1,892 research outputs found
The structure of borders in a small world
Geographic borders are not only essential for the effective functioning of
government, the distribution of administrative responsibilities and the
allocation of public resources, they also influence the interregional flow of
information, cross-border trade operations, the diffusion of innovation and
technology, and the spatial spread of infectious diseases. However, as growing
interactions and mobility across long distances, cultural, and political
borders continue to amplify the small world effect and effectively decrease the
relative importance of local interactions, it is difficult to assess the
location and structure of effective borders that may play the most significant
role in mobility-driven processes. The paradigm of spatially coherent
communities may no longer be a plausible one, and it is unclear what structures
emerge from the interplay of interactions and activities across spatial scales.
Here we analyse a multi-scale proxy network for human mobility that
incorporates travel across a few to a few thousand kilometres. We determine an
effective system of geographically continuous borders implicitly encoded in
multi-scale mobility patterns. We find that effective large scale boundaries
define spatially coherent subdivisions and only partially coincide with
administrative borders. We find that spatial coherence is partially lost if
only long range traffic is taken into account and show that prevalent models
for multi-scale mobility networks cannot account for the observed patterns.
These results will allow for new types of quantitative, comparative analyses of
multi-scale interaction networks in general and may provide insight into a
multitude of spatiotemporal phenomena generated by human activity.Comment: 9 page
Accelerating random walks by disorder
We investigate the dynamic impact of heterogeneous environments on
superdiffusive random walks known as L\'evy flights. We devote particular
attention to the relative weight of source and target locations on the rates
for spatial displacements of the random walk. Unlike ordinary random walks
which are slowed down for all values of the relative weight of source and
target, non-local superdiffusive processes show distinct regimes of attenuation
and acceleration for increased source and target weight, respectively.
Consequently, spatial inhomogeneities can facilitate the spread of
superdiffusive processes, in contrast to common belief that external disorder
generally slows down stochastic processes. Our results are based on a novel
type of fractional Fokker-Planck equation which we investigate numerically and
by perturbation theory for weak disorder.Comment: 8 pages, 5 figure
Tradable SO-2-permits in the European Union: a practicable scheme for public utilities
In this paper, a practicable scheme of SO2-emission permits for European power producers is developed. Background is the second UN-ECE Sulphur Protocol from 1994 (Protocol of Oslo). After discussing some theoretical models of spatially differentiated permit schemes, evaluating the U.S. Acid Rain and RECLAIM Program, and considering the setting in the EU-15 countries, a scheme of locally undifferentiated emission permits is proposed which is distinguished by a high degree of both economic efficiency and market functioning. However, as our model simulations indicate, national deposition targets will be violated in all probability due to the scheme?s missing differentiation regarding the receptors. The risk of hot spots is addressed adequately by a differentiated bundle of countermeasures. The general economic impact of an EU-wide permit scheme is low, and, in terms of change in GDP, lower compared to a non-coordinated SO2 policy. The proposed mode of the initial permit allocation allows for early price signals and guarantees maximum static and dynamic efficiency. Balancing the interests of existing and new emitters, a long-term transition from the grandfathering to the free auction procedure is chosen. --
Neutron rich nuclei in density dependent relativistic Hartree-Fock theory with isovector mesons
Density dependent relativistic Hartree-Fock theory has been extended to
describe properties of exotic nuclei. The effects of Fock exchange terms and of
pi - and rho - meson contributions are discussed. These effects are found to be
more important for neutron rich nuclei than for nuclei near the valley of
stability.Comment: 10 pages, 5 figures, LaTeX, macro packages graphicx and time
Effective DBHF Method for Asymmetric Nuclear Matter and Finite Nuclei
A new decomposition of the Dirac structure of nucleon self-energies in the
Dirac Brueckner-Hartree-Fock (DBHF) approach is adopted to investigate the
equation of state for asymmetric nuclear matter. The effective coupling
constants of , , and mesons with a density
dependence in the relativistic mean field approach are deduced by reproducing
the nucleon self-energy resulting from the DBHF at each density for symmetric
and asymmetric nuclear matter. With these couplings the properties of finite
nuclei are investigated. The agreement of charge radii and binding energies of
finite nuclei with the experimental data are improved simultaneously in
comparison with the projection method. It seems that the properties of finite
nuclei are sensitive to the scheme used for the DBHF self-energy extraction. We
may conclude that the properties of the asymmetric nuclear matter and finite
nuclei could be well described by the new decomposition approach of the G
matrix.Comment: 16 pages, 5 figure
Relativistic Structure of the Nucleon Self-Energy in Asymmetric Nuclei
The Dirac structure of the nucleon self-energy in asymmetric nuclear matter
cannot reliably be deduced from the momentum dependence of the single-particle
energies. It is demonstrated that such attempts yield an isospin dependence
with even a wrong sign. Relativistic studies of finite nuclei have been based
on such studies of asymmetric nuclear matter. The effects of these isospin
components on the results for finite nuclei are investigated.Comment: 9 pages, Latex 4 figures include
Levy Flights in Inhomogeneous Media
We investigate the impact of external periodic potentials on superdiffusive
random walks known as Levy flights and show that even strongly superdiffusive
transport is substantially affected by the external field. Unlike ordinary
random walks, Levy flights are surprisingly sensitive to the shape of the
potential while their asymptotic behavior ceases to depend on the Levy index
. Our analysis is based on a novel generalization of the Fokker-Planck
equation suitable for systems in thermal equilibrium. Thus, the results
presented are applicable to the large class of situations in which
superdiffusion is caused by topological complexity, such as diffusion on folded
polymers and scale-free networks.Comment: 4 pages, 4 figure
Dirac Hartree-Fock for Finite Nuclei Employing realistic Forces
We discuss two different approximation schemes for the self-consistent
solution of the {\it relativistic} Brueckner-Hartree-Fock equation for finite
nuclei. In the first scheme, the Dirac effects are deduced from corresponding
nuclear matter calculations, whereas in the second approach the local-density
approximation is used to account for the effects of correlations. The results
obtained by the two methods are very similar. Employing a realistic
one-boson-exchange potential (Bonn~A), the predictions for energies and radii
of O and Ca come out in substantially better agreement with
experiment as compared to non-relativistic approaches. As a by-product of our
study, it turns out that the Fock exchange-terms, ignored in a previous
investigation, are not negligible.Comment:
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