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Diagonals and algebraicity modulo : a sharper degree bound
No full textTo appear in the Annales scientifiques de l'École normale supérieure. A longer version of this work is available at https://arxiv.org/abs/2306.02640International audienceIn 1984, Deligne proved that for any prime number , the reduction modulo of the diagonal of a multivariate algebraic power series with integer coefficients is algebraic over the field of rational functions with coefficients in . Moreover, he conjectured that the algebraic degrees of these functions should grow at most polynomially in . In this article, we provide a new and elementary proof of Deligne's theorem, which yields the first general polynomial bound on with an explicit and reasonable degree.En 1984, Deligne a montr\'e que pour tout nombre premier~, la r\'eduction modulo de la diagonale d'une s\'erie formelle alg\'ebrique de plusieurs variables \`a coefficients entiers est alg\'ebrique sur le corps des fonctions rationnelles \`a coefficients dans . De plus, il a sugg\'er\'e que les degr\'es d'alg\'ebricit\'e de ces fonctions devaient cro\^itre au plus polynomialement en fonction de . Dans cet article, nous pr\'esentons une nouvelle preuve du th\'eor\`eme de Deligne qui est \'el\'ementaire et permet d'\'etablir la premi\`ere borne g\'en\'erale polynomiale avec un degr\'e raisonnable
The contributions of various calcifying plankton to the South Atlantic calcium carbonate stock
No full textInternational audiencePelagic calcifying plankton play an important role in the marine carbon cycle. However, field studies quantifying the contributions of multiple calcifying plankton groups to particulate inorganic carbon (PIC) stocks and export into the ocean interior are scarce. Most studies target one specific plankton group and adjust their sampling strategy accordingly, hampering comparisons. Furthermore, the literature is strongly biased towards foraminifera and coccolithophores, so aragonite contributions (e.g., gastropods) remain virtually unconstrained. A holistic view is required for future projections of marine carbon cycle changes. Here, we present the contributions of three main calcifying plankton groups – coccolithophores, foraminifera and planktonic gastropods (comprising heteropods and pteropods) – to PIC stocks and fluxes throughout the water column during a sampling campaign in the South Atlantic Ocean. Coccolithophore calcite dominated the depth-integrated PIC standing stock (∼ 80 %), followed by aragonite from planktonic gastropods (∼ 17 %) and calcite from foraminifera (∼ 3 %). The estimated production and export of the calcifying plankton largely depend on assumed turnover times and sinking speeds, which both have large uncertainties. Coccolithophores contributed 92 %–99 % of the produced PIC, depending on planktonic gastropod turnover time, and from 52 % to 99 % of the exported PIC, depending on their mode of sinking. Both the standing stock and export of planktonic gastropods was significantly larger than that of foraminifera. Similarity between our results and those from different ocean basins suggests that these patterns are global in nature, implying that not only coccolithophores but also gastropods may be a more important contributor to the oceans PIC inventory than foraminifera, challenging a longstanding paradigm
Moving Vehicle Obstacle Pre-Detection with Telemetry Sensing Signal Prediction by Negative Group Delay Method
No full textInternational audienceThis article develops a vehicle-obstacle anti-collision system based on negative group delay (NGD) function. As theory, the anti-collision pre-detector design and implementation are described by exploiting the NGD time-advance ta to anticipate obstacle positioning relatively to a moving object and required safety distance. The NGD pre-detector design equation and methodology are formulated as function of ta. The NGD anti-collision system is validated with realistic demonstration constituted by a mini-vehicle equipped with a telemetry sensor moving along an 85-cm track towards an obstacle. NGD circuit implemented on microcontroller and Arduino UNO platform in good agreement with theory confirms obstacle pre-detection at 30-cm and 50-cm safety distances with ta={-0.1, -0.2, -0.3} seconds. The NGD anti-collision system has notable benefits as ta flexibility, low-power consumption, learning phase unnecessity and embedding simplicity. As future work, the NGD method will be integrated in anti-accident industrial systems by providing optimal safety distance allowing to react quickly and effectively to obstacle warnings by maximizing response time
Reassessment of DLVO theory at sub-nanometric scale: Application to Na-smectite
No full textInternational audienceThe nearly century-old DLVO (Derjaguin, Landau, Verview, and Overbeek) theory decomposes the interaction forces between solid surfaces separated by a fluid into attractive and repulsive components. In its standard version, it applies beyond solid surface separation distances of around 1 nm. For sub-nanometer distances, additional so-called hydration forces were widely invoked to explain the discrepancy between theory and measurements. Alternatively to this semi-empirical approach, either the validity of the theoretical expression for the electrostatic component of the disjoining pressure, or the adaptation of the Boltzmann distribution usually considered in the application of DLVO theory are questioned. Here, it is suggested that the introduction of a hydration component whose parameters cannot be predetermined but are a matter of calibration is unnecessary. Hydration effects, introduced in the calculation of ionic concentrations, directly influence ions distributions and therefore the repulsion of adjacent electric double layers. This effect is thus part of the electrostatic component of the disjoining pressure. An analytical expression extending DLVO theory to small surface separation distances together with a simplified electrical model, both proposed here, enable reproducing data for Na-smectite. Results are also in good agreement with thermodynamic and molecular dynamics calculations. It is confirmed that the discrepancy between theory and observations is mainly due to the underestimation of counterions concentration at the mid-plane identified using an inappropriate electrical model. For ions with radii around 0.1 nm, commonly found in natural media, the standard DLVO expression can still be used, but with mid-plane concentrations calculated using an appropriate electrical mode
Former à l’ère de l’IA : lucidité critique pour éviter les illusions
No full textInternational audienc
À l’école de l’évaluation-exclusion
No full textLe Café pédagogique https://www.cafepedagogique.net/2026/01/12/a-lecole-de-levaluation-exclusion/?utm_campaign=Lexpresso_12-01-2026&utm_medium=email&utm_source=ExpressoL’Ecole transformerait-elle l’évaluation en terrible instrument d’exclusion ? Entrelacs des inégalités sociales et scolaires, baccalauréat déclassé au profit du nébuleux système de sélection Parcoursup, « durcissement » des conditions d’obtention du bac et des modalités du contrôle continu, poids anxiogène d’une évaluation permanente, « harmonisation » qui fait de l’enseignant·e une machine à noter des productions standardisées… Cette tribune se veut un appel : à « (ré)inventer une évaluation au service de tous », à préférer la « démo-cratie éducative » au « management de l’exclusion »
Rad51 determines pathway usage in post-replication repair
No full textInternational audienceStalled replication forks are processed in post-replication repair by homologous recombination, fork regression, and translesion DNA synthesis. However, the regulation of pathway usage is not fully understood. Rad51 protein maintains genomic stability through its roles in recombination and in protecting stalled replication forks. We report isolation of mutations in Saccharomyces cerevisiae Rad51 that shift post-replication repair from recombination to alternate pathways including mutagenic translesion synthesis. Rad51-E135D and Rad51-K305N show near normal in vitro recombination despite changes in their DNA binding profiles, in particular to dsDNA. The mutants lead to a defect in Rad51 recruitment to stalled forks in vivo as well as a defect in the protection of dsDNA from degradation by Dna2-Sgs1 and Exo1 in vitro. Together, the evidence suggests that Rad51 binding to duplex DNA is critical to control pathway usage at stalled replication forks
Intra-night optical polarization monitoring of blazars
No full textInternational audienceBlazars are known for their extreme variability across the electromagnetic spectrum. Variability at very short timescales can push the boundaries between competing models offering us much needed discriminating power. This is particularly true for polarization variability that allows us to probe particle acceleration and high-energy emission models in blazars. Here we present results from the first pilot study of intra-night optical polarization monitoring conducted using RoboPol at the Skinakas Observatory and supplemented by observations from the Calar Alto, Perkins, and Sierra Nevada observatories. Our results show that while variability patterns can widely vary between sources, variability on timescales as short as minutes is prevalent in blazar jets. The amplitude of variations are typically small, a few percent for the polarization degree and less than 20 degrees for the polarization angle, pointing to a significant contribution to the optical emission from a turbulent magnetic field component, while the overall stability of the polarization angle over time points to a preferred magnetic field orientation
CFD simulation of new FPC demonstrating different configurations of thermal performance improvement techniques followed by economic analysis
No full textInternational audienceWe have first studied the effect of the fins attached to the air duct with a selective coating on the absorber on the performance improvement. The results obtained in this part allowed us to conclude that the application of a selective absorber combined with three rows of fins allows to reach an efficiency of 80%, while maintaining the low weight of the collector, operating in forced convection mode. Secondly, simulations were carried out to study the effect of the transparent insulating material slats attached to the transparent cover on the reduction of the convective losses and, consequently, on the optimization of the collector efficiency. The results of this study have allowed to propose a new design of the flat plate air collector operating in forced convection mode, which can reach an efficiency of 81%, associated with the collector inclined at 45°, containing six parallel PMMA lamellas and inclined at –45°. In order to remedy the limitations to the widespread use of air collectors, our study is based primarily on the aim of establishing the most effective design for maximum thermal efficiency and loss control. It is in this context that this work will focus essentially on intensifying transfers between the absorber and the air while decreasing heat losses between the absorber and the transparent cover. The numerical modeling chosen, based on the GAMBIT and FLUENT software, will be elaborated. To improve the performance of the solar collector, we studied the effect of the selective coating on the absorber, and then the thickness of the air space. We are interested in minimizing heat loss toward the front of the collector. In this chapter, we propose an innovative solution to minimize the collector's convective losses and consequently improve its efficiency in forced convection mode. The innovative front-facing flat-plate solar collectors where parallel slats of transparent insulating material are placed between the transparent cover and the absorber plate. The new collector not only has a higher efficiency than commercially available collectors, but is also a cost-effective solution because its cost is considerably low and its weight is considerably reduced