93,014 research outputs found
Gaseous diffusion in glassy polymers
A model for gaseous diffusion in glassy polymers is developed with a view to accounting for the observations made in dual sorption and certain other phenomena in polymers below their glass transition temperature. In this paper a preliminary study of the effects of both the immobilizing mechanism and the generalized diffusion mechanism on travelling waves and the diffusive wavefronts is made
Feature Selection via Coalitional Game Theory
We present and study the contribution-selection algorithm (CSA), a novel algorithm for feature selection. The algorithm is based on the multiperturbation shapley analysis (MSA), a framework that relies on game theory to estimate usefulness. The algorithm iteratively estimates the usefulness of features and selects them accordingly, using either forward selection or backward elimination. It can optimize various performance measures over unseen data such as accuracy, balanced error rate, and area under receiver-operator-characteristic curve. Empirical comparison with several other existing feature selection methods shows that the backward elimination variant of CSA leads to the most accurate classification results on an array of data sets
Atmospheric turbulence and superstatistics
Nonequilibrium systems with large-scale fluctuations of a suitable system
parameter are often effectively described by a superposition of two statistics,
a superstatistics. Here we illustrate this concept by analysing experimental
data of fluctuations in atmospheric wind velocity differences at Florence
airport.Comment: 9 pages, 4 figures. New version to appear in Europhysics News (2005
Enhancing efficiency of single, large-aperture antennas
Numerical analysis method provides means of describing energy distribution in focal plane of parabolic surface in terms of phase and wavelength. Two approaches for enhancing antenna efficiency include single, large reflector focused to feeding element, and array of smaller apertures whose individual outputs are summed
MEDUSA - New Model of Internet Topology Using k-shell Decomposition
The k-shell decomposition of a random graph provides a different and more
insightful separation of the roles of the different nodes in such a graph than
does the usual analysis in terms of node degrees. We develop this approach in
order to analyze the Internet's structure at a coarse level, that of the
"Autonomous Systems" or ASes, the subnetworks out of which the Internet is
assembled. We employ new data from DIMES (see http://www.netdimes.org), a
distributed agent-based mapping effort which at present has attracted over 3800
volunteers running more than 7300 DIMES clients in over 85 countries. We
combine this data with the AS graph information available from the RouteViews
project at Univ. Oregon, and have obtained an Internet map with far more detail
than any previous effort.
The data suggests a new picture of the AS-graph structure, which
distinguishes a relatively large, redundantly connected core of nearly 100 ASes
and two components that flow data in and out from this core. One component is
fractally interconnected through peer links; the second makes direct
connections to the core only. The model which results has superficial
similarities with and important differences from the "Jellyfish" structure
proposed by Tauro et al., so we call it a "Medusa." We plan to use this picture
as a framework for measuring and extrapolating changes in the Internet's
physical structure. Our k-shell analysis may also be relevant for estimating
the function of nodes in the "scale-free" graphs extracted from other
naturally-occurring processes.Comment: 24 pages, 17 figure
First-principles thermal equation of state and thermoelasticity of hcp Fe at high pressures
We investigate the equation of state and elastic properties of hcp iron at
high pressures and high temperatures using first principles linear response
linear-muffin-tin-orbital method in the generalized-gradient approximation. We
calculate the Helmholtz free energy as a function of volume, temperature, and
volume-conserving strains, including the electronic excitation contributions
from band structures and lattice vibrational contributions from quasi-harmonic
lattice dynamics. We perform detailed investigations on the behavior of elastic
moduli and equation of state properties as functions of temperature and
pressure, including the pressure-volume equation of state, bulk modulus, the
thermal expansion coefficient, the Gruneisen ratio, and the shock Hugoniot.
Detailed comparison has been made with available experimental measurements and
theoretical predictions.Comment: 33 pages, 12 figure
Trends in Elasticity and Electronic Structure of Transition-Metal Nitrides and Carbides from First Principles
The elastic properties of the -structured transition-metal nitrides and
their carbide counterparts are studied using the {\it ab initio\} density
functional perturbation theory. The linear response results of elastic
constants are in excellent agreement with those obtained from numerical
derivative methods, and are also consistent with measured data. We find the
following trends: (1) Bulk moduli and tetragonal shear moduli
, increase and lattice constants decrease
rightward or downward on the Periodic Table for the metal component or if C is
replaced by N; (2) The inequality holds for
; (3) depends strongly on the number of valence electrons per
unit cell (). From the fitted curve of as a function of , we
can predict that MoN is unstable in structure, and transition-metal
carbonitrides ( ZrCN) and di-transition-metal carbides
( HfTaC) have maximum at .Comment: 4 pages, 2 figures, submitted to PRL. 2 typos in ref. 15 were
correcte
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