Showing papers by "University of Luxembourg published in 2006"

Modelling the global ocean tides: modern insights from FES2004

Abstract: During the 1990s, a large number of new tidal atlases were developed, primarily to provide accurate tidal corrections for satellite altimetry applications. During this decade, the French tidal group (FTG), led by C. Le Provost, produced a series of finite element solutions (FES) tidal atlases, among which FES2004 is the latest release, computed from the tidal hydrodynamic equations and data assimilation. The aim of this paper is to review the state of the art of tidal modelling and the progress achieved during this past decade. The first sections summarise the general FTG approach to modelling the global tides. In the following sections, we introduce the FES2004 tidal atlas and validate the model against in situ and satellite data. We demonstrate the higher accuracy of the FES2004 release compared to earlier FES tidal atlases, and we recommend its use in tidal applications. The final section focuses on the new dissipation term added to the equations, which aims to account for the conversion of barotropic energy into internal tidal energy. There is a huge improvement in the hydrodynamic tidal solution and energy budget obtained when this term is taken into account.

Beyond Learning: Democratic Education for a Human Future

Abstract: Learning Chapter 2: Coming into Presence Chapter 3: The Community of Those Who Have Nothing in Common Chapter 4: How Difficult Should Education Be? Chapter 5: The Architecture of Education Chapter 6: Education and the Democratic Person Epilogue: A Pedagogy of Interruption.

Collaborative analysis of alpha-synuclein gene promoter variability and Parkinson disease.

Abstract: ContextIdentification and replication of susceptibility genes for Parkinson disease at the population level have been hampered by small studies with potential biases. α-Synuclein (SNCA) has been one of the most promising susceptibility genes, but large-scale studies have been lacking.ObjectiveTo determine whether allele-length variability in the dinucleotide repeat sequence (REP1) of the SNCA gene promoter is associated with Parkinson disease susceptibility, whether SNCA promoter haplotypes are associated with Parkinson disease, and whether REP1 variability modifies age at onset.Design, Setting, and ParticipantsWe performed a collaborative analysis of individual-level data on SNCA REP1 and flanking markers in patients with Parkinson disease and controls. Study site recruitment, data collection, and analyses were performed between April 5, 2004, and December 31, 2005. Eighteen participating sites of a global genetics consortium provided clinical data. Genotyping was performed for SNCA REP1, −770, and −116 markers at individual sites; however, each site also provided 20 DNA samples for regenotyping centrally.Main Outcome MeasuresMeasures included estimations of Hardy-Weinberg equilibrium in controls; a test of heterogeneity; analyses for association of single variants or haplotypes; and survival analyses for age at onset.ResultsOf the 18 sites, 11 met stringent criteria for concordance with Hardy-Weinberg equilibrium and low genotyping error rate. These 11 sites provided complete data for 2692 cases and 2652 controls. There was no heterogeneity across studies (P>.60). The SNCA REP1 alleles differed in frequency for cases and controls (P<.001). Genotypes defined by the 263 base-pair allele were associated with Parkinson disease (odds ratio, 1.43; 95% confidence interval, 1.22-1.69; P<.001 for trend). Multilocus haplotypes differed in frequency for cases and controls (global score statistic, P<.001). Two-loci haplotypes were associated with Parkinson disease only when they included REP1 as one of the loci. However, genotypes defined by REP1 alleles did not modify age at onset (P = .55).ConclusionThis large-scale collaborative analysis demonstrates that SNCA REP1 allele-length variability is associated with an increased risk of Parkinson disease.

Climatic Change and Rural-Urban Migration: The Case of Sub-Saharan Africa

Abstract: We investigate the role that climatic change has played in the pattern of urbanization in sub-Saharan African countries compared to the rest of the developing world. To this end we assemble a cross-country panel data set that allows us to estimate the determinants of urbanization. The results of our econometric analysis suggest that climatic change, as proxied by rainfall, has acted to change urbanization in sub-Saharan Africa but not elsewhere in the developing world. Moreover, this link has become stronger since decolonization, which is likely due to the often simultaneous lifting of legislation prohibiting the free internal movement of native Africans.

Collaborative Multiagent Reinforcement Learning by Payoff Propagation

Abstract: In this article we describe a set of scalable techniques for learning the behavior of a group of agents in a collaborative multiagent setting. As a basis we use the framework of coordination graphs of Guestrin, Koller, and Parr (2002a) which exploits the dependencies between agents to decompose the global payoff function into a sum of local terms. First, we deal with the single-state case and describe a payoff propagation algorithm that computes the individual actions that approximately maximize the global payoff function. The method can be viewed as the decision-making analogue of belief propagation in Bayesian networks. Second, we focus on learning the behavior of the agents in sequential decision-making tasks. We introduce different model-free reinforcement-learning techniques, unitedly called Sparse Cooperative Q-learning, which approximate the global action-value function based on the topology of a coordination graph, and perform updates using the contribution of the individual agents to the maximal global action value. The combined use of an edge-based decomposition of the action-value function and the payoff propagation algorithm for efficient action selection, result in an approach that scales only linearly in the problem size. We provide experimental evidence that our method outperforms related multiagent reinforcement-learning methods based on temporal differences.

Raman spectroscopy of single-wall boron nitride nanotubes.

Abstract: Single-wall boron nitride nanotubes samples synthesized by laser vaporization of a hexagonal BN target under a nitrogen atmosphere are studied by UV and visible Raman spectroscopy. We show that resonant conditions are necessary for investigating phonon modes of BNNTs. Raman excitation in the UV (229 nm) provides preresonant conditions, allowing the identification of the A1 tangential mode at 1370 cm-1. This is 5 cm-1 higher than the E2g mode in bulk h-BN. Ab initio calculations show that the lower frequency of bulk h-BN with respect to large diameter nanotubes and the single sheet of h-BN is related to a softening of the sp2 bonds in the bulk due to interlayer interaction.

Point-Based Value Iteration for Continuous POMDPs

Abstract: We propose a novel approach to optimize Partially Observable Markov Decisions Processes (POMDPs) defined on continuous spaces. To date, most algorithms for model-based POMDPs are restricted to discrete states, actions, and observations, but many real-world problems such as, for instance, robot navigation, are naturally defined on continuous spaces. In this work, we demonstrate that the value function for continuous POMDPs is convex in the beliefs over continuous state spaces, and piecewise-linear convex for the particular case of discrete observations and actions but still continuous states. We also demonstrate that continuous Bellman backups are contracting and isotonic ensuring the monotonic convergence of value-iteration algorithms. Relying on those properties, we extend the algorithm, originally developed for discrete POMDPs, to work in continuous state spaces by representing the observation, transition, and reward models using Gaussian mixtures, and the beliefs using Gaussian mixtures or particle sets. With these representations, the integrals that appear in the Bellman backup can be computed in closed form and, therefore, the algorithm is computationally feasible. Finally, we further extend to deal with continuous action and observation sets by designing effective sampling approaches.

Introduction to normative multiagent systems

Abstract: This article introduces the research issues related to and definition of normative multiagent systems. It also describes the papers selected from NorMAS05 that are part of this double special issue and relates the papers to each other.

Unemployment Insurance in Europe: Unemployment Duration and Subsequent Employment Stability

Abstract: The empirical literature on unemployment insurance has focused on its direct effect on unemployment duration, while the potential indirect effect on employment stability through a more efficient matching process, as the unemployed can search for a longer period, has attracted much less attention. In the European context this is surprising as reform proposals of the unemployment insurance system aiming at reducing high European unemployment rates should consider both effects. This paper provides evidence on the effect of unemployment benefits on unemployment and employment duration in Europe, using individual data from the European Community Household Panel for eight countries. Country specific estimates based on a multivariate discrete proportional hazard model, controlling for observed and unobserved individual heterogeneity, suggest that even if receiving benefits has a direct negative effect increasing the duration of unemployment spells, there is also a positive indirect effect of benefits on subsequent employment duration. This indirect effect is pronounced in countries with relatively generous benefit systems, and for recipients who have remained unemployed for at least six months. In terms of the magnitude of the effect, recipients remain employed on average two to four months longer than non-recipients. This represents a ten to twenty per cent increase relative to the average employment duration, compensating for the additional time spent in unemployment. These findings are in line with theories suggesting a matching effect of unemployment insurance.

Between-breed variability of stillbirth and its relationship with sow and piglet characteristics.

Abstract: Litter characteristics at birth were recorded in 4 genetic types of sows with differing maternal abilities. Eighty-two litters from F(1) Duroc x Large White sows, 651 litters from Large White sows, 63 litters from Meishan sows, and 173 litters from Laconie sows were considered. Statistical models included random effects of sow, litter, or both; fixed effects of sow genetic type, parity, birth assistance, and piglet sex, as well as gestation length, farrowing duration, piglet birth weight, and litter size as linear covariates. The quadratic components of the last 2 factors were also considered. For statistical analyses, GLM were first considered, assuming a binomial distribution of stillbirth. Hierarchical models were also fitted to the data to take into account correlations among piglets from the same litter. Model selection was performed based on deviance and deviance information criterion. Finally, standard and robust generalized estimating equations (GEE) procedures were applied to quantify the importance of each effect on a piglet's probability of stillbirth. The 5 most important factors involved were, in decreasing order (contribution of each effect to variance reduction): difference between piglet birth weight and the litter mean (2.36%), individual birth weight (2.25%), piglet sex (1.01%), farrowing duration (0.99%), and sow genetic type (0.94%). Probability of stillbirth was greater for lighter piglets, for male piglets, and for piglets from small or very large litters. Probability of stillbirth increased with sow parity number and with farrowing duration. Piglets born from Meishan sows had a lower risk of stillbirth (P < 0.0001) and were little affected by the sources of variation mentioned above compared with the 3 other sow genetic types. Standard and robust GEE approaches gave similar results despite some disequilibrium in the data set structure highlighted with the robust GEE approach.

Absence of alpha 7 integrin in dystrophin-deficient mice causes a myopathy similar to Duchenne muscular dystrophy.

Abstract: Both the dystrophin-glycoprotein complex and alpha 7 beta 1 integrin have critical roles in the maintenance of muscle integrity via the provision of mechanical links between muscle fibres and the basement membrane. Absence of either dystrophin or alpha 7 integrin results in a muscular dystrophy. To clarify the role of alpha 7 integrin and dystrophin in muscle development and function, we generated integrin alpha 7/dystrophin double-mutant knockout (DKO) mice. Surprisingly, DKO mice survived post-natally and were indistinguishable from wild-type, integrin alpha 7-deficient and mdx mice at birth, but died within 24-28 days. Histological analysis revealed a severe muscular dystrophy in DKO mice with endomysial fibrosis and ectopic calcification. Weight loss was correlated with the loss of muscle fibres, indicating that progressive muscle wasting in the double mutant was most likely due to inadequate muscle regeneration. The data further support that premature death of DKO mice is due to cardiac and/or respiratory failure. The integrin alpha 7/dystrophin-deficient mouse model, therefore, resembles the pathological changes seen in Duchenne muscular dystrophy and suggests that the different clinical severity of dystrophin deficiency in human and mouse may be due to a fine-tuned difference in expression of dystrophin and integrin alpha 7 in both species. Together, these findings indicate an essential role for integrin alpha 7 in the maintenance of dystrophin-deficient muscles.

From Clinicogenetic Studies of Maturity-Onset Diabetes of the Young to Unraveling Complex Mechanisms of Glucokinase Regulation

Abstract: Glucokinase functions as a glucose sensor in pancreatic beta-cells and regulates hepatic glucose metabolism. A total of 83 probands were referred for a diagnostic screening of mutations in the glucokinase (GCK) gene. We found 11 different mutations (V62A, G72R, L146R, A208T, M210K, Y215X, S263P, E339G, R377C, S453L, and IVS5 + 1G>C) in 14 probands. Functional characterization of recombinant glutathionyl S-transferase-G72R glucokinase showed slightly increased activity, whereas S263P and G264S had near-normal activity. The other point mutations were inactivating. S263P showed marked thermal instability, whereas the stability of G72R and G264S differed only slightly from that of wild type. G72R and M210K did not respond to an allosteric glucokinase activator (GKA) or the hepatic glucokinase regulatory protein (GKRP). Mutation analysis of the role of glycine at position 72 by substituting E, F, K, M, S, or Q showed that G is unique since all these mutants had very low or no activity and were refractory to GKRP and GKA. Structural analysis provided plausible explanations for the drug resistance of G72R and M210K. Our study provides further evidence that protein instability in combination with loss of control by a putative endogenous activator and GKRP could be involved in the development of hyperglycemia in maturity-onset diabetes of the young, type 2. Furthermore, based on data obtained on G264S, we propose that other and still unknown mechanisms participate in the regulation of glucokinase.

On the security of HMAC and NMAC based on HAVAL, MD4, MD5, SHA-0 and SHA-1 (extended abstract)

Abstract: HMAC is a widely used message authentication code and a pseudorandom function generator based on cryptographic hash functions such as MD5 and SHA-1. It has been standardized by ANSI, IETF, ISO and NIST. HMAC is proved to be secure as long as the compression function of the underlying hash function is a pseudorandom function. In this paper we devise two new distinguishers of the structure of HMAC, called differential and rectangle distinguishers, and use them to discuss the security of HMAC based on HAVAL, MD4, MD5, SHA-0 and SHA-1. We show how to distinguish HMAC with reduced or full versions of these cryptographic hash functions from a random function or from HMAC with a random function. We also show how to use our differential distinguisher to devise a forgery attack on HMAC. Our distinguishing and forgery attacks can also be mounted on NMAC based on HAVAL, MD4, MD5, SHA-0 and SHA-1.

Party System(s) and Electoral Behaviour in Belgium: From Stability to Balkanisation

Abstract: De Winter, L., Swyngedouw, M. & Dumont, P. “Party System(s) and Electoral Behaviour in Belgium: From Stability to Balkanisation”, in M. Brans, L. De Winter, W. Swenden (eds.) The Politics of Belgium, London, Routledge, 2009, 71-94

Policy with or without parties? A comparative analysis of policy priorities and policy change in Belgium, 1991 to 2000

Abstract: This paper confronts two models of policy: the party model states that policy-making is an orderly process initiated by parties implementing their party programme and carrying out their electoral promises; the external pressure model contends that policy change is a non-orderly process but rather a disjoint process coming in large bursts that are difficult to predict. Drawing upon eight policy agendas in Belgium covering the period from 1991 to 2000 we put both models to the test. Policy measures are operationalized via the budget and legislation. We found that budgets are as good as disconnected from any other policy agenda in Belgium. Legislation and the evolving legislative attention for issues in Belgium can be traced back to some extent to parties and external pressure at the same time. In terms of static policy priorities, we found that the party model indicators, party programmes and government agreements, are fairly good predictors of the legislative attention an issue will receive during...

Do Belgian Parties Undermine the Democratic Chain of Delegation

Abstract: De Winter, L., and Dumont, P. “Do Belgian Parties Undermine the Democratic Chain of Delegation?”, in M. Brans, L. De Winter, W. Swenden (eds.) The Politics of Belgium, London, Routledge, 2009, 95-114