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 $\rho$ and the long term current density $J$. As such, control problems may be reformulated as variational problems over $\rho$ and $J$. We discuss several optimization problems: the problem in which both $\rho$ and $J$ are varied freely, the problem in which $\rho$ is fixed and the one in which $J$ 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 \ud 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 \ud 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 \ud 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