53,413 research outputs found
Dynamic models of residential segregation: brief review, analytical resolution and study of the introduction of coordination
In his 1971's Dynamic Models of Segregation paper, the economist Thomas C.
Schelling showed that a small preference for one's neighbors to be of the same
color could lead to total segregation, even if total segregation does not
correspond to individual preferences and to a residential configuration
maximizing the collective utility.
The present work is aimed at deepening the understanding of the properties of
dynamic models of segregation based on Schelling's hypotheses. Its main
contributions are (i) to offer a comprehensive and up-to-date review of this
family of models; (ii) to provide an analytical solution to the most general
form of this model under rather general assumptions; to the best of our
knowledge, such a solution did not exist so far; (iii) to analyse the effect of
two devices aimed at decreasing segregation in such a model.Comment: 52 pages, 21 figures, working pape
The Tully-Fisher relation of distant field galaxies
We examine the evolution of the Tully-Fisher relation (TFR) using a sample of
89 field spirals, with 0.1 < z < 1, for which we have measured confident
rotation velocities (Vrot). By plotting the residuals from the local TFR versus
redshift, or alternatively fitting the TFR to our data in several redshift
bins, we find evidence that luminous spiral galaxies are increasingly offset
from the local TFR with redshift, reaching a brightening of -1.0+-0.5 mag, for
a given Vrot, by approximately z = 1. Since selection effects would generally
increase the fraction of intrinsically-bright galaxies at higher redshifts, we
argue that the observed evolution is probably an upper limit.
Previous studies have used an observed correlation between the TFR residuals
and Vrot to argue that low mass galaxies have evolved significantly more than
those with higher mass. However, we demonstrate that such a correlation may
exist purely due to an intrinsic coupling between the Vrot scatter and TFR
residuals, acting in combination with the TFR scatter and restrictions on the
magnitude range of the data, and therefore it does not necessarily indicate a
physical difference in the evolution of galaxies with different Vrot.
Finally, if we interpret the luminosity evolution derived from the TFR as due
to the evolution of the star formation rate (SFR) in these luminous spiral
galaxies, we find that SFR(z) is proportional to (1+z)^(1.7+-1.1), slower than
commonly derived for the overall field galaxy population. This suggests that
the rapid evolution in the SFR density of the universe observed since
approximately z = 1 is not driven by the evolution of the SFR in individual
bright spiral galaxies. (Abridged.)Comment: 14 pages, 10 figures, accepted by MNRA
Orientational phase transitions in anisotropic rare-earth magnets at low temperatures
Orientational phase transitions are investigated within the Heisenberg model
with single-site anisotropy. The temperature dependence of the cone angle is
calculated within the spin-wave theory. The role of the quantum
renormalizations of anisotropy constants is discussed. A comparison with the
experimental data on the cone-plane orientational transition in holmium is
performed.Comment: 9 pages, LaTeX, 3 figure
Generic Sandpile Models Have Directed Percolation Exponents
We study sandpile models with stochastic toppling rules and having sticky
grains so that with a non-zero probability no toppling occurs, even if the
local height of pile exceeds the threshold value. Dissipation is introduced by
adding a small probability of particle loss at each toppling. Generically, for
models with a preferred direction, the avalanche exponents are those of
critical directed percolation clusters. For undirected models, avalanche
exponents are those of directed percolation clusters in one higher dimension.Comment: 4 pages, 4 figures, minor change
Self-organisation to criticality in a system without conservation law
We numerically investigate the approach to the stationary state in the
nonconservative Olami-Feder-Christensen (OFC) model for earthquakes. Starting
from initially random configurations, we monitor the average earthquake size in
different portions of the system as a function of time (the time is defined as
the input energy per site in the system). We find that the process of
self-organisation develops from the boundaries of the system and it is
controlled by a dynamical critical exponent z~1.3 that appears to be universal
over a range of dissipation levels of the local dynamics. We show moreover that
the transient time of the system scales with system size L as . We argue that the (non-trivial) scaling of the transient time in the
OFC model is associated to the establishment of long-range spatial correlations
in the steady state.Comment: 10 pages, 6 figures; accepted for publication in Journal of Physics
Constraints on Superfluid Hydrodynamics from Equilibrium Partition Functions
Following up on recent work in the context of ordinary fluids, we study the
equilibrium partition function of a 3+1 dimensional superfluid on an arbitrary
stationary background spacetime, and with arbitrary stationary background gauge
fields, in the long wavelength expansion. We argue that this partition function
is generated by a 3 dimensional Euclidean effective action for the massless
Goldstone field. We parameterize the general form of this action at first order
in the derivative expansion. We demonstrate that the constitutive relations of
relativistic superfluid hydrodynamics are significantly constrained by the
requirement of consistency with such an effective action. At first order in the
derivative expansion we demonstrate that the resultant constraints on
constitutive relations coincide precisely with the equalities between
hydrodynamical transport coefficients recently derived from the second law of
thermodynamics.Comment: 46 page
Phonon-induced quadrupolar ordering of the magnetic superconductor TmNiBC
We present synchrotron x-ray diffraction studies revealing that the lattice
of thulium borocarbide is distorted below T_Q = 13.5 K at zero field. T_Q
increases and the amplitude of the displacements is drastically enhanced, by a
factor of 10 at 60 kOe, when a magnetic field is applied along [100]. The
distortion occurs at the same wave vector as the antiferromagnetic ordering
induced by the a-axis field. A model is presented that accounts for the
properties of the quadrupolar phase and explains the peculiar behavior of the
antiferromagnetic ordering previously observed in this compound.Comment: submitted to PR
New Insights on Interstellar Gas-Phase Iron
In this paper, we report on the gas-phase abundance of singly-ionized iron
(Fe II) for 51 lines of sight, using data from the Far Ultraviolet
Spectroscopic Explorer (FUSE). Fe II column densities are derived by measuring
the equivalent widths of several ultraviolet absorption lines and subsequently
fitting those to a curve of growth. Our derivation of Fe II column densities
and abundances creates the largest sample of iron abundances in moderately- to
highly-reddened lines of sight explored with FUSE, lines of sight that are on
average more reddened than lines of sight in previous Copernicus studies. We
present three major results. First, we observe the well-established correlation
between iron depletion and and also find trends between iron depletion
and other line of sight parameters (e.g. f(H_2), E_(B-V), and A_V), and examine
the significance of these trends. Of note, a few of our lines of sight probe
larger densities than previously explored and we do not see significantly
enhanced depletion effects. Second, we present two detections of an extremely
weak Fe II line at 1901.773 A in the archival STIS spectra of two lines of
sight (HD 24534 and HD 93222). We compare these detections to the column
densities derived through FUSE spectra and comment on the line's f-value and
utility for future studies of Fe II. Lastly, we present strong anecdotal
evidence that the Fe II f-values derived empirically through FUSE data are more
accurate than previous values that have been theoretically calculated, with the
probable exception of f_1112.Comment: Accepted for publication in ApJ, 669, 378; see ApJ version for small
updates. 53 total pages (preprint format), 7 tables, 11 figure
Tracking and data systems support for the Helios project. Volume 3: DSN support of Project Helios May 1976 - June 1977
Spacecraft extended mission coverage does not generally carry a high priority, but Helios was fortunate in that a combination of separated viewperiods and unique utilization of the STDN Goldstone antenna have provided a considerable amount of additional science data return, particularly at key times such a perihelion and/or solar occultation
Guide to fisheries policy research in Cambodia : the institutional and legal context
Fishery policy, Fishery management, Legal aspects, Cambodia,
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