308 research outputs found
Collective versus hub activation of epidemic phases on networks
We consider a general criterion to discern the nature of the threshold in
epidemic models on scale-free (SF) networks. Comparing the epidemic lifespan of
the nodes with largest degrees with the infection time between them, we propose
a general dual scenario, in which the epidemic transition is either ruled by a
hub activation process, leading to a null threshold in the thermodynamic limit,
or given by a collective activation process, corresponding to a standard phase
transition with a finite threshold. We validate the proposed criterion applying
it to different epidemic models, with waning immunity or heterogeneous
infection rates in both synthetic and real SF networks. In particular, a waning
immunity, irrespective of its strength, leads to collective activation with
finite threshold in scale-free networks with large exponent, at odds with
canonical theoretical approaches.Comment: Revised version accepted for publication in PR
Phase transitions with infinitely many absorbing states in complex networks
We instigate the properties of the threshold contact process (TCP), a process
showing an absorbing-state phase transition with infinitely many absorbing
states, on random complex networks. The finite size scaling exponents
characterizing the transition are obtained in a heterogeneous mean field (HMF)
approximation and compared with extensive simulations, particularly in the case
of heterogeneous scale-free networks. We observe that the TCP exhibits the same
critical properties as the contact process (CP), which undergoes an
absorbing-state phase transition to a single absorbing state. The accordance
among the critical exponents of different models and networks leads to
conjecture that the critical behavior of the contact process in a HMF theory is
a universal feature of absorbing state phase transitions in complex networks,
depending only on the locality of the interactions and independent of the
number of absorbing states. The conditions for the applicability of the
conjecture are discussed considering a parallel with the
susceptible-infected-susceptible epidemic spreading model, which in fact
belongs to a different universality class in complex networks.Comment: 9 pages, 6 figures to appear in Phys Rev
Quantifying echo chamber effects in information spreading over political communication networks
Echo chambers in online social networks, in which users prefer to interact
only with ideologically-aligned peers, are believed to facilitate
misinformation spreading and contribute to radicalize political discourse. In
this paper, we gauge the effects of echo chambers in information spreading
phenomena over political communication networks. Mining 12 million Twitter
messages, we reconstruct a network in which users interchange opinions related
to the impeachment of the former Brazilian President Dilma Rousseff. We define
a continuous {political position} parameter, independent of the network's
structure, that allows to quantify the presence of echo chambers in the
strongly connected component of the network, reflected in two well-separated
communities of similar sizes with opposite views of the impeachment process. By
means of simple spreading models, we show that the capability of users in
propagating the content they produce, measured by the associated spreadability,
strongly depends on their attitude. Users expressing pro-impeachment sentiments
are capable to transmit information, on average, to a larger audience than
users expressing anti-impeachment sentiments. Furthermore, the users'
spreadability is correlated to the diversity, in terms of political position,
of the audience reached. Our method can be exploited to identify the presence
of echo chambers and their effects across different contexts and shed light
upon the mechanisms allowing to break echo chambers.Comment: 9 pages, 4 figures. Supplementary Information available as ancillary
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Irrelevance of information outflow in opinion dynamics models
The Sznajd model for opinion dynamics has attracted a large interest as a
simple realization of the psychological principle of social validation. As its
most salient feature, it has been claimed that the Sznajd model is
qualitatively different from other ordering processes, because it is the only
one featuring outflow of information as opposed to inflow. We show that this
claim is unfounded by presenting a generalized zero-temperature Glauber-type of
dynamics which yields results indistinguishable from those of the Sznajd model.
In one-dimension we also derive an exact expression for the exit probability of
the Sznajd model, that turns out to coincide with the result of an analytical
approach based on the Kirkwood approximation. This observation raises
interesting questions about the applicability and limitations of this approach.Comment: 5 pages, 4 figure
La declaración unilateral de independencia de Kosovo a la luz de la Opinión Consultiva de la Corte Internacional de Justicia, de 22 de julio de 2010 y de las declaraciones, opiniones individuales y disidentes a la misma
The Opinion of the International Court of Justice on the unilateral declaration of independence of Kosovo is an example of what the Court should not do. Saying only that international law does not prohibit declarations of independence and refusing to answer the matters related to this claim, the Court's opinion adopts a vacuum of legal terms and irresponsible
Voter models on weighted networks
We study the dynamics of the voter and Moran processes running on top of
complex network substrates where each edge has a weight depending on the degree
of the nodes it connects. For each elementary dynamical step the first node is
chosen at random and the second is selected with probability proportional to
the weight of the connecting edge. We present a heterogeneous mean-field
approach allowing to identify conservation laws and to calculate exit
probabilities along with consensus times. In the specific case when the weight
is given by the product of nodes' degree raised to a power theta, we derive a
rich phase-diagram, with the consensus time exhibiting various scaling laws
depending on theta and on the exponent of the degree distribution gamma.
Numerical simulations give very good agreement for small values of |theta|. An
additional analytical treatment (heterogeneous pair approximation) improves the
agreement with numerics, but the theoretical understanding of the behavior in
the limit of large |theta| remains an open challenge.Comment: 21 double-spaced pages, 6 figure
Effects of local population structure in a reaction-diffusion model of a contact process on metapopulation networks
We investigate the effects of local population structure in reaction-diffusion processes representing a contact process (CP) on metapopulations represented as complex networks. Considering a model in which the nodes of a large scale network represent local populations defined in terms of a homogeneous graph, we show by means of extensive numerical simulations that the critical properties of the reaction-diffusion system are independent of the local population structure, even when this one is given by a ordered linear chain. This independence is confirmed by the perfect matching between numerical critical exponents and the results from a heterogeneous mean-field theory suited, in principle, to describe situations of local homogeneous mixing. The analysis of several variations of the reaction-diffusion process allows us to conclude the independence from population structure of the critical properties of CP-like models on metapopulations, and thus of the universality of the reaction-diffusion description of this kind of models.Postprint (published version
Nasopharyngeal angiofibroma: review of literature
Angiofibroma of nasopharynx is a highly vascular, rare and benign tumor, with a high a incidence of recurrence and local invasity which nature always defied the researchers on the subject. The controversies start on its site of origin, till therapeutical modalities, passing by clinical and diagnostic aspects. The most serious and baffling feature, however, is the occurrence of hemorrhagic outbreaks, sometimes very intense, which may lead the patient to death. In view of this, the efforts developed towards of minimizing this problematic fact have been many, aiming adequate control of the illness. The present article presents a global review on this subject and intends to discuss the main aspects related to angiofibroma of nasopharynx in a simple and didatic form.O angiofibroma de nasofaringe é um tumor benigno, raro, altamente vascularizado, com elevado Ãndice de recorrência e invasividade local, cuja natureza sempre desafiou os estudiosos do assunto. As controvérsias vão desde o seu local de origem, até modalidades terapêuticas, passando por aspectos clÃnicos e diagnósticos. A caracterÃstica mais grave e desconcertante, no entanto, é a ocorrência de surtos hemorrágicos, por vezes muito intensos, podendo levar o paciente à morte. Frente a esse fato, muitos foram os esforços desenvolvidos no sentido de minimizar esse problema, visando o controle adequado da doença. Esse estudo apresenta uma revisão global sobre o tema e se propõe a expor de forma simples e didática os principais aspectos relacionados ao angiofibroma nasofarÃngeo.Instituto de Assistência Médica ao Servidor Público Estadual de São PauloUNIFESP-EPMHSPE FMO Departamento de OtorrinolaringologiaHospital do Servidor Público Municipal Serviço de OtorrinolaringologiaUniversidade de São Paulo Faculdade de MedicinaHospital do Servidor Público Estadual FMO Departamento de OtorrinolaringologiaUNIFESP, EPMSciEL
Degradation in AlGaN/GaN heterojunction field effect transistors upon electrical stress: Effects of field and temperature
AlGaN/GaN heterojunction field effect transistors (HFETs) with 2 μm gate length were subjected to on-state-high-field (high drain bias and drain current) and reverse-gate-bias (no drain currentand reverse gate bias) stress at room and elevated temperatures for up to 10 h. The resulting degradation of the HFETs was studied by direct current and uniquely phase noise before and after stress. A series of drain and gate voltages was applied during the on-state-high-field and reverse-gate-bias stress conditions, respectively, to examine the effect of electric field on degradation of the HFET devices passivated with SiNx. The degradation behaviors under these two types of stress conditions were analyzed and compared. In order to isolate the effect of self-heating/temperature on device degradation, stress experiments were conducted at base plate temperatures up to 150 °C. It was found that the electric field induced by reverse-gate-bias mainly generated trap(s), most likely in the AlGaN barrier, which initially were manifested as generation-recombination (G-R) peak(s) in the phase noise spectra near 103 Hz. Meanwhileelectric field induced by on-state-high-field stress mainly generated hot-electron and hot-phonon effects, which result in a nearly frequency independent increase of noise spectra. The external base plate temperatures promote trap generation as evidenced by increased G-R peak intensities
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