652 research outputs found
Toward source region tomography with inter-source interferometry: Shear wave velocity from 2018 West Bohemia swarm earthquakes
The concept of seismic interferometry embraces the construction of waves traveling between receivers or sources with cross‐correlation techniques. In the present study cross correlations of coda waves are used to measure traveltimes of shear waves between earthquake locations for five event clusters of the 2018 West Bohemia earthquake swarm. With the help of a high‐quality earthquake catalog, I was able to determine the shear wave velocity in the region of the five clusters separately. The shear wave velocities range between 3.5 and 4.2 km/s. The resolution of this novel method is given by the extent of the clusters and better than for a comparable classical tomography. It is suggested to use the method in a tomographic inversion and map the shear wave velocity in the source region with unprecedented resolution. Furthermore, the influence of focal mechanisms and the attenuation properties on the polarity and location of the maxima in the cross‐correlation functions is discussed. The intracluster ratio of P wave to S wave velocity is approximately fixed at 1.68
Co-trimoxazole prophylaxis in HIV : the evidence
BibliographyHuman immunodeficiency virus (HIV) damages the body’s
immune system, making secondary (or opportunistic)
infections more common. Treatment and prevention of such
infections is integral to the management of patients with HIV
infection. Co-trimoxazole is a prophylactic treatment that has a
wide range of action against common bacteria, parasites, fungi
and yeasts. As part of a minimum care package, UNAIDS/
WHO recommends co-trimoxazole prophylaxis for HIVinfected
adults with symptomatic disease (WHO stage II, III
or IV), or asymptomatic individuals with CD4 counts ≤500
cells/μl, and for all HIV-positive pregnant women after the
first trimester.1 Co-trimoxazole is also recommended for use in
children with proven HIV infection and infants exposed to HIV
(from 4 - 6 weeks of age until infection with HIV is ruled out).2
The object of this report is to summarise the effects of
co-trimoxazole prophylaxis on morbidity and mortality among
HIV-infected individuals
In-depth analysis of the Naming Game dynamics: the homogeneous mixing case
Language emergence and evolution has recently gained growing attention
through multi-agent models and mathematical frameworks to study their behavior.
Here we investigate further the Naming Game, a model able to account for the
emergence of a shared vocabulary of form-meaning associations through
social/cultural learning. Due to the simplicity of both the structure of the
agents and their interaction rules, the dynamics of this model can be analyzed
in great detail using numerical simulations and analytical arguments. This
paper first reviews some existing results and then presents a new overall
understanding.Comment: 30 pages, 19 figures (few in reduced definition). In press in IJMP
Hofstadter butterflies of carbon nanotubes: Pseudofractality of the magnetoelectronic spectrum
The electronic spectrum of a two-dimensional square lattice in a
perpendicular magnetic field has become known as the Hofstadter butterfly
[Hofstadter, Phys. Rev. B 14, 2239 (1976).]. We have calculated
quasi-one-dimensional analogs of the Hofstadter butterfly for carbon nanotubes
(CNTs). For the case of single-wall CNTs, it is straightforward to implement
magnetic fields parallel to the tube axis by means of zone folding in the
graphene reciprocal lattice. We have also studied perpendicular magnetic fields
which, in contrast to the parallel case, lead to a much richer, pseudofractal
spectrum. Moreover, we have investigated magnetic fields piercing double-wall
CNTs and found strong signatures of interwall interaction in the resulting
Hofstadter butterfly spectrum, which can be understood with the help of a
minimal model. Ubiquitous to all perpendicular magnetic field spectra is the
presence of cusp catastrophes at specific values of energy and magnetic field.
Resolving the density of states along the tube circumference allows recognition
of the snake states already predicted for nonuniform magnetic fields in the
two-dimensional electron gas. An analytic model of the magnetic spectrum of
electrons on a cylindrical surface is used to explain some of the results.Comment: 14 pages, 12 figures update to published versio
Sharp transition towards shared vocabularies in multi-agent systems
What processes can explain how very large populations are able to converge on
the use of a particular word or grammatical construction without global
coordination? Answering this question helps to understand why new language
constructs usually propagate along an S-shaped curve with a rather sudden
transition towards global agreement. It also helps to analyze and design new
technologies that support or orchestrate self-organizing communication systems,
such as recent social tagging systems for the web. The article introduces and
studies a microscopic model of communicating autonomous agents performing
language games without any central control. We show that the system undergoes a
disorder/order transition, going trough a sharp symmetry breaking process to
reach a shared set of conventions. Before the transition, the system builds up
non-trivial scale-invariant correlations, for instance in the distribution of
competing synonyms, which display a Zipf-like law. These correlations make the
system ready for the transition towards shared conventions, which, observed on
the time-scale of collective behaviors, becomes sharper and sharper with system
size. This surprising result not only explains why human language can scale up
to very large populations but also suggests ways to optimize artificial
semiotic dynamics.Comment: 12 pages, 4 figure
Calibration of optimal execution of financial transactions in the presence of transient market impact
Trading large volumes of a financial asset in order driven markets requires
the use of algorithmic execution dividing the volume in many transactions in
order to minimize costs due to market impact. A proper design of an optimal
execution strategy strongly depends on a careful modeling of market impact,
i.e. how the price reacts to trades. In this paper we consider a recently
introduced market impact model (Bouchaud et al., 2004), which has the property
of describing both the volume and the temporal dependence of price change due
to trading. We show how this model can be used to describe price impact also in
aggregated trade time or in real time. We then solve analytically and calibrate
with real data the optimal execution problem both for risk neutral and for risk
averse investors and we derive an efficient frontier of optimal execution. When
we include spread costs the problem must be solved numerically and we show that
the introduction of such costs regularizes the solution.Comment: 31 pages, 8 figure
ParkIndex: Validation and Application of a Pragmatic Measure of Park Access and Use
Composite metrics integrating park availability, features, and quality for a given address or neighborhood are lacking. The purposes of this study were to describe the validation, application, and demonstration of ParkIndex in four diverse communities. This study occurred in Fall 2018 in 128 census block groups within Seattle(WA), Brooklyn(NY), Raleigh(NC), and Greenville County(SC). All parks within a half-mile buffer were audited to calculate a composite park quality score, and select households provided data about use of proximal parks via an online, map-based survey. For each household, the number of parks, total park acreage, and average park quality score within one half-mile were calculated using GIS. Logistic regression was used to identify a parsimonious model predicting park use. ParkIndex values (representing the probability of park use) were mapped for all study areas and after scenarios involving the addition and renovation/improvement of parks. Out of 360 participants, 23.3% reported visiting a park within the past 30 days. The number of parks (OR = 1.36, 95% CI = 1.15–1.62), total park acreage (OR = 1.13, 95% CI = 1.07–1.19), and average park quality score (OR = 1.04, 95% CI = 1.01–1.06) within one half-mile were all associated with park use. Composite ParkIndex values across the study areas ranged from 0 to 100. Hypothetical additions of or renovations to study area parks resulted in ParkIndex increases of 22.7% and 19.2%, respectively. ParkIndex has substantial value for park and urban planners, citizens, and researchers as a common metric to facilitate awareness, decision-making, and intervention planning related to park access, environmental justice, and community health
Recombination rate and selection strength in HIV intra-patient evolution
The evolutionary dynamics of HIV during the chronic phase of infection is
driven by the host immune response and by selective pressures exerted through
drug treatment. To understand and model the evolution of HIV quantitatively,
the parameters governing genetic diversification and the strength of selection
need to be known. While mutation rates can be measured in single replication
cycles, the relevant effective recombination rate depends on the probability of
coinfection of a cell with more than one virus and can only be inferred from
population data. However, most population genetic estimators for recombination
rates assume absence of selection and are hence of limited applicability to
HIV, since positive and purifying selection are important in HIV evolution.
Here, we estimate the rate of recombination and the distribution of selection
coefficients from time-resolved sequence data tracking the evolution of HIV
within single patients. By examining temporal changes in the genetic
composition of the population, we estimate the effective recombination to be
r=1.4e-5 recombinations per site and generation. Furthermore, we provide
evidence that selection coefficients of at least 15% of the observed
non-synonymous polymorphisms exceed 0.8% per generation. These results provide
a basis for a more detailed understanding of the evolution of HIV. A
particularly interesting case is evolution in response to drug treatment, where
recombination can facilitate the rapid acquisition of multiple resistance
mutations. With the methods developed here, more precise and more detailed
studies will be possible, as soon as data with higher time resolution and
greater sample sizes is available.Comment: to appear in PLoS Computational Biolog
Information transmission in oscillatory neural activity
Periodic neural activity not locked to the stimulus or to motor responses is
usually ignored. Here, we present new tools for modeling and quantifying the
information transmission based on periodic neural activity that occurs with
quasi-random phase relative to the stimulus. We propose a model to reproduce
characteristic features of oscillatory spike trains, such as histograms of
inter-spike intervals and phase locking of spikes to an oscillatory influence.
The proposed model is based on an inhomogeneous Gamma process governed by a
density function that is a product of the usual stimulus-dependent rate and a
quasi-periodic function. Further, we present an analysis method generalizing
the direct method (Rieke et al, 1999; Brenner et al, 2000) to assess the
information content in such data. We demonstrate these tools on recordings from
relay cells in the lateral geniculate nucleus of the cat.Comment: 18 pages, 8 figures, to appear in Biological Cybernetic
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