1,837 research outputs found

    Oscillatory wave fronts in chains of coupled nonlinear oscillators

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    Wave front pinning and propagation in damped chains of coupled oscillators are studied. There are two important thresholds for an applied constant stress FF: for F<Fcd|F|<F_{cd} (dynamic Peierls stress), wave fronts fail to propagate, for Fcd<F<FcsF_{cd} < |F| < F_{cs} stable static and moving wave fronts coexist, and for F>Fcs|F| > F_{cs} (static Peierls stress) there are only stable moving wave fronts. For piecewise linear models, extending an exact method of Atkinson and Cabrera's to chains with damped dynamics corroborates this description. For smooth nonlinearities, an approximate analytical description is found by means of the active point theory. Generically for small or zero damping, stable wave front profiles are non-monotone and become wavy (oscillatory) in one of their tails.Comment: 18 pages, 21 figures, 2 column revtex. To appear in Phys. Rev.

    Can Crop Insurance Premiums Be Reliably Estimated?

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    This paper develops and applies a methodology to assess the accuracy of historical loss-cost rating procedures, similar to those used by the U.S. Department of Agriculture’s Risk Management Agency (RMA), versus alternative parametric premium estimation methods. It finds that the accuracy of loss-cost procedures leaves much to be desired, but can be markedly improved through the use of alternative methods and increased farm-level yield sample sizes. Evidence suggests that the high degree of inaccuracy in crop insurance premium estimations through historical loss-cost procedures identified in the paper might be a major factor behind the need for substantial government subsidies to keep the program solvent.agricultural subsidies, crop insurance premium estimation, loss-cost procedures, Risk Management Agency, Agricultural and Food Policy, Agricultural Finance, Farm Management, Risk and Uncertainty,

    Effects of disorder on the wave front depinning transition in spatially discrete systems

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    Pinning and depinning of wave fronts are ubiquitous features of spatially discrete systems describing a host of phenomena in physics, biology, etc. A large class of discrete systems is described by overdamped chains of nonlinear oscillators with nearest-neighbor coupling and subject to random external forces. The presence of weak randomness shrinks the pinning interval and it changes the critical exponent of the wave front depinning transition from 1/2 to 3/2. This effect is derived by means of a recent asymptotic theory of the depinning transition, extended to discrete drift-diffusion models of transport in semiconductor superlattices and confirmed by numerical calculations.Comment: 4 pages, 3 figures, to appear as a Rapid Commun. in Phys. Rev.

    Nonlinear stability of oscillatory wave fronts in chains of coupled oscillators

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    We present a stability theory for kink propagation in chains of coupled oscillators and a new algorithm for the numerical study of kink dynamics. The numerical solutions are computed using an equivalent integral equation instead of a system of differential equations. This avoids uncertainty about the impact of artificial boundary conditions and discretization in time. Stability results also follow from the integral version. Stable kinks have a monotone leading edge and move with a velocity larger than a critical value which depends on the damping strength.Comment: 11 figure

    Evolución y características sedimentológicas de las facies fluviales basales del Buntsandstein de Olesa de Montserrat (Provincia de Barcelona)

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    [ES] La sección del Buntsandstein de Olesa de Montserrat está constituida en sus tramos basales por una serie de depósitos generados por corrientes anastomosadas con un material de lecho progresivamente más fino. Se precisan las características peculiares de los depósitos originados por el sistema anastomosado de lecho arenoso y a la luz de estos datos se realizan algunas consideraciones sobre el modelo proximal-distal en corrientes anastomosadas.[FR] La coupe du Buntsandstein d’Olesa de Montserrat est constituée, dans sa partia basale, por un ensamble de dépóts originés par des courants anastomosés ayee materiel du lit progressivement plus fin. On precise les caractéres particuliers des dépóts originés par un systéme anastomosé de lit sableaux et, ayee ces données, on arrive á des considérations génerales sur le modéle proximal-distal en courants anastomosés.[EN] The lower part of tha Buntsandstein in Olasa de Montserrat is formad by a set of braidad streams deposits with bed load material fining up progresivaly in the section. Special features of tbe deposits originated by braided systems with sandy bed load are precisad, and on the light of thase data, soma considarations about the proximal-distal modal in braidad streams are pointed out.Peer reviewe

    Molecular line probes of activity in galaxies

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    The use of specific tracers of the dense molecular gas phase can help to explore the feedback of activity on the interstellar medium (ISM) in galaxies. This information is a key to any quantitative assessment of the efficiency of the star formation process in galaxies. We present the results of a survey devoted to probe the feedback of activity through the study of the excitation and chemistry of the dense molecular gas in a sample of local universe starbursts and active galactic nuclei (AGNs). Our sample includes also 17 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). From the analysis of the LIRGs/ULIRGs subsample, published in Gracia-Carpio et al.(2007) we find the first clear observational evidence that the star formation efficiency of the dense gas, measured by the L_FIR/L_HCN ratio, is significantly higher in LIRGs and ULIRGs than in normal galaxies. Mounting evidence of overabundant HCN in active environments would even reinforce the reported trend, pointing to a significant turn upward in the Kennicutt-Schmidt law around L_FIR=10^11 L_sun. This result has major implications for the use of HCN as a tracer of the dense gas in local and high-redshift luminous infrared galaxies.Comment: 4 pages, 2 figures, contributed paper to Far-Infrared Workshop 07 (FIR 2007

    The Economic Value of Basin Protection to Improve the Quality and Reliability of Potable Water Supply: Some Evidence from Ecuador

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    This study estimates the willingness to pay (WTP) of Loja’s households to protect two micro-basins that supply over 40 percent of potable water to the city. Results indicate that households have an average WTP of $5.80 per month, which corresponds to a 25 percent increase in the self-reported monthly water bill, to preserve the basins.Basin protection, contingent valuation, Loja, Ecuador, Environmental Economics and Policy, Land Economics/Use,

    Intersection local times of fractional Brownian motions with H(0,1)H\in(0,1) as generalized white noise functionals

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    In Rd\R^d, for any dimension d1d\geq 1, expansions of self-intersection local times of fractional Brownian motions with arbitrary Hurst coefficients in (0,1)(0,1) are presented. The expansions are in terms of Wick powers of white noises (corresponding to multiple Wiener integrals), being well-defined in the sense of generalized white noise functionals.Comment: 17 page

    Searching for Communities in Bipartite Networks

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    Bipartite networks are a useful tool for representing and investigating interaction networks. We consider methods for identifying communities in bipartite networks. Intuitive notions of network community groups are made explicit using Newman's modularity measure. A specialized version of the modularity, adapted to be appropriate for bipartite networks, is presented; a corresponding algorithm is described for identifying community groups through maximizing this measure. The algorithm is applied to networks derived from the EU Framework Programs on Research and Technological Development. Community groups identified are compared using information-theoretic methods.Comment: 12 pages, 4 figures, to appear in "Proceedings of the 5th Jagna International Workshop: Stochastic and Quantum Dynamics of Biomolecular Systems," C. C. Bernido and M. V. Carpio-Bernido, editors. A version with full-quality figures and larger file size is available at http://ccm.uma.pt/publications/Barber-Faria-Streit-Strogan-2008.pd
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