4,152 research outputs found

    An information theoretic approach to statistical dependence: copula information

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    We discuss the connection between information and copula theories by showing that a copula can be employed to decompose the information content of a multivariate distribution into marginal and dependence components, with the latter quantified by the mutual information. We define the information excess as a measure of deviation from a maximum entropy distribution. The idea of marginal invariant dependence measures is also discussed and used to show that empirical linear correlation underestimates the amplitude of the actual correlation in the case of non-Gaussian marginals. The mutual information is shown to provide an upper bound for the asymptotic empirical log-likelihood of a copula. An analytical expression for the information excess of T-copulas is provided, allowing for simple model identification within this family. We illustrate the framework in a financial data set.Comment: to appear in Europhysics Letter

    Evaluation of local thermal discomfort in a classroom equipped with cross flow ventilation

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    This paper presents an evaluation of the local thermal discomfort level in a classroom equipped with cross ventilation, for a typical moderate summer day in Portugal. Three different ventilation configurations based on window and door opening were considered. In each, the thermal comfort, air quality and acoustical comfort conditions were also evaluated. This experimental study was made in the South of Portugal, exposed to a Mediterranean climate. Thermal comfort was based on the PMV index, the air quality was based on the air renovation rate and acoustical comfort levels were based on the reverberation time, voice clarity, definition and early reflection ratio. The detailed local thermal discomfort analysis was based on draught risk and uncomfortable air velocity fluctuations. Other measurements included relative humidity, the radiative mean temperature, carbon dioxide concentration (tracer gas decay), and noise level decay of impulsive response. Results showed that for the warmest of weather open windows and classroom door gave the best air quality and comfort conditions

    Statistical Mechanics Analysis of LDPC Coding in MIMO Gaussian Channels

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    Using analytical methods of statistical mechanics, we analyse the typical behaviour of a multiple-input multiple-output (MIMO) Gaussian channel with binary inputs under LDPC network coding and joint decoding. The saddle point equations for the replica symmetric solution are found in particular realizations of this channel, including a small and large number of transmitters and receivers. In particular, we examine the cases of a single transmitter, a single receiver and the symmetric and asymmetric interference channels. Both dynamical and thermodynamical transitions from the ferromagnetic solution of perfect decoding to a non-ferromagnetic solution are identified for the cases considered, marking the practical and theoretical limits of the system under the current coding scheme. Numerical results are provided, showing the typical level of improvement/deterioration achieved with respect to the single transmitter/receiver result, for the various cases.Comment: 25 pages, 7 figure

    Self-control in Sparsely Coded Networks

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    A complete self-control mechanism is proposed in the dynamics of neural networks through the introduction of a time-dependent threshold, determined in function of both the noise and the pattern activity in the network. Especially for sparsely coded models this mechanism is shown to considerably improve the storage capacity, the basins of attraction and the mutual information content of the network.Comment: 4 pages, 6 Postscript figure

    Opinion Dynamics of Learning Agents: Does Seeking Consensus Lead to Disagreement?

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    We study opinion dynamics in a population of interacting adaptive agents voting on a set of complex multidimensional issues. We consider agents which can classify issues into for or against. The agents arrive at the opinions about each issue in question using an adaptive algorithm. Adaptation comes from learning and the information for the learning process comes from interacting with other neighboring agents and trying to change the internal state in order to concur with their opinions. The change in the internal state is driven by the information contained in the issue and in the opinion of the other agent. We present results in a simple yet rich context where each agent uses a Boolean Perceptron to state its opinion. If there is no internal clock, so the update occurs with asynchronously exchanged information among pairs of agents, then the typical case, if the number of issues is kept small, is the evolution into a society thorn by the emergence of factions with extreme opposite beliefs. This occurs even when seeking consensus with agents with opposite opinions. The curious result is that it is learning from those that hold the same opinions that drives the emergence of factions. This results follows from the fact that factions are prevented by not learning at all from those agents that hold the same opinion. If the number of issues is large, the dynamics becomes trapped and the society does not evolve into factions and a distribution of moderate opinions is observed. We also study the less realistic, but technically simpler synchronous case showing that global consensus is a fixed point. However, the approach to this consensus is glassy in the limit of large societies if agents adapt even in the case of agreement.Comment: 16 pages, 10 figures, revised versio

    Landscape complexity analysis based on texture patterns and satellite image for a São Paulo's Cerrado site.

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    We analysed the spatial heterogeneity complexity of Cerrado vegetation of São Paulo. Spatial heterogeneity was obtained by quantity texture patterns on ASTER images for Jataí Ecological Station (JES; Luiz Antônio ‐ SP) conservation unit. Two hillsides of JES where cerradão physiognomy was present were analyzed, and for each of them we sampled three positions: base, medium and top. Vertical structure and canopy cover density varied along hillsides. For each site, we extracted the texture pattern for two spatial scales (150x150m and 75x75m). Spatial complexity were estimated by two landscape metrics, both based on informational entropy: a) maximum entropy (H/Hmax), in which high values of complexity are assigned to patterns more disordered; and b) convex function of entropy (LMC), which attribute high values of complexity to patterns situated in intermediate range between order and disorder. Comparing different sites of the same hillside, both metrics had identical results in relation to greatest and smallest values of complexity. In hillside 1, the top side showed greater values. In these area, the cerradão shows high trees with less density of canopy in comparison to others sites of this hillside, which provides more spatial heterogeneity. For the smaller values of complexity, there was difference in comparison of analyzed extensions in hillside 1: for 150 x 150m spatial scale, was attributed medium hillside site to smaller values of complexity; for 75 x 75m, the base had the smallest values. However, the value of complexity of medium and low hillsides for the same spatial extension were very close. In hillside 2, on the base of site, which shows a cerradão with shorter trees and lesser canopy cover density in relation to others sites of hillside, texture patterns had highest values of complexity for both entropy measures. The most homogeneity site, located in medium position of hillside and with presence of high trees and canopy density, smallest values were recorded. Based on our findings we can concluded that there is a tendency of sites situated in the top and in the base of hillside showed great values of complexity, while sites located in medium hillside tend to be less complex in terms of spatial heterogeneity. Also, for local scale, the canopy density is more relevant than tree vertical structure on determining the complexity of texture patterns of vegetation

    Exponential energy decay of solutions for a system of viscoelastic wave equations of Kirchhoff type with strong damping

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    The initial boundary value problem for a system of viscoelastic wave equations of Kirchhoff type with strong damping is considered. We prove that, under suitable assumptions on relaxation functions and certain initial data, the decay rate of the solutions energy is exponential

    Spatially Resolved Chemistry in Nearby Galaxies I. The Center of IC 342

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    We have imaged emission from the millimeter lines of eight molecules--C2H, C34S, N2H+, CH3OH, HNCO, HNC, HC3N, and SO--in the central half kpc of the nearby spiral galaxy IC 342. The 5" (~50 pc) resolution images were made with OVRO. Using these maps we obtain a picture of the chemistry within the nuclear region on the sizescales of individual GMCs. Bright emission is detected from all but SO. There are marked differences in morphology for the different molecules. A principal component analysis is performed to quantify similarities and differences among the images. This analysis reveals that while all molecules are to zeroth order correlated, that is, they are all found in dense molecular clouds, there are three distinct groups of molecules distinguished by the location of their emission within the nuclear region. N2H+, C18O, HNC and HCN are widespread and bright, good overall tracers of dense molecular gas. C2H and C34S, tracers of PDR chemistry, originate exclusively from the central 50-100 pc region, where radiation fields are high. The third group of molecules, CH3OH and HNCO, correlates well with the expected locations of bar-induced orbital shocks. The good correlation of HNCO with the established shock tracer molecule CH3OH is evidence that this molecule, whose chemistry has been uncertain, is indeed produced by processing of grains. HC3N is observed to correlate tightly with 3mm continuum emission, demonstrating that the young starbursts are the sites of the warmest and densest molecular gas. We compare our HNC images with the HCN images of Downes et al. (1992) to produce the first high resolution, extragalactic HCN/HNC map: the HNC/HCN ratio is near unity across the nucleus and the correlation of both of these gas tracers with the star formation is excellent. (Abridged).Comment: 54 pages including 10 figures and 8 tables. Accepted for publication in Ap

    In-medium modifications of the ππ\pi\pi interaction in photon-induced reactions

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    Differential cross sections of the reactions (γ,ππ)(\gamma,\pi^\circ\pi^\circ) and (γ,ππ++ππ)(\gamma,\pi^\circ\pi^++\pi^\circ\pi^-) have been measured for several nuclei (1^1H,12^{12}C, and nat^{\rm nat}Pb) at an incident-photon energy of EγE_{\gamma}=400-460 MeV at the tagged-photon facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the ππ\pi\pi invariant-mass distribution is found in the ππ\pi^\circ\pi^\circ channel. This dependence is not observed in the ππ+/\pi^\circ\pi^{+/-} channel and is consistent with an in-medium modification of the ππ\pi\pi interaction in the II=JJ=0 channel. The data are compared to π\pi-induced measurements and to calculations within a chiral-unitary approach

    Spectral density of random graphs with topological constraints

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    The spectral density of random graphs with topological constraints is analysed using the replica method. We consider graph ensembles featuring generalised degree-degree correlations, as well as those with a community structure. In each case an exact solution is found for the spectral density in the form of consistency equations depending on the statistical properties of the graph ensemble in question. We highlight the effect of these topological constraints on the resulting spectral density.Comment: 24 pages, 6 figure
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