2,117 research outputs found
Active Brownian Particles. From Individual to Collective Stochastic Dynamics
We review theoretical models of individual motility as well as collective
dynamics and pattern formation of active particles. We focus on simple models
of active dynamics with a particular emphasis on nonlinear and stochastic
dynamics of such self-propelled entities in the framework of statistical
mechanics. Examples of such active units in complex physico-chemical and
biological systems are chemically powered nano-rods, localized patterns in
reaction-diffusion system, motile cells or macroscopic animals. Based on the
description of individual motion of point-like active particles by stochastic
differential equations, we discuss different velocity-dependent friction
functions, the impact of various types of fluctuations and calculate
characteristic observables such as stationary velocity distributions or
diffusion coefficients. Finally, we consider not only the free and confined
individual active dynamics but also different types of interaction between
active particles. The resulting collective dynamical behavior of large
assemblies and aggregates of active units is discussed and an overview over
some recent results on spatiotemporal pattern formation in such systems is
given.Comment: 161 pages, Review, Eur Phys J Special-Topics, accepte
Automatic Metadata Generation using Associative Networks
In spite of its tremendous value, metadata is generally sparse and
incomplete, thereby hampering the effectiveness of digital information
services. Many of the existing mechanisms for the automated creation of
metadata rely primarily on content analysis which can be costly and
inefficient. The automatic metadata generation system proposed in this article
leverages resource relationships generated from existing metadata as a medium
for propagation from metadata-rich to metadata-poor resources. Because of its
independence from content analysis, it can be applied to a wide variety of
resource media types and is shown to be computationally inexpensive. The
proposed method operates through two distinct phases. Occurrence and
co-occurrence algorithms first generate an associative network of repository
resources leveraging existing repository metadata. Second, using the
associative network as a substrate, metadata associated with metadata-rich
resources is propagated to metadata-poor resources by means of a discrete-form
spreading activation algorithm. This article discusses the general framework
for building associative networks, an algorithm for disseminating metadata
through such networks, and the results of an experiment and validation of the
proposed method using a standard bibliographic dataset
Quantum Fluids and Classical Determinants
A "quasiclassical" approximation to the quantum spectrum of the Schroedinger
equation is obtained from the trace of a quasiclassical evolution operator for
the "hydrodynamical" version of the theory, in which the dynamical evolution
takes place in the extended phase space . The quasiclassical evolution operator is
multiplicative along the classical flow, the corresponding quasiclassical zeta
function is entire for nice hyperbolic flows, and its eigenvalue spectrum
contains the spectrum of the semiclassical zeta function. The advantage of the
quasiclassical zeta function is that it has a larger analyticity domain than
the original semiclassical zeta function; the disadvantage is that it contains
eigenvalues extraneous to the quantum problem. Numerical investigations
indicate that the presence of these extraneous eigenvalues renders the original
Gutzwiller-Voros semiclassical zeta function preferable in practice to the
quasiclassical zeta function presented here. The cumulant expansion of the
exact quantum mechanical scattering kernel and the cycle expansion of the
corresponding semiclassical zeta function part ways at a threshold given by the
topological entropy; beyond this threshold quantum mechanics cannot resolve
fine details of the classical chaotic dynamics.Comment: 33 pages, LaTeX with lamuphys.sty, epsf.sty, epsfig.sty macros,
available at http://www.nbi.dk/~predrag
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