledge about social networks.
First, the emergence of a giant component in the network seems to be controlled
by whether or not the network spans over the entire urban border, almost
independently of the population or geographic extension of the city. Second,
urban communities are much less geographically clustered than expected. These
two findings shed new light on the widely-studied searchability in
self-organized networks. By exhaustive simulation of decentralized search
strategies we conclude that urban networks are searchable not through
geographical proximity as their country-wide counterparts, but through an
homophily-driven community structure
Efficiency of Human Activity on Information Spreading on Twitter
Understanding the collective reaction to individual actions is key to
effectively spread information in social media. In this work we define
efficiency on Twitter, as the ratio between the emergent spreading process and
the activity employed by the user. We characterize this property by means of a
quantitative analysis of the structural and dynamical patterns emergent from
human interactions, and show it to be universal across several Twitter
conversations. We found that some influential users efficiently cause
remarkable collective reactions by each message sent, while the majority of
users must employ extremely larger efforts to reach similar effects. Next we
propose a model that reproduces the retweet cascades occurring on Twitter to
explain the emergent distribution of the user efficiency. The model shows that
the dynamical patterns of the conversations are strongly conditioned by the
topology of the underlying network. We conclude that the appearance of a small
fraction of extremely efficient users results from the heterogeneity of the
followers network and independently of the individual user behavior.Comment: 29 pages, 10 figure
Further Improvements in the Understanding of Isotropic Loop Quantum Cosmology
The flat, homogeneous, and isotropic universe with a massless scalar field is
a paradigmatic model in Loop Quantum Cosmology. In spite of the prominent role
that the model has played in the development of this branch of physics, there
still remain some aspects of its quantization which deserve a more detailed
discussion. These aspects include the kinematical resolution of the
cosmological singularity, the precise relation between the solutions of the
densitized and non-densitized versions of the quantum Hamiltonian constraint,
the possibility of identifying superselection sectors which are as simple as
possible, and a clear comprehension of the Wheeler-DeWitt (WDW) limit
associated with the theory in those sectors. We propose an alternative operator
to represent the Hamiltonian constraint which is specially suitable to deal
with these issues in a satisfactory way. In particular, with our constraint
operator, the singularity decouples in the kinematical Hilbert space and can be
removed already at this level. Thanks to this fact, we can densitize the
quantum Hamiltonian constraint in a rigorous manner. Besides, together with the
physical observables, this constraint superselects simple sectors for the
universe volume, with a support contained in a single semiaxis of the real line
and for which the basic functions that encode the information about the
geometry possess optimal physical properties. Namely, they provide a
no-boundary description around the cosmological singularity and admit a
well-defined WDW limit in terms of standing waves. Both properties explain the
presence of a generic quantum bounce replacing the singularity at a fundamental
level, in contrast with previous studies where the bounce was proved in
concrete regimes and focusing on states with a marked semiclassical behavior.Comment: 13 pages, version accepted for publication in Physical Review
Millimeter Wave Substrate Integrated Waveguide Antennas: Design and Fabrication Analysis
The paper presents a new concept in antenna design, whereby a photo-imageable thick-film process is used to integrate a waveguide antenna within a multilayer structure. This has yielded a very compact, high performance antenna working at high millimeter-wave (mm-wave) frequencies, with a high degree of repeatability and reliability in antenna construction. Theoretical and experimental results for 70 GHz mm-wave integrated antennas, fabricated using the new technique are presented. The antennas were formed from miniature slotted waveguide arrays using up to 18 layers of photo-imageable material. To enhance the electrical performance a novel folded waveguide array was also investigated. The fabrication process is analysed in detail and the critical issues involved in the fabrication cycle are discussed. The losses in the substrate integrated waveguide have been calculated. The performance of the new integrated antenna is compared to conventional metallic, air-filled waveguide antennas, and also to conventional microstrip antenna arrays operating at the same frequencies
On the microstructure, growth pattern and original porosity of belemnite rostra: insights from calcitic Jurassic belemnites
[EN] Calcitic belemnite rostra are usually employed to perform paleoenvironmental studies based on geochemical data. However, several
questions, such as their original porosity and microstructure, remain open, despite they are essential to make accurate interpretations based
on geochemical analyses.
This paper revisits and enlightens some of these questions. Petrographic data demonstrate that calcite crystals of the rostrum solidum of
belemnites grow from spherulites that successively develop along the apical line, resulting in a “regular spherulithic prismatic” microstructure.
Radially arranged calcite crystals emerge and diverge from the spherulites: towards the apex, crystals grow until a new spherulite is
formed; towards the external walls of the rostrum, the crystals become progressively bigger and prismatic. Adjacent crystals slightly vary
in their c-axis orientation, resulting in undulose extinction. Concentric growth layering develops at different scales and is superimposed and
traversed by a radial pattern, which results in the micro-fibrous texture that is observed in the calcite crystals in the rostra.
Petrographic data demonstrate that single calcite crystals in the rostra have a composite nature, which strongly suggests that the belemnite
rostra were originally porous. Single crystals consistently comprise two distinct zones or sectors in optical continuity: 1) the inner
zone is fluorescent, has relatively low optical relief under transmitted light (TL) microscopy, a dark-grey color under backscatter electron
microscopy (BSEM), a commonly triangular shape, a “patchy” appearance and relatively high Mg and Na contents; 2) the outer sector is
non-fluorescent, has relatively high optical relief under TL, a light-grey color under BSEM and low Mg and Na contents. The inner and
fluorescent sectors are interpreted to have formed first as a product of biologically controlled mineralization during belemnite skeletal
growth and the non-fluorescent outer sectors as overgrowths of the former, filling the intra- and inter-crystalline porosity. This question has
important implications for making paleoenvironmental and/or paleoclimatic interpretations based on geochemical analyses of belemnite
rostra.
Finally, the petrographic features of composite calcite crystals in the rostra also suggest the non-classical crystallization of belemnite
rostra, as previously suggested by other authors.[ES] Los análisis geoquĂmicos obtenidos en rostros de los belemnites se utilizan habitualmente para llevar a cabo estudios paleoambientales.
Sin embargo, hay cuestiones esenciales, como la porosidad y la microestructura original de los belemnites, que están todavĂa en discusiĂłn
y que hay que tener en cuenta si se quieren realizar interpretaciones geoquĂmicas adecuadas.
Los datos petrográficos obtenidos en este trabajo demuestran que los cristales de calcita del rostrum solidum de los belemnites crecen a
partir de esferulitos que sucesivamente se desarrollan a lo largo de la lĂnea apical, dando lugar a una microestructura “esferulĂtica prismática
regular”. Los cristales de calcita surgen y divergen radialmente desde los esferulitos: hacia el apex, hasta que se forma un nuevo esferulito
y hacia los bordes externos del rostro donde los cristales se van haciendo progresivamente mayores y prismáticos. La ligera variación de la
orientaciĂłn del eje-c de los cristales adyacentes explica la extinciĂłn ondulante. Al patrĂłn radial, que da lugar a la textura micro-fibrosa de southlos
cristales de calcita del rostro, se le superpone un bandeado de crecimiento concéntrico.
Los datos petrográficos también indican que los rostros de los belemnites eran originalmente porosos como lo demuestra la naturaleza
compuesta de los cristales de calcita que están formados sistemáticamente por dos zonas distintas que están en continuidad óptica: 1) la
zona interna es fluorescente, tiene relieve Ăłptico bajo, en microscopia de luz transmitida (TL), color gris oscuro en microscopia electrĂłnica
con electrones retrodispersados (BSEM), morfologĂa triangular y contenidos en Mg y Na relativamente altos; 2) la zona exterior no es
fluorescente, tiene relieve Ăłptico alto (TL), color gris claro (BSEM), y contenidos en Mg y Na relativamente bajos. La zona fluorescente
interior se formĂł primero, durante el crecimiento de los belemnites y la zona no fluorescente precipitĂł como un cemento rellenando la
porosidad intra- e intercristalina.
Las caracterĂsticas petrográficas de los cristales de calcita tambiĂ©n sugieren que la cristalizaciĂłn no clásica del esqueleto de los belemnites,
como han sugerido algunos autores previamente.This research was carried out with the financial support of
the projects CGL2011-22709, CGL2014-52670-P and RYC-
2009-04316 (RamĂłn y Cajal Program, Ministerio de Ciencia
e Innovación), and by the “Sedimentary Basin Analysis”
and “Paleoclimatology and Global Change” research groups
of the Complutense University of Madrid.Peer reviewe
Inhomogeneous Loop Quantum Cosmology: Hybrid Quantization of the Gowdy Model
The Gowdy cosmologies provide a suitable arena to further develop Loop
Quantum Cosmology, allowing the presence of inhomogeneities. For the particular
case of Gowdy spacetimes with the spatial topology of a three-torus and a
content of linearly polarized gravitational waves, we detail a hybrid quantum
theory in which we combine a loop quantization of the degrees of freedom that
parametrize the subfamily of homogeneous solutions, which represent Bianchi I
spacetimes, and a Fock quantization of the inhomogeneities. Two different
theories are constructed and compared, corresponding to two different schemes
for the quantization of the Bianchi I model within the {\sl improved dynamics}
formalism of Loop Quantum Cosmology. One of these schemes has been recently put
forward by Ashtekar and Wilson-Ewing. We address several issues including the
quantum resolution of the cosmological singularity, the structure of the
superselection sectors in the quantum system, or the construction of the
Hilbert space of physical states.Comment: 16 pages, version accepted for publication in Physical Review
Biphasic effect of insulin on beta cell apoptosis depending on glucose deprivation
AbstractInsulin resistant states are associated with an increase in the beta cell mass and also high levels of circulating insulin. Ultimately the beta cells undergo a failure that leads to diabetes. At this stage, a question arises if those persistent high levels of circulating insulin may contribute to beta cell damage. To address this important issue, we submitted beta cells to a prolonged effect of increasing concentrations of insulin. We observed that a prolonged effect of high levels of insulin on the presence of serum (15–24h) in glucose-deprived beta cells induced apoptosis. This apoptotic effect was both dose- and cycloheximide-dependent
Signatures of homoclinic motion in quantum chaos
Homoclinic motion plays a key role in the organization of classical chaos in
Hamiltonian systems. In this Letter, we show that it also imprints a clear
signature in the corresponding quantum spectra. By numerically studying the
fluctuations of the widths of wavefunctions localized along periodic orbits we
reveal the existence of an oscillatory behavior, that is explained solely in
terms of the primary homoclinic motion. Furthermore, our results indicate that
it survives the semiclassical limit.Comment: 5 pages, 4 figure
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