5,744 research outputs found
Sufficient conditions for convergence of the Sum-Product Algorithm
We derive novel conditions that guarantee convergence of the Sum-Product
algorithm (also known as Loopy Belief Propagation or simply Belief Propagation)
to a unique fixed point, irrespective of the initial messages. The
computational complexity of the conditions is polynomial in the number of
variables. In contrast with previously existing conditions, our results are
directly applicable to arbitrary factor graphs (with discrete variables) and
are shown to be valid also in the case of factors containing zeros, under some
additional conditions. We compare our bounds with existing ones, numerically
and, if possible, analytically. For binary variables with pairwise
interactions, we derive sufficient conditions that take into account local
evidence (i.e., single variable factors) and the type of pair interactions
(attractive or repulsive). It is shown empirically that this bound outperforms
existing bounds.Comment: 15 pages, 5 figures. Major changes and new results in this revised
version. Submitted to IEEE Transactions on Information Theor
Modelling water flow and seasonal soil moisture dynamics in analluvial groundwater-fed wetland
Complex interactions occur in riparian wetlands between groundwater, surface water and climatic conditions. Knowledge of the hydrology of these systems is necessary to understand their functioning and their value and models are a useful and probably essential tool to capture their hydrological complexity. In this study, a 2D-model describing saturated-unsaturated water flow is applied to a transect through a groundwater-fed riparian wetland located along the middle reach of the river Dijle. The transect has high levees close to the river and a depression further into the floodplain. Scaling factors are introduced to describe the variability of soil hydraulic properties along the transect. Preliminary model calculations for one year show a good agreement between model calculations and measurements and demonstrate the capability of the model to capture the internal groundwater dynamics. Seasonal variations in soil moisture are reproduced well by the model thus translating external hydrological boundary conditions to root zone conditions. The model proves to be a promising tool for assessing effects of changes in hydrological boundary conditions on vegetation type distribution and to gain more insight in the highly variable internal flow processes of riparian wetlands.</p> <p style='line-height: 20px;'><b>Keywords: </b>riparian wetland,eco-hydrology, upward seepage, floodplain hydrolog
Linear PDEs and eigenvalue problems corresponding to ergodic stochastic optimization problems on compact manifolds
We consider long term average or `ergodic' optimal control poblems with a
special structure: Control is exerted in all directions and the control costs
are proportional to the square of the norm of the control field with respect to
the metric induced by the noise. The long term stochastic dynamics on the
manifold will be completely characterized by the long term density and
the long term current density . As such, control problems may be
reformulated as variational problems over and . We discuss several
optimization problems: the problem in which both and are varied
freely, the problem in which is fixed and the one in which is fixed.
These problems lead to different kinds of operator problems: linear PDEs in the
first two cases and a nonlinear PDE in the latter case. These results are
obtained through through variational principle using infinite dimensional
Lagrange multipliers. In the case where the initial dynamics are reversible we
obtain the result that the optimally controlled diffusion is also
symmetrizable. The particular case of constraining the dynamics to be
reversible of the optimally controlled process leads to a linear eigenvalue
problem for the square root of the density process
ICT literacy: a technical or a non-technical issue?
In this short reply to Riis’ paper I first deal with his perceptive defence of ICT literacy, to which I fully subscribe, showing how his ideas might gain from highlighting the ‘technical’ dimensions involved in literacy practices. Second, this will allow me to make some comments regarding the curricular and organizational aspects of contemporary (school) education, which forms the largest part of his paper. My main line of criticism towards Riis’ paper is that I defend a ‘technical’ rather than a ‘non-technical’ account of digital literacy
Alcohol outlets near schools in a midsize Romanian city : prevalence and characteristics
Objective: alcohol availability is one of the strongest predictors of adolescent alcohol use, and subsequent harm. Alcohol outlets near schools are an important indicator of three types of availability related to adolescent alcohol use; physical (number), economic (price), and legal (compliance with age limits).\ud
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Method: two teams with trained students (16 and 17 years old) visited all 37 schools in a 200,000 inhabitant Romanian city (Pitesti). On the spot all alcohol outlets were visited and data was collected on outlet characteristics and visitors. Also, by conducting mystery shopping purchase attempts by the researchers, compliance on the age limits for alcohol sales was tested.\ud
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Results: a total of 40 outlets were found within a 250 meter distance around 23 schools. Alcohol turns out to be cheap, and commercial alcohol brand signs are dominantly visible. With respect to compliance with the 18-year-old Romanian age limit for alcohol sales, only eight (20%) outlets refused to sell alcohol to under aged decoy customers.\ud
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Conclusion: adolescent alcohol availability is high on the physical, economic and legal aspect. Pitesti is the first city in\ud
Romania where an international alcohol prevention project has started to reduce alcohol related consequences. This project\ud
involves all relevant stakeholders, and the first new legislation on this subject had been implemented
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