Université de Cergy-Pontoise
  • Cergy-Pontoise, France
Recent publications
Let (Zn)n≥0 be a supercritical Galton–Watson process. The Lotka–Nagaev estimator Zn+1/Zn is a common estimator for the offspring mean. In this paper, we establish some Cramér moderate deviation results for the Lotka–Nagaev estimator via a martingale method. Applications to construction of confidence intervals are also given.
Organic thermoelectric (OTE) materials promise convenient energy conversion between heat gradients and voltage with flexible and wearable power‐supplying devices at a low price. Although a variety of OTE materials are investigated, the TE performance is still far from practical application. To achieve high TE performance, a thorough understanding of the structure–property relationship in OTE materials is necessary. In this comprehensive review, the fundamentals of OTEs are summarized, the recent achievements of OTE materials are reviewed, and the relationship between structure and properties in high‐performance OTE materials is discussed. Furthermore, how the molecular backbones, side chains, energy levels, molecular packing, and heteroatom effect all play vital roles in thermoelectric properties is addressed. Finally, the future direction of research on OTE materials is envisaged.
Salt crystallization is a major cause of damage in porous building materials. Accelerated salt weathering tests carried out in the laboratory are among the most common methods to assess the durability of material to salt decay. However, existing standards and recommendations for salt weathering tests have limitations in terms of effectiveness and/or reliability. In the framework of the RILEM Technical Committee 271-ASC, a procedure has been developed which proposes a new approach to salt crystallization tests. It starts from the consideration that salt damage can be seen as a process developing in two phases: accumulation of the salt in the material and propagation of the decay. In the first phase, salts are introduced in the material and accumulate close to the evaporation surface, while in the second phase damage propagates because of repeated dissolution and crystallization cycles, induced by re-wetting with liquid water and by relative humidity changes. In this paper, the procedure is described and the results of a first round robin validation of the test, carried out on 7 materials and involving 10 laboratories, are presented. The results show that the procedure is effective to cause decay within the time period of the test (about 3 months) and that the decay increases with subsequent cycles. The decay observed differs in type and severity depending on the salt type and concentration and on the type of substrate. The decay types detected in the laboratories are generally representative of those observed in the field for the selected substrates. The differences in durability between the various substrates, as assessed at the end of the test, are in line with the durability expected based on field observation. The reproducibility of the results in terms of decay type is good; some differences have been observed in terms of material loss. These are more significant in the case of NaCl contaminated specimens. Based on the results, proposals for fine-tuning of the procedure are given.
This paper documents a large-scale crowdsourcing operation based on nineteenth-century French military maps using the Open Historical Map (OHM) interface. It explains how the project was conceived, and offers a template for future large-scale collaborative mapping projects.
Sensing is the process of detecting and monitoring any physico‐chemical environmental parameters. Herein, new self‐powered iontronic sensors, which utilize touch‐induced ionic charge separation in ionically conductive hydrogels, are introduced for potential use in object mapping, recognition, and localization. This is accomplished using high‐resolution stereolithography (SLA) 3D printing of stacked ionic assemblies consisting of discrete compartments having different ion transport properties. The latter assemblies readily allow programming the output voltage magnitude and polarity by means of variations in ion type, charge density, and cross‐linking density within the iontronic device. Voltages of up to 70 mV are generated on application of compressive strains of as much as 50% (≈22.5 kPa), with the magnitude directly proportional to stress, and the polarity dependent on the sign of the mobile ion. As a proof‐of‐concept demonstration, the resulting touch sensors are integrated on the fingertip to enable the tactile feedback, mimicking the tactile perception of objects for recognition applications. In addition, it is proposed that streaming potential is the underlying mechanism behind the iontronic touch sensors. The electromechanical response is therein consistent with a streaming potential model.
We consider continuous space–time decay–surge population models, which are semi-stochastic processes for which deterministically declining populations, bound to fade away, are reinvigorated at random times by bursts or surges of random sizes. In a particular separable framework (in a sense made precise below) we provide explicit formulae for the scale (or harmonic) function and the speed measure of the process. The behavior of the scale function at infinity allows us to formulate conditions under which such processes either explode or are transient at infinity, or Harris recurrent. A description of the structures of both the discrete-time embedded chain and extreme record chain of such continuous-time processes is supplied.
Swedish gymnastics has been reduced to a directory of progressive analytical movements intended to form the basis of what is known in French physical education as a global gymnastics. This article explains how Philippe Tissié was inspired by Swedish gymnastics in his development of a hybrid vision between the Swedish method of physical education and the French model that was largely derived from the works of Jean Saint-Martin Amoros and Philippe Sarremejane. The paper demonstrates how Tissié’s French gymnastics was not only limited to analytical movements but also included the practice of sports. At the same time, it explains how the creation of this hybrid model meshed scientific findings from life sciences (biology and physiology) with human and social sciences (psychology and sociology). Between 1886 and 1935, Tissié’s appropriation, thus, enabled him to structure his conceptions of physical education and to move from the Swedish to the Franco-Swedish method.
The formation of long-lived, multicellular clusters is a fundamental step in the physiopathology of many disease-causing bacteria. Experiments on abiotic surfaces suggest that bacterial colonization, including initial cluster formation, requires (1) irreversible adhesion, (2) cell proliferation, and (3) a phenotypic transition. However, here we show that on infection of a polarized MDCK epithelium, Pseudomonas aeruginosa (PA) forms long-lived – i.e., permanent – bacterial clusters without requiring irreversible adhesion, cell proliferation, or a phenotypic transition. By combining experiments and a mathematical model, we reveal that the cluster formation process is mediated by type IV pili (T4P). Furthermore, we unveil how T4P quantitatively operate during adhesion, finding that it is a stochastic process that involves an activation time, requires the retraction of pili, and results in reversible attachment. We explain how such reversible attachment process leads to the formation of permanent bacterial clusters and quantify the cluster growth dynamics.
Résumé Nous nous intéressons au développement professionnel des enseignants stagiaires dans une formation à distance, via une plateforme e-space qui apparait comme un hub social et éducatif (Gobert, 2009, 2020). Nous abordons la question du développement en articulant deux théories : l'approche écologique de Bronfenbrenner et l'approche instrumentale de Rabardel. Ces deux théories perçoivent le développement comme le produit des interactions mutuelles du sujet avec son environnement. Au-delà de la simple maîtrise de l’outil numérique, nous cherchons à montrer comment l’appropriation d’un environnement numérique contribue au développement professionnel. Nous étudions l’hypothèse selon laquelle le sujet-apprenant (ici enseignant stagiaire) interagit avec les composantes de cet environnement et que c’est lors de ces interactions qu'il apprend, construit et se construit. Les résultats montrent que les changements des caractéristiques de l’environnement, provoqués par l’intégration d’un artefact extérieur (la vidéo), influent sur la coconstruction d’une culture numérique.
Notre contribution s’intéresse à l’activité des enseignant·es en formation en situation d’exercice de l’autorité en la traitant en trois temps. Après avoir montré que la notion de gestion de classe est évolutive et développé en quoi celle d’exercice de l’autorité s’en distingue ou la rejoint, nous considérons les apports de deux paradigmes (behavioriste, constructiviste) et de deux approches (analyse des pratiques, analyse de l’activité) qui se sont succédé dans les recherches et les formations en gestion de classe depuis un demi-siècle. Au regard de ces travaux, nous présentons ensuite les apports de notre recherche empirique, qui s’inscrit dans une entrée « analyse de l’activité ». Nous exposons en quoi cette approche, qui implique la mise en place d’observations filmées suivies d’entretiens d’autoconfrontation, permet de mieux comprendre les préoccupations des enseignant·es au plus proche de leur activité réelle en classe. D’une part, celle-ci nous permet d’identifier deux préoccupations typiques et conjointes chez les enseignant·es en formation : enrôler/maintenir le ou les élève(s) dans la tâche et gérer un ou des comportement(s) d’un ou plusieurs élève(s) perçu(s) comme inapproprié(s). D’autre part, nous décrivons les trois façons dont s’articulent ces deux préoccupations : imbriquées, indépendantes ou incompatibles. Enfin, ces résultats ouvrent la discussion sur plusieurs pistes fécondes pour concevoir des dispositifs de formation articulés aux situations de classe et aux préoccupations des enseignant·es qui entrent dans le métier.
Channels of energy estimates control the energy of an initial data from that which it radiates outside a light cone. For the linearised energy critical wave equation, they have been obtained in the radial case in odd dimensions, first in three dimensions in [8], then for the general case in [10]. We consider even dimensions, for which such estimates are known to fail [2]. We propose a weaker version of these estimates, around a single ground state as well as around a multisoliton. This allows us in [1] to prove the soliton resolution conjecture in six dimensions.
This paper extends the subjective expected utility model of decision making under uncertainty to include incomplete beliefs and tastes in Savage’s framework. The main result is an axiomatization of the subjective multi-prior expected multi-utility representations of preference relations under uncertainty. In contrast with the existing works, the setup allows for flexibility regarding the outcome space. The approach of this paper also provides theoretical foundations for subjective Knightian uncertainty and the expected multi-utility model with complete beliefs.
This article deals with an introduction of probability at school through sensory modalities. This approach is based on sampling fluctuation with empirical observations of frequencies. Initially, we analyze how students from high school work on such a task by using dice, pencil, and paper. We then identify the use of schemas and data visualization by students. Considering schemas as a primitive form of algorithmic approaches, we observe interaction with mathematics. In class, students know how to use notion of frequency, but linking frequencies with probability appears to be a difficult issue. This absence of a statistical link leads us to identify entry points for probability teaching. We then decide to analyze primary school students when they have to explore a random experiment by using statistics.
We consider a regulator willing to drive individual choices towards increasing social welfare by providing incentives to a large population of individuals. For that purpose, we formalize and solve the problem of finding an optimal personalized-incentive policy: optimal in the sense that it maximizes social welfare under an incentive budget constraint, personalized in the sense that the incentives proposed depend on the alternatives available to each individual, as well as her preferences. We propose a polynomial time approximation algorithm that computes a policy within few seconds and we analytically prove that it is boundedly close to the optimum. We then extend the problem to efficiently calculate the Maximum Social Welfare Curve, which gives the maximum social welfare achievable for a range of incentive budgets (not just one value). This curve is a valuable practical tool for the regulator to determine the right incentive budget to invest. Finally, we simulate a large-scale application to mode choice in a French department (about 200 thousands individuals) and illustrate the effectiveness of the proposed personalized-incentive policy in reducing CO 2 emissions.
The effects of measuring devices/sensors on improving the power quality (PQ) of electric networks are studied in this paper. In this context, improving the performance of an LCL-type grid connected to a three-phase three-wire shunt active filter (SAF) in the presence of voltage perturbations is studied. In order to ensure the high-quality performance of LCL-SAF in the presence of voltage perturbations, the robust continuous second-order sliding mode controller (2-SMC), including twisting and super-twisting controllers, and continuous higher-order sliding mode controller (C-HOSMC)-based approaches are employed. These controllers, whose outputs are processed by pulse-width modulation (PWM), allow minimization of the phase shift and prevent the generation of discontinuous chattering commands, which can severely damage the VSI components. Moreover, an integration of a generalized instantaneous power identification algorithm with an advanced phase locked loop (PLL) was proposed and experimentally tested to validate the effective performances of SAF under severe perturbations. Additionally, the studied approaches were tested via simulations taking into account a conventional nonlinear industrial load in a real textile factory environment, using measurements provided by power quality analyzers. Finally, the effects of the measuring devices, including the current and voltage sensors, on the accuracy and reliability of the SAF and, consequently, on the PQ of the electric power grid were studied via simulations and experimentally. The results of this study support the validity of the recently published patent.
We obtain quantitative stochastic homogenization results for Hamilton–Jacobi equations arising in front propagation problems which move in the normal direction with a possible unbounded velocity. More precisely, we establish error estimates and rates of convergence for homogenization and effective Hamiltonian. The main idea is to perturb our unbounded problem by a bounded one, and to establish stability results in this context. Then, we combine the estimates that we find with the ones from the bounded case.
The levelized cost of energy (LCOE) of offshore wind energy conversion systems using permanent magnet and fully superconducting generators is compared. Scaling laws are used to calculate the masses and the costs of all the components, to the exception of the generators that are designed and optimized by using the finite element method. The comparisons are done for various turbine’s nominal power, ranging from 2 to 23 MW. The results show that for each generator type, there exists an optimal nominal power that minimizes the LCOE. In addition, the use of a superconducting generator could significantly lower (about 7% at 20 MW) the LCOE compared with using a permanent magnet generator.
Indolo[3,2-b]carbazole-based hole transporting materials (HTM1–3) are developed for dopant-free perovskite solar cells (PSCs). The newly synthesized compounds are studied as alternatives of conventional hole-transporting materials which typically require additives, are characterized by low resistivity to penetration of water, complicated synthesis and purification. The influence of substituents of derivatives of indolo[3,2-b]carbazole on their physical properties, e.g. ionization potentials, hole mobilities, the temperatures of thermal transitions, is investigated using experimental and theoretical tools. Ionization potentials in the order HTM2<HTM1<HTM3 indicate good energy level alignment with the valence band maximum of the perovskite layer. Time-of-flight hole mobilities in the order HTM3 (5.26 ×10⁻³ cm²V⁻¹s⁻¹) > HTM1 (1.1 ×10⁻³ cm²V⁻¹s⁻¹) > HTM2 (0.55 ×10⁻³ cm²V⁻¹s⁻¹) without additives indicate good hole transporting properties, principally stemming from their small degrees of energetic disorder following the order HTM3 (73.4 meV) ~ HTM2 (73.2 meV) > HTM1 (59.5 meV). The influence of different combinations of these parameters results in the different power conversion efficiencies of the developed dopant-free PSCs: [19.45% for the device containing HTM2] ~ [18.75% for PCS containing HTM3] > [14.46% for the device containing HTM1]. The devices demonstrate considerably higher stability and practically comparable efficiency as additives-containing reference PSCs with conventional hole-transporting material spiro-OMeTAD.
Tradable credit schemes (or tolling in tokens) are a form of quantity control, which promise to be an appealing alternative to congestion pricing (or tolling in dollars) owing to considerations of revenue neutrality, equity, reduced infrastructure costs, and political acceptability. The comparative performance of the two instruments under uncertainty in demand and supply has only recently received attention in the transportation setting, despite being widely studied for emission markets. In this paper, we add to this literature by considering a tradable credit scheme in a departure time context wherein users are provided an initial endowment of tokens by the regulator and incur a token charge (determined prior to all departures) to travel in a specific time-period. Tokens can be bought and sold within a marketplace at a price determined endogenously by token demand and supply. Two key features of the market model are: (1) the time-of-day dynamics of price is explicitly modeled through a smooth market clearing mechanism in each period, and (2) the selling decisions of users, which determine the distribution of token supply in the market over the day are explicitly modeled. This enables us to study the impact of selling behavior on performance of the credit system. Travel demand is modeled using a logit mixture model and supply consists of static congestion. Extensive experiments under stochastic demand show that when the tolls (in dollars and tokens) are not day-to-day adaptive, tolling in tokens outperforms tolling in dollars when congestion effects are more severe (e.g., realistic BPR models and steep congestion functions, high demand levels and high day-to-day variability). Importantly, we find that this result is robust with respect to selling behavior in the market, although there can be welfare losses in the quantity control system when selling behavior in the market is excessively irrational. These findings underscore the importance of examining disaggregate market behavior when designing tradable credit schemes. Moreover, when the supply of tokens can be adapted from day to day, the credit system was found to be superior in all tested scenarios, provided the selling behavior of individuals is rational. Finally, even in the case when toll revenues in the price instrument are equally redistributed (often difficult in practice), tolling in tokens (when tokens are equally distributed) is marginally more equitable in scenarios where congestion effects are more severe. These findings make a case for tolling in tokens.
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1,269 members
Damien Masson
  • Department of Geography and History
Olivier Donni
  • THEMA Research Center
Andréa Pimenta
  • Departement of Biology
Gregory Chaume
  • Departement of Chemistry
Emmanuel Pauthe
  • Departement of Biology
33 boulevard du port, F-95000, Cergy-Pontoise, France
Head of institution
François Germinet