7,473 research outputs found
Dynamics underlying Box-office: Movie Competition on Recommender Systems
We introduce a simple model to study movie competition in the recommender
systems. Movies of heterogeneous quality compete against each other through
viewers' reviews and generate interesting dynamics of box-office. By assuming
mean-field interactions between the competing movies, we show that run-away
effect of popularity spreading is triggered by defeating the average review
score, leading to hits in box-office. The average review score thus
characterizes the critical movie quality necessary for transition from
box-office bombs to blockbusters. The major factors affecting the critical
review score are examined. By iterating the mean-field dynamical equations, we
obtain qualitative agreements with simulations and real systems in the
dynamical forms of box-office, revealing the significant role of competition in
understanding box-office dynamics.Comment: 8 pages, 6 figure
Swimming Efficiency of Bacterium Escherichia Coli
We use in vivo measurements of swimming bacteria in an optical trap to
determine fundamental properties of bacterial propulsion. In particular, we
determine the propulsion matrix, which relates the angular velocity of the
flagellum to the torques and forces propelling the bacterium. From the
propulsion matrix dynamical properties such as forces, torques, swimming speed
and power can be obtained from measurements of the angular velocity of the
motor. We find significant heterogeneities among different individuals even
though all bacteria started from a single colony. The propulsive efficiency,
defined as the ratio of the propulsive power output to the rotary power input
provided by the motors, is found to be 0.2%.Comment: 6 page
Optimal Location of Sources in Transportation Networks
We consider the problem of optimizing the locations of source nodes in
transportation networks. A reduction of the fraction of surplus nodes induces a
glassy transition. In contrast to most constraint satisfaction problems
involving discrete variables, our problem involves continuous variables which
lead to cavity fields in the form of functions. The one-step replica symmetry
breaking (1RSB) solution involves solving a stable distribution of functionals,
which is in general infeasible. In this paper, we obtain small closed sets of
functional cavity fields and demonstrate how functional recursions are
converted to simple recursions of probabilities, which make the 1RSB solution
feasible. The physical results in the replica symmetric (RS) and the 1RSB
frameworks are thus derived and the stability of the RS and 1RSB solutions are
examined.Comment: 38 pages, 18 figure
Improved Lagrangian mixing models for passive scalars in isotropic turbulence
Lagrangian data for velocity, scalars, and energy and scalar dissipation from direct numerical simulations are used to validate Lagrangian mixing models for inert passive scalars in stationary isotropic turbulence. The scalar fluctuations are nearly Gaussian, and, as a result of production by uniform mean gradients, statistically stationary. Comparisons are made for Taylor-scale Reynolds numbers in the range 38 to about 240 and Schmidt numbers in the range 1/8 to 1. Model predictions for one-point, one-time Eulerian statistics ~Eulerian correspondence! and one-particle, two-time Lagrangian statistics ~Lagrangian correspondence! are examined. Two scalar mixing models, namely the Lagrangian Fokker–Planck model and the Lagrangian colored-noise ~LCN! model, are proposed and written in terms of stochastic differential equations ~SDE! with specified drift and diffusion terms. Both of these models rely on statistics of the scalar field conditioned upon the energy dissipation, as provided by the Lagrangian spectral relaxation ~LSR! model. With the exception of the scalar dissipation, the models are shown to capture the Reynolds and Schmidt-number dependence of the Lagrangian integral time scales. However, the LCN model provides a more realistic description of the Lagrangian scalar fluctuations as differentiable time series having the correct form of the scalar autocorrelation function. Further extensions of the new mixing models to non-Gaussian scalars are conceptually straightforward, but require a closure for the scalar-conditioned scalar dissipation rate matrix. Likewise, accurate prediction of joint statistics for differential diffusion between different scalars with unequal molecular diffusivities will require the formulation of a multiscale SDE similar to the LSR model
Measurement of Lagrangian velocity in fully developed turbulence
We have developed a new experimental technique to measure the Lagrangian
velocity of tracer particles in a turbulent flow, based on ultrasonic Doppler
tracking. This method yields a direct access to the velocity of a single
particule at a turbulent Reynolds number . Its dynamics is
analyzed with two decades of time resolution, below the Lagrangian correlation
time. We observe that the Lagrangian velocity spectrum has a Lorentzian form
, in agreement
with a Kolmogorov-like scaling in the inertial range. The probability density
function (PDF) of the velocity time increments displays a change of shape from
quasi-Gaussian a integral time scale to stretched exponential tails at the
smallest time increments. This intermittency, when measured from relative
scaling exponents of structure functions, is more pronounced than in the
Eulerian framework.Comment: 4 pages, 5 figures. to appear in PR
Self-organization in social tagging systems
Individuals often imitate each other to fall into the typical group, leading to a self-organized state of typical behaviors in a community. In this paper, we model self-organization in social tagging systems and illustrate the underlying interaction and dynamics. Specifically, we introduce a model in which individuals adjust their own tagging tendency to imitate the average tagging tendency. We found that when users are of low confidence, they tend to imitate others and lead to a self-organized state with active tagging. On the other hand, when users are of high confidence and are stubborn to change, tagging becomes inactive. We observe a phase transition at a critical level of user confidence when the system changes from one regime to the other. The distributions of post length obtained from the model are compared to real data, which show good agreement
Implementation of a national HIV pre-exposure prophylaxis service is associated with changes in characteristics of people with newly diagnosed HIV: a retrospective cohort study
OBJECTIVES: To review characteristics of individuals newly diagnosed with HIV following implementation of a national pre-exposure prophylaxis (PrEP) programme (comprehensive PrEP services, delivered in sexual health clinics) to inform future delivery and broader HIV prevention strategies. METHODS: We extracted data from national HIV databases (July 2015-June 2018). We compared sociodemographic characteristics of individuals diagnosed in the period before and after PrEP implementation, and determined the proportion of 'potentially preventable' infections with the sexual health clinic-based PrEP delivery model used. RESULTS: Those diagnosed with HIV before PrEP implementation were more likely to be male (342/418, 81.8% vs 142/197, 72.1%, p=0.005), be white indigenous (327/418, 78.2% vs 126/197, 64.0%, p<0.001), report transmission route as sex between men (219/418, 52.4% vs 81/197, 41.1%, p=0.014), and have acquired HIV in the country of the programme (302/418, 72.2% vs 114/197, 57.9% p<0.001) and less likely to report transmission through heterosexual sex (114/418, 27.3% vs 77/197, 39.1%, p=0.002) than after implementation.Pre-implementation, 8.6% (36/418) diagnoses were 'potentially preventable' with the PrEP model used. Post-implementation, this was 6.6% (13/197), but higher among those with recently acquired HIV (49/170, 28.8%). Overall, individuals with 'potentially preventable' infections were more likely to be male (49/49, 100% vs 435/566, 76.9%, p<0.001), aged <40 years (37/49, 75.5% vs 307/566, 54.2%, p=0.004), report transmission route as sex between men (49/49, 100% vs 251/566, 44.3%, p<0.001), have previously received post-exposure prophylaxis (12/49, 24.5% vs 7/566, 1.2%, p<0.001) and less likely to be black African (0/49, 0% vs 67/566, 11.8%, p=0.010) than those not meeting this definition. CONCLUSIONS: The sexual health clinic-based national PrEP delivery model appeared to best suit men who have sex with men and white indigenous individuals but had limited reach into other key vulnerable groups. Enhanced models of delivery and HIV combination prevention are required to widen access to individuals not benefiting from PrEP at present
Evolution of speckle during spinodal decomposition
Time-dependent properties of the speckled intensity patterns created by
scattering coherent radiation from materials undergoing spinodal decomposition
are investigated by numerical integration of the Cahn-Hilliard-Cook equation.
For binary systems which obey a local conservation law, the characteristic
domain size is known to grow in time as with n=1/3,
where B is a constant. The intensities of individual speckles are found to be
nonstationary, persistent time series. The two-time intensity covariance at
wave vector can be collapsed onto a scaling function , where and . Both analytically and numerically, the covariance
is found to depend on only through in the
small- limit and in the large-
limit, consistent with a simple theory of moving interfaces that applies to any
universality class described by a scalar order parameter. The speckle-intensity
covariance is numerically demonstrated to be equal to the square of the
two-time structure factor of the scattering material, for which an analytic
scaling function is obtained for large In addition, the two-time,
two-point order-parameter correlation function is found to scale as
, even for quite large
distances . The asymptotic power-law exponent for the autocorrelation
function is found to be , violating an upper bound
conjectured by Fisher and Huse.Comment: RevTex: 11 pages + 12 figures, submitted to PR
Optimal Resource Allocation in Random Networks with Transportation Bandwidths
We apply statistical physics to study the task of resource allocation in
random sparse networks with limited bandwidths for the transportation of
resources along the links. Useful algorithms are obtained from recursive
relations. Bottlenecks emerge when the bandwidths are small, causing an
increase in the fraction of idle links. For a given total bandwidth per node,
the efficiency of allocation increases with the network connectivity. In the
high connectivity limit, we find a phase transition at a critical bandwidth,
above which clusters of balanced nodes appear, characterised by a profile of
homogenized resource allocation similar to the Maxwell's construction.Comment: 28 pages, 11 figure
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