44,671 research outputs found
Modelling rail track deterioration and maintenance: current practices and future needs
As commercialisation and privatisation of railway systems reach the political agendas in a number of countries, including Australia, the separation of infrastructure from operating business dictates that track costs need to be shared on an equitable basis. There is also a world-wide trend towards increased pressures on rail track infrastructure through increases in axle loads and train speeds. Such productivity and customer service driven pressures inevitably lead to reductions in the life of track components and increases in track maintenance costs. This paper provides a state-of-the-art review of track degradation modeling, as well as an overview of track maintenance decision support systems currently in use in North America and Europe. The essential elements of a maintenance optimisation model currently under development are also highlighted
Exclusive electroproduction of J/psi mesons
A nonperturbative calculation of elastic electroproduction of the J/psi meson
is presented and compared to the experimental data.
Our model describes well the observed dependences of the cross sections on
the photon virtuality Q2 and on the energy, and the measured ratio R of
longitudinal to transverse cross sections.Comment: Five *.eps figure
Nonperturbative QCD treatment of photoproduction
We present a nonperturbative QCD calculation of elastic meson
production in photon-proton scattering at high energies. Using light cone wave
functions of the photon and vector mesons, and the framework of the model of
the stochastic QCD vacuum, we calculate the differential and integrated elastic
cross sections for \gamma p \goto J/\psi p . With an energy dependence
following the two-pomeron model we are able to give a consistent description of
the integrated cross sections and the differential cross sections at low
in the range from 20 GeV up to the highest HERA energies. We discuss different
approaches to introduce saturation and find no specific effects up to energies
presently available. We also calculate and compare to experiments the cross
section for photoproduction.Comment: 24 pages, 10 figures, 4 table
Economic transition and the distributions of income and wealth
Using a model of wealth distribution dynamics and occupational choice, the author investigates the distributional consequences of policies and developments associated with the transition from central planning to a market system. The model suggests that even an efficient privatization designed to be egalitarian may lead to increases in inequality (and possibly poverty), both during the transition and in the new steady state. Creating new markets in services that are also supplied by the public sector may also contribute to an increase in inequality. So can labor market reforms that lead to a decompression of the earnings structure and to greater flexibility in employment. The results underline the importance of retaining government provision of basic public goods and services, removing barriers that prevent the participation of the poor in the new private sector, and ensuring that suitable safety nets are in place.Environmental Economics&Policies,Economic Theory&Research,Banks&Banking Reform,Fiscal&Monetary Policy,Labor Policies,Banks&Banking Reform,Health Economics&Finance,Economic Theory&Research,Inequality,Environmental Economics&Policies
On the Benjamini--Hochberg method
We investigate the properties of the Benjamini--Hochberg method for multiple
testing and of a variant of Storey's generalization of it, extending and
complementing the asymptotic and exact results available in the literature.
Results are obtained under two different sets of assumptions and include
asymptotic and exact expressions and bounds for the proportion of rejections,
the proportion of incorrect rejections out of all rejections and two other
proportions used to quantify the efficacy of the method.Comment: Published at http://dx.doi.org/10.1214/009053606000000425 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Distributions in motion: economic growth, inequality, and poverty dynamics
The joint determination of aggregate economic growth and distributional change has been studied empirically from at least three different perspectives. A macroeconomic approach that relies on cross-country data on poverty, inequality, and growth rates has generated some interesting stylized facts about the correlations between these variables, but has not shed much light on the underlying determinants."Meso-"and microeconomic approaches have fared somewhat better. The microeconomic approach, in particular, builds on the observation that growth, changes in poverty, and changes in inequality are simply different aggregations of information on the incidence of economic growth along the income distribution. This paper reviews the evolution of attempts to understand the nature of growth incidence curves, from the statistical decompositions associated with generalizations of the Oaxaca-Blinder method, to more recent efforts to generate"economically consistent"counterfactuals, drawing on structural, reduced-form, and computable general equilibrium models.Rural Poverty Reduction,Achieving Shared Growth,Inequality,Services&Transfers to Poor,Economic Theory&Research
Activation thresholds in epidemic spreading with motile infectious agents on scale-free networks
We investigate a fermionic susceptible-infected-susceptible model with
mobility of infected individuals on uncorrelated scale-free networks with
power-law degree distributions of exponents
. Two diffusive processes with diffusion rate of an infected
vertex are considered. In the \textit{standard diffusion}, one of the
nearest-neighbors is chosen with equal chance while in the \textit{biased
diffusion} this choice happens with probability proportional to the neighbor's
degree. A non-monotonic dependence of the epidemic threshold on with an
optimum diffusion rate , for which the epidemic spreading is more
efficient, is found for standard diffusion while monotonic decays are observed
in the biased case. The epidemic thresholds go to zero as the network size is
increased and the form that this happens depends on the diffusion rule and
degree exponent. We analytically investigated the dynamics using quenched and
heterogeneous mean-field theories. The former presents, in general, a better
performance for standard and the latter for biased diffusion models, indicating
different activation mechanisms of the epidemic phases that are rationalized in
terms of hubs or max -core subgraphs.Comment: 9 pages, 4 figure
Are Magnetic Wind-Driving Disks Inherently Unstable?
There have been claims in the literature that accretion disks in which a
centrifugally driven wind is the dominant mode of angular momentum transport
are inherently unstable. This issue is considered here by applying an
equilibrium-curve analysis to the wind-driving, ambipolar diffusion-dominated,
magnetic disk model of Wardle & Konigl (1993). The equilibrium solution curves
for this class of models typically exhibit two distinct branches. It is argued
that only one of these branches represents unstable equilibria and that a real
disk/wind system likely corresponds to a stable solution.Comment: 5 pages, 2 figures, to be published in ApJ, vol. 617 (2004 Dec 20).
Uses emulateapj.cl
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