5,939 research outputs found
How Many Communities Are There?
Stochastic blockmodels and variants thereof are among the most widely used
approaches to community detection for social networks and relational data. A
stochastic blockmodel partitions the nodes of a network into disjoint sets,
called communities. The approach is inherently related to clustering with
mixture models; and raises a similar model selection problem for the number of
communities. The Bayesian information criterion (BIC) is a popular solution,
however, for stochastic blockmodels, the conditional independence assumption
given the communities of the endpoints among different edges is usually
violated in practice. In this regard, we propose composite likelihood BIC
(CL-BIC) to select the number of communities, and we show it is robust against
possible misspecifications in the underlying stochastic blockmodel assumptions.
We derive the requisite methodology and illustrate the approach using both
simulated and real data. Supplementary materials containing the relevant
computer code are available online.Comment: 26 pages, 3 figure
Change in the Magnetic Domain Alignment Process at the Onset of a Frustrated Magnetic State in Ferrimagnetic La2Ni(Ni1/3Sb2/3)O6 Double Perovskite
We have performed a combined study of magnetization hysteresis loops and time
dependence of the magnetization in a broad temperature range for the
ferrimagnetic La2Ni(Ni1/3Sb2/3)O6 double perovskite. This material has a
ferrimagnetic order transition at ~100 K and at lower temperatures (~ 20 K)
shows the signature of a frustrated state due to the presence of two competing
magnetic exchange interactions. The temperature dependence of the coercive
field shows an important upturn below the point where the frustrated state sets
in. The use of the magnetization vs. applied magnetic field hysteresis data,
together with the magnetization vs. time data provides a unique opportunity to
distinguish between different scenarios for the low temperature regime. From
our analysis, a strong domain wall pinning results the best scenario for the
low temperature regime. For temperatures larger than 20K the adequate scenario
seems to correspond to a weak domain wall pinning.Comment: 4 pages, 5 figures included. Manuscript submitted to IEEE
Transactions on Magnetics, proceedings of the LAW3M 2013 conferenc
Pitx2; un viaje desde la regulación embrionaria a la arritmogenésis cardiaca
Conferenciante invitado al ciclo de conferencias del Máster en Biología Celular y MolecularNuestra línea de investigación se ha centrado en diseccionar el papel funcional de factor de transcripción Pitx2 durante en desarrollo cardiovascular y también la miogenesis esquelética. Más recientemente hemos descubierto el papel esencial de este factor de transcripción en procesos regenerativos y como controlador en la expresión y función de distintos microRNAs. Estos resultados han permitido desarrollar y registrar un patente sobre el uso de microRNAs en cardiopatías arritmogénicas y en la actualidad hemos sometido una segunda patente. En la actualidad encamínanos nuestros esfuerzos por tanto en dilucidar el papel funcional de los microRNAs en el contexto muscular, tanto esquelético como cardiaco.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech
Collective identities in reconfigured urban spaces
En este trabajo se trata de materializar la identidad colectiva, reto¬mando los conceptos de memoria y cul-tura como proveedoras de significados y destacando el papel de los colectivos en relación a las “ciudades medias” transformadas a metrópolis, partiendo del análisis teórico-conceptual de otros autores, con el objetivo de construir aproximaciones metodológicas para su aplicación a casos de estudio concre¬tos; como las sociedades que habitan espacios urbanos de mediano tamaño, expuestos a transformaciones socio-espaciales continuas, por factores de crecimiento económico-globales.In this communication, we try to gain a clear understanding of col¬lective identity, returning to the concepts of memory and culture as providers of meaning and highlighting the role of groups in relation to “medium cities” transformed into metropolises. Our ap¬proach has been to start from theoreti¬cal-conceptual analyses of other authors with the objective of building methodo¬logical approaches for its application to specific case studies such as societies that inhabit urban spaces of medium size and are exposed to continuous socio-spatial transformations due to fac¬tors of global economic growth
Multiplexity and multireciprocity in directed multiplexes
Real-world multi-layer networks feature nontrivial dependencies among links
of different layers. Here we argue that, if links are directed, dependencies
are twofold. Besides the ordinary tendency of links of different layers to
align as the result of `multiplexity', there is also a tendency to anti-align
as the result of what we call `multireciprocity', i.e. the fact that links in
one layer can be reciprocated by \emph{opposite} links in a different layer.
Multireciprocity generalizes the scalar definition of single-layer reciprocity
to that of a square matrix involving all pairs of layers. We introduce
multiplexity and multireciprocity matrices for both binary and weighted
multiplexes and validate their statistical significance against maximum-entropy
null models that filter out the effects of node heterogeneity. We then perform
a detailed empirical analysis of the World Trade Multiplex (WTM), representing
the import-export relationships between world countries in different
commodities. We show that the WTM exhibits strong multiplexity and
multireciprocity, an effect which is however largely encoded into the degree or
strength sequences of individual layers. The residual effects are still
significant and allow to classify pairs of commodities according to their
tendency to be traded together in the same direction and/or in opposite ones.
We also find that the multireciprocity of the WTM is significantly lower than
the usual reciprocity measured on the aggregate network. Moreover, layers with
low (high) internal reciprocity are embedded within sets of layers with
comparably low (high) mutual multireciprocity. This suggests that, in the WTM,
reciprocity is inherent to groups of related commodities rather than to
individual commodities. We discuss the implications for international trade
research focusing on product taxonomies, the product space, and
fitness/complexity metrics.Comment: 20 pages, 8 figure
The role of distances in the World Trade Web
In the economic literature, geographic distances are considered fundamental
factors to be included in any theoretical model whose aim is the quantification
of the trade between countries. Quantitatively, distances enter into the
so-called gravity models that successfully predict the weight of non-zero trade
flows. However, it has been recently shown that gravity models fail to
reproduce the binary topology of the World Trade Web. In this paper a different
approach is presented: the formalism of exponential random graphs is used and
the distances are treated as constraints, to be imposed on a previously chosen
ensemble of graphs. Then, the information encoded in the geographical distances
is used to explain the binary structure of the World Trade Web, by testing it
on the degree-degree correlations and the reciprocity structure. This leads to
the definition of a novel null model that combines spatial and non-spatial
effects. The effectiveness of spatial constraints is compared to that of
nonspatial ones by means of the Akaike Information Criterion and the Bayesian
Information Criterion. Even if it is commonly believed that the World Trade Web
is strongly dependent on the distances, what emerges from our analysis is that
distances do not play a crucial role in shaping the World Trade Web binary
structure and that the information encoded into the reciprocity is far more
useful in explaining the observed patterns.Comment: Preprint, accepted for SITIS 2012 (http://www.sitis-conf.org/). Final
version to be published by IEEE Computer Society as conference proceeding
Energy Efficiency in Greenhouse Evaporative Cooling Techniques: Cooling Boxes versus Cellulose Pads
Evaporative cooling systems using a combination of evaporative pads and extractor fans require greenhouses to be hermetic. The greatest concentration of greenhouses in the world is located in southeast Spain, but these tend not to be hermetic structures and consequently can only rely on fogging systems as evaporative cooling techniques. Evaporative cooling boxes provide an alternative to such systems. Using a low-speed wind tunnel, the present work has compared the performance of this system with four pads of differing geometry and thickness manufactured by two different companies. The results obtained show that the plastic packing in the cooling unit produces a pressure drop of 11.05 Pa at 2 m·s−1, which is between 51.27% and 94.87% lower than that produced by the cellulose pads. This pressure drop was not influenced by increases in the water flow. The evaporative cooling boxes presented greater saturation efficiency at the same flow, namely 82.63%, as opposed to an average figure of 65% for the cellulose pads; and also had a lower specific consumption of water, at around 3.05 L·h−1·m−2·°C−1. Consequently, we conclude that evaporative cooling boxes are a good option for cooling non-hermetic greenhouses such as those most frequently used in the Mediterranean basin
Relations between entanglement and purity in non-Markovian dynamics
Knowledge of the relationships among different features of quantumness, like
entanglement and state purity, is important from both fundamental and practical
viewpoints. Yet, this issue remains little explored in dynamical contexts for
open quantum systems. We address this problem by studying the dynamics of
entanglement and purity for two-qubit systems using paradigmatic models of
radiation-matter interaction, with a qubit being isolated from the environment
(spectator configuration). We show the effects of the corresponding local
quantum channels on an initial two-qubit pure entangled state in the
concurrence-purity diagram and find the conditions which enable dynamical
closed formulas of concurrence, used to quantify entanglement, as a function of
purity. We finally discuss the usefulness of these relations in assessing
entanglement and purity thresholds which allow noisy quantum teleportation. Our
results provide new insights about how different properties of composite open
quantum systems behave and relate each other during quantum evolutions.Comment: 16 Pages, 10 Figures. One author added. Improved version with more
references and comment
On Suggesting Entities as Web Search Queries
The Web of Data is growing in popularity and dimension,
and named entity exploitation is gaining importance in many research
fields. In this paper, we explore the use of entities that can be extracted
from a query log to enhance query recommendation. In particular, we
extend a state-of-the-art recommendation algorithm to take into account
the semantic information associated with submitted queries. Our novel
method generates highly related and diversified suggestions that we as-
sess by means of a new evaluation technique. The manually annotated
dataset used for performance comparisons has been made available to
the research community to favor the repeatability of experiments
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