2,190 research outputs found
Classes of random walks on temporal networks with competing timescales
Random walks find applications in many areas of science and are the heart of
essential network analytic tools. When defined on temporal networks, even basic
random walk models may exhibit a rich spectrum of behaviours, due to the
co-existence of different timescales in the system. Here, we introduce random
walks on general stochastic temporal networks allowing for lasting
interactions, with up to three competing timescales. We then compare the mean
resting time and stationary state of different models. We also discuss the
accuracy of the mathematical analysis depending on the random walk model and
the structure of the underlying network, and pay particular attention to the
emergence of non-Markovian behaviour, even when all dynamical entities are
governed by memoryless distributions.Comment: 16 pages, 5 figure
A refutation of the practice style hypothesis: the case of antibiotics prescription by French general practitioners for acute rhinopharyngitis
Many researches in France or abroad have highlighted the medical practice variation (MPV)phenomenon, or even the inappropriateness of certain medical decisions. There is no consensus on the origin of this MPV between preference-centred versus opportunities and constraints approaches. This study principal purpose is to refute hypothesis which assume that physicians adopt for their patient a uniform practice style for each similar clinical decision beyond the time. More specifically, multilevel models are estimated: First to measure variability of antibiotics prescription by French general practitioners for acute rhinopharyngitis, a clinical decision making context with weak uncertainty, and to tests its significance; Second to prioritize its determinants, especially those relating to GP or its practice setting environment, by controlling visit or patient confounders. The study was based on the 2001 activity data, added by an ad hoc questionnaire, of a sample of 778 GPs arising from a panel of 1006 computerized French GPs. We observe that a great part of the total variation was due to intra-physician variability (70%). Hence, in the French general practice context, we find empirical support for the rejection of the ‘practice style’, the ’enthusiasm’ or the ‘surgical signature’ hypothesis. Thus, it is patients' characteristics that largely explain the prescription, even if physicians' characteristics (area of practice, level of activity, network participation, participation in ongoing medical training) and environmental factors (recent visit from pharmaceutical sales representatives) also exert considerable influence. The latter suggest that MPV are partly caused by differences in the type of dissemination or diffusion of information. Such findings may help us to develop and identify facilitators for promoting a better use of antibiotics in France and, more generally, for influencing GPs practice when it is of interest.Medical practice variation, Multilevel analysis, Upper respiratory tract infections, Rhinopharyngitis, Antibiotics, General practitioners, Panel, France
The x-ray corona and jet of cygnus x-1
Evidence is presented indicating that in the hard state of Cygnus X-1, the
coronal mag- netic field might be below equipartition with radiation
(suggesting that the corona is not powered by magnetic field dissipation) and
that the ion temperature in the corona is significantly lower than what
predicted by ADAF like models. It is also shown that the current estimates of
the jet power set interesting contraints on the jet velocity (which is at least
mildly relativistic), the accretion efficiency (which is large in both spectral
states), and the nature of the X-ray emitting region (which is unlikely to be
the jet).Comment: 8 pages, 1 figure. Accepted for publication in Journal of Modern
Physics D, Proceedings of HEPRO II conference, Buenos Aires, Argentina,
October 26-30, 200
Random walk on temporal networks with lasting edges
We consider random walks on dynamical networks where edges appear and
disappear during finite time intervals. The process is grounded on three
independent stochastic processes determining the walker's waiting-time, the
up-time and down-time of edges activation. We first propose a comprehensive
analytical and numerical treatment on directed acyclic graphs. Once cycles are
allowed in the network, non-Markovian trajectories may emerge, remarkably even
if the walker and the evolution of the network edges are governed by memoryless
Poisson processes. We then introduce a general analytical framework to
characterize such non-Markovian walks and validate our findings with numerical
simulations.Comment: 18 pages, 18 figure
Contribution of fictitious forces to polarization drag in rotating media
Models for polarization drag - mechanical analog of the Faraday effect - are
extended to include inertial corrections to the dielectrics properties of the
rotating medium in its rest-frame. Instead of the Coriolis-Faraday term
originally proposed by Baranova & Zel'dovich, inertia corrections due to the
fictitious Coriolis and centrifugal forces are here derived by considering the
effect of rotation on both the Lorentz and plasma dielectric models. These
modified rest-frame properties are subsequently used to deduce laboratory
properties. Although elegant and insightful, it is shown that the
Coriolis-Faraday correction inferred from Larmor's theorem is limited in that
it can only capture inertial corrections to polarization drag when the
equivalent Faraday rotation is defined at the wave frequency of interest. This
is notably not the case for low frequency polarization drag in a rotating
magnetized plasma, although it is verified here using the more general
phenomenological models that the impact of fictitious forces is in general
negligible in these conditions.Comment: 10 pages, 4 figure
Using infrared/X-ray flare statistics to probe the emission regions near the event horizon of Sgr A*
The supermassive black hole at the centre of the Galaxy flares at least daily
in the infrared (IR) and X-ray bands, yet the process driving these flares is
still unknown. So far detailed analysis has only been performed on a few bright
flares. In particular, the broadband spectral modelling suffers from a strong
lack of simultaneous data. However, new monitoring campaigns now provide data
on thousands of flaring events, allowing a statistical analysis of the flare
properties. In this paper, we investigate the X-ray and IR flux distributions
of the flare events. Using a self-consistent calculation of the particle
distribution, we model the statistical properties of the flares. Based on a
previous work on single flares, we consider two families of models: pure
synchrotron models and synchrotron self-Compton (SSC) models. We investigate
the effect of fluctuations in some relevant parameters (e.g. acceleration
properties, density, magnetic field) on the flux distributions. The
distribution of these parameters is readily derived from the flux distributions
observed at different wavelengths. In both scenarios, we find that fluctuations
of the power injected in accelerated particles plays a major role. This must be
distributed as a power-law (with different indices in each model). In the
synchrotron dominated scenario, we derive the most extreme values of the
acceleration power required to reproduce the brightest flares. In that model,
the distribution of the acceleration slope fluctuations is constrained and in
the SSC scenario we constrain the distributions of the correlated magnetic
field and flow density variations.Comment: 9 pages, 3 tables, 6 figures, MNRAS, June 201
Merging for Impact: Radically rethinking how we deliver impact
Is your organisation fit to tackle today's challenges? The mission of a non-profit organisation is vast and complex, and the environment in which it operates is in constant evolution. Operational models that seemed relevant a few years ago might not be the best ones today, and the sector should explore innovative ways to deliver impact.This publication explores one radical option: merging with another actor in the field. Mergers are still uncommon in the non-profit sector, often met with fierce resistance. However, we are convinced that they can increase impact in many situations, and that they should thus be considered more often.In 2015, MAVA Foundation merged with FIBA, becoming just one legal entity under the name of MAVA Foundation. Drawing from our experiences, 'Merging for Impact: Radically rethinking how we deliver impact' outlines what it takes to approach such a change in structure, then explores the impact we have measured from our merger through feedback from the field. Looking back, we are convinced the venture has been a success
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