8,519 research outputs found
Distributed Learning from Interactions in Social Networks
We consider a network scenario in which agents can evaluate each other
according to a score graph that models some interactions. The goal is to design
a distributed protocol, run by the agents, that allows them to learn their
unknown state among a finite set of possible values. We propose a Bayesian
framework in which scores and states are associated to probabilistic events
with unknown parameters and hyperparameters, respectively. We show that each
agent can learn its state by means of a local Bayesian classifier and a
(centralized) Maximum-Likelihood (ML) estimator of parameter-hyperparameter
that combines plain ML and Empirical Bayes approaches. By using tools from
graphical models, which allow us to gain insight on conditional dependencies of
scores and states, we provide a relaxed probabilistic model that ultimately
leads to a parameter-hyperparameter estimator amenable to distributed
computation. To highlight the appropriateness of the proposed relaxation, we
demonstrate the distributed estimators on a social interaction set-up for user
profiling.Comment: This submission is a shorter work (for conference publication) of a
more comprehensive paper, already submitted as arXiv:1706.04081 (under review
for journal publication). In this short submission only one social set-up is
considered and only one of the relaxed estimators is proposed. Moreover, the
exhaustive analysis, carried out in the longer manuscript, is completely
missing in this versio
Modelling solar low-lying cool loops with optically thick radiative losses
We investigate the increase of the DEM (differential emission measure)
towards the chromosphere due to small and cool magnetic loops (height
~Mm, ~K). In a previous paper we analysed the
conditions of existence and stability of these loops through hydrodynamic
simulations, focusing on their dependence on the details of the optically thin
radiative loss function used. In this paper, we extend those hydrodynamic
simulations to verify if this class of loops exists and it is stable when using
an optically thick radiative loss function. We study two cases: constant
background heating and a heating depending on the density. The contribution to
the transition region EUV output of these loops is also calculated and
presented. We find that stable, quasi-static cool loops can be obtained by
using an optically thick radiative loss function and a background heating
depending on the density. The DEMs of these loops, however, fail to reproduce
the observed DEM for temperatures between . We also show the
transient phase of a dynamic loop obtained by considering constant heating rate
and find that its average DEM, interpreted as a set of evolving dynamic loops,
reproduces quite well the observed DEM.Comment: Accepted for publication in A&A on Aug 21st 2015. arXiv admin note:
text overlap with arXiv:1112.030
Diffraction effects in length measurements by laser interferometry
High-accuracy dimensional measurements by laser interferometers require
corrections because of diffraction, which makes the effective fringe-period
different from the wavelength of a plane (or spherical) wave . By
using a combined X-ray and optical interferometer as a tool to investigate
diffraction across a laser beam, we observed wavelength variations as large as
. We show that they originate from the wavefront evolution
under paraxial propagation in the presence of wavefront- and intensity-profile
perturbations.Comment: preprint, 10 pages, 6 figures, submitted to Optics Expres
Longterm Influence of Inertia on the Diffusion of a Brownian Particle
We demonstrate experimentally that a Brownian particle is subject to inertial
effects at long time scales. By using a blinking optical tweezers, we extend
the range of previous experiments by several orders of magnitude up to a few
seconds. The measured mean square displacement of a freely diffusing Brownian
particle in a liquid shows a deviation from the Einstein-Smoluchowsky theory
that diverges with time. These results are consistent with a generalized theory
that takes into account not only the particle inertia but also the inertia of
the fluid surrounding the particle. This can lead to a bias in the estimation
of the diffusion coefficient from finite-time measurements. We show that the
decay of the relative error is polynomial and not exponential and, therefore,
can have significant effects at time scales relevant for experiments.Comment: 5 pages, 4 figure
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