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Mobilités et transformation numérique : l’exemple de Kigali au Rwanda
No full textInternational audienceSince the early 2010s, the Rwandan government has been actively pursuing a policy of modernizing public transportation in the capital city of Kigali. Based on fieldwork the authors conducted in 2019, this note examines the role of digital technology in this transformation. The analysis highlights a comprehensive overhaul of the transportation sector, including the ban on informal minibuses, modernization of the bus fleet, restructuring of bus terminals, and the installation of bus stops. Digital tools, such as smart cards for ticketing and the mandatory installation of electronic meters in motorcycle taxis and taxis, have supported these changes. This shift emphasizes increased control over business transactions and mobilities, while also exacerbating the risk of economic or technical exclusion to certain populations.Le gouvernement rwandais mène depuis le début des années 2010 une politique volontariste de modernisation de l’offre en transports en commun dans la capitale du Rwanda, Kigali. À partir d’un travail de terrain mené en 2019, la note analyse le rôle du numérique dans cette transformation. Elle conclut à une réforme en profondeur du secteur des transports (minibus informels interdits de circulation, modernisation de la flotte de bus, gares routières restructurées et arrêts de bus installés), en partie adossée à des outils numériques (cartes à puce pour la billettique, obligation d’équiper les taxis-motos et taxis de compteurs électroniques). Ces changements accentuent les contrôles sur les transactions commerciales et les mobilités, allant jusqu’à exacerber le risque d’exclusion économique ou technique de certaines populations
Résolution des transferts de chaleur par Monte-Carlo dans une géométrie urbaine lors d'une vague de chaleur
No full textInternational audienceLa modélisation des transferts de chaleur dans une géométrie urbaine constitue une étape clé pour prévoir l’ı̂lot de chaleur urbain, le confort extérieur des piétons et la consommation énergétique des bâtiments ainsi que pour quantifier l’effet des mesures d’atténuation et d’adaptation au changement climatique. Une difficulté réside dans le passage à l’échelle de ces simulations qui doivent intégrer la morphologie urbaine complexe constituée de divers matériaux. Pour répondre à cet enjeu, une approche probabiliste de la résolution des transferts de chaleur est présentée qui utilise la méthode de Monte-Carlo connue pour son insensibilité aux géométries et conditions aux limites complexes. La formulation intégrale est présentéeavec les marches aléatoires pour la conduction et les rayonnements thermique et solaire. Une validation numérique est présentée dans une application de pont thermique en géométrie tridimensionnelle. Ensuite, le modèle probabiliste de transfert de chaleur est appliqué à un scénario de vague de chaleur où sont calculées : la température radiante moyenne extérieure montrant l’influence des arbres ; et la température moyenne intérieure des murs montrant l’influence des apports solaires à travers les ouvertures
Error estimates and variance reduction for nonequilibrium stochastic dynamics
No full textInternational audienceEquilibrium properties in statistical physics are obtained by computing averages with respect to Boltzmann-Gibbs measures, sampled in practice using ergodic dynamics such as the Langevin dynamics. Some quantities however cannot be computed by simply sampling the Boltzmann-Gibbs measure, in particular transport coefficients, which relate the current of some physical quantity of interest to the forcing needed to induce it. For instance, a temperature difference induces an energy current, the proportionality factor between these two quantities being the thermal conductivity. From an abstract point of view, transport coefficients can also be considered as some form of sensitivity analysis with respect to an added forcing to the baseline dynamics. There are various numerical techniques to estimate transport coefficients, which all suffer from large errors, in particular large statistical errors. This contribution reviews the most popular methods, namely the Green-Kubo approach where the transport coefficient is expressed as some time-integrated correlation function, and the approach based on longtime averages of the stochastic dynamics perturbed by an external driving (so-called nonequilibrium molecular dynamics). In each case, the various sources of errors are made precise, in particular the bias related to the time discretization of the underlying continuous dynamics, and the variance of the associated Monte Carlo estimators. Some recent alternative techniques to estimate transport coefficients are also discussed
Oak beams in medieval frameworks: constraints and advantages for restoration
No full textInternational audienceDuring the fire of Notre-Dame de Paris, timber frames coming from the Middle ages period, in the nave and choir, were burnt. It was decided to proceed to an identical reconstruction, but many questions arise about the timber frame. Is wood outdated as a material for beams? Is oak wood the best solution for timber frame? Are we able to find similar long slender oak beams in today French forest? Is it safe to use green wood for the building?In fact, the forested area in France is the same today as in the Middle ages period, but two times higher than during the 19 th century. Results from the forest inventory proves that 40 million slender oak trees are available in order to sort around 1500 trees for the restoration. Cathedral heritage provided a demonstration of the high efficiency of carpenter’s knowhow. Most of oak beams kept their functionality for more than 800 years. This is a nice experimental result for a very long mechanical testing. Wood, emerged as material 300 million years ago as a result of long time evolution, is used to build trees, which are a collection of wooden beams able to resist flexure forces, for a long time.Technical know-how about timber frame is old in human societies and was improved during the Middle ages, but scientific knowledge about wood as an anisotropic material is very recent. The background in mechanics for such material has been developed in early 20 th century and observation tools for material structures at the nanometre scale appeared in late 20 th century. Wood can be considered as an archetypal hightech material using polymer-with-additive technology combined to nanostructured fibrecomposite within a honeycomb-like micro-structured material. This explains why nature solution for tree beams is very efficient in terms of bending resistance per weight. The mechanical behaviour of wooden beams is highly sensitive to the orientation of the main direction of anisotropy (grain direction) relative to bending forces. Natural beams in trees are built by a kind of 3D printing process so that grain orientation at beam periphery is always optimum. Mechanical bending performance of these beams (round-wood), in spite of branching, is equivalent to the performance of standard clearwood specimens documented in databases. Squaring at the minimum this round-wood should be done while keeping grain angle closest to zero-degree value, which is performed by specific axe-hewn log squaring. The result is “coarse” slightly curved beams much more resistant than “nicely” straight sawn beams with local grain angle above 10°. Using directly green wood in the building process is effective for time of building and energy consumption. The drying of wood induces long deep longitudinal cracks and a reduction in beam section but a bigger increase in wood mechanical performance. If the green frame resists, the dried one will resist all the following years. Finally, today carpenters have kept the building know-how and there are many modern non-destructive techniques available for the best sorting of trees and mechanical evaluation of beams
A Theory of Self-Prospection
No full textA present-biased decision maker (DM) faces a two-armed bandit problem whose risky arm generates random payoffs at exponentially distributed times. The DM learns about payoff arrivals through informative feedback. At the unique stationary Markov perfect equilibrium of the multi-self game, positive feedback supports greater equilibrium welfare than both negative and transparent feedback. Regardless of the form of feedback, the DM's behavior exhibits indecision, deriving from their desire to procrastinate. We relate our findings to the theory of self-prospection -the process of imagining future goals and outcomes when seeking motivation in the present
Logement abordable et fiscalité locale : acceptions, intrications, disjonctions ?
No full textNational audienc
Generalizing Roberts' Characterization of Unit Interval Graphs
No full textInternational audienceFor any natural number d, a graph G is a (disjoint) d-interval graph if it is the intersection graph of (disjoint) d-intervals, the union of d (disjoint) intervals on the real line. Two important subclasses of d-interval graphs are unit and balanced d-interval graphs (where every interval has unit length or all the intervals associated to a same vertex have the same length, respectively). A celebrated result by Roberts gives a simple characterization of unit interval graphs being exactly claw-free interval graphs. Here, we study the generalization of this characterization for d-interval graphs. In particular, we prove that for any d ⩾ 2, if G is a K_{1,2d+1}-free interval graph, then G is a unit d-interval graph. However, somehow surprisingly, under the same assumptions, G is not always a disjoint unit d-interval graph. This implies that the class of disjoint unit d-interval graphs is strictly included in the class of unit d-interval graphs. Finally, we study the relationships between the classes obtained under disjoint and non-disjoint d-intervals in the balanced case and show that the classes of disjoint balanced 2-intervals and balanced 2-intervals coincide, but this is no longer true for d > 2
PGD_ MIAM_ Altération microbienne et préservation des monuments en zone urbaine
No full textThe preservation of built cultural heritage in an urban area is a socio-economic challenge for municipalities. During their lifetime, buildings out of stone are exposed to chemical and biological actions that lead to different alteration patterns.In order to evaluate the treatments developed in the laboratory by the ICMPE team, under controlled and real conditions. The polymers will be applied to healthy coupons in the presence of microbial communities in the laboratory under controlled conditions as well as on the roof of UPEC. The application of treatments will take place in real conditions at Père Lachaise Cemetery on pre-aged coupons as well as on graves (in agreement with the cemetery manager). This would make it possible to see the curative effect and preventive effect of the treatments developed in the laboratory and its effectiveness on the different microbial communities (bacteria, fungi and microalgae). The safety of the treatments on the material as well as on the surrounding environment will be evaluated.La préservation du patrimoine culturel bâti en zone urbaine est un défi socio-économique. Durant leur vie, les bâtiments en pierre sont exposés à des actions chimiques et biologiques qui conduisent à différents faciès d’altération.Afin d’évaluer les traitements développés au laboratoire par l’équipe ICMPE, en conditions contrôlées et réelles. Les polymères seront appliqués sur des coupons sains en présence de communautés microbiennes au laboratoire en conditions contrôlées ainsi que sur le toit de l’UPEC. L’application des traitements aura lieu en conditions réelles au cimetière Père Lachaise sur des coupons pré-vieillis ainsi que sur des tombes (en accord avec le gestionnaire du Cimetière). Cela permettrait de voir l’effet curatif et l’effet préventif des traitements développés au laboratoire et son efficacité sur les différentes communautés microbiennes (Bactéries, champignons et micro-algues). L’innocuité des traitements sur le matériau ainsi que sur le milieu environnant sera évalué
Growth conditions to ensure input-to-state stability of time-delay systems under point-wise dissipation
No full textInternational audienceWe propose relaxed Lyapunov-based conditions to ensure input-to-state stability (ISS) of nonlinear time-delay systems. Their strength lies in the fact that the dissipation rate of the Lyapunov-Krasovskii functional (LKF) involves only the current value of solution's norm rather than the LKF itself. The additional requirement takes the form of a growth condition between the dissipation rate and its maximal increase along the system's solutions. We show through examples that the obtained conditions are more general than existing techniques, including the strictification method through the addition of an exponential term in the integral kernel of the LKF, whose limitations are highlighted through a counter-example
Impacts of repetitive droughts and the key role of experience : evidence from Nigeria
No full textWestern African Sahel faced severe droughts in the 1980s, affecting agricultural production and food security. In recent decades, farmers have faced uncertainty in the timing and amount of rainy seasons and are confronted with erratic rainfall with high interannual variations. Can the experience of past dry events reduce the vulnerability of households to short-term rainfall shocks? In this paper, I match three waves of panel household surveys focusing on agriculture in Nigeria (GHS, from 2010-2016) and high temporal resolution precipitation data set from the Climate Hazard Center (CHIRPS). I show evidence of the extreme importance of the long-dry period of the 1980s and identify more recent droughts in 2013/2015, which are in line with a change in the characteristics of the rainfall trends. Through a two-way-fixed effect strategy, I exploit the spatial variation of the exposition to the 2015 drought. First, I look at the short-term effects of being hit by a drought on agricultural production and food security indicators. I show that being hit by a drought decreases yields by 14%, and decreases the food diversity of households by around 1%. Second, I look at the impacts' heterogeneity according to the plot's experience, using the timing of the year of acquisition of the plot. I compare short-term droughts' effects on households that acquired their first plot before the 1980s dry period to those that acquired it after. Results suggest that acquiring the land before 1985 attenuates the harmful effects of a climate shock, as these particular households have only a 3% reduction in their yields due to the 2015 drought. This is especially the case when households were severely hit in the 1980s. This result might suggest that having a long-lasting experience under extreme dry events on cultivated land reduces vulnerability to rainfall variability