HAL-Polytechnique
Not a member yet
51406 research outputs found
Sort by
MAGICARPP: ModulAr GeneralIzed Check of Age Requirement while Preserving Privacy
No full textInternational audienceAge verification is increasingly mandated by law, especially in online services. In this paper, we present a proof-of-concept protocol for privacy-preserving age verification. Our approach allows users to prove compliance with age requirements without revealing their identity to the service provider. Furthermore, the website remains unaware of which authority issued the age certification, and the certification authority does not learn which website is making the request or the specific age being verified. This design ensures strong privacy guarantees for all parties involved. Its properties served as a baseline for part of the reference document published by the French regulator ARCOM
Heatwave Intensification in the Cameroonian Sahelian Zones: Biodiversity Survival Index, Required Vegetation Cover Standards in Human-Settled Environments, and Community Resilience Capacities
No full textInternational audienceRecent extreme heatwaves in the Sahel, with maximum temperatures exceeding 45 °C, are exposing biodiversity, human populations, and agricultural systems to severe thermal stress. This article synthesizes recent climatic evidence demonstrating the intensification of heatwaves in the region. It proposes a methodological framework combining in situ observations, remote sensing, and modeling to estimate a Biodiversity Survival Index (BSI) under 35-45 °C temperature scenarios. The results indicate that operational vegetation cover standards for human-inhabited environments (defined through minimum thresholds and desirable target levels) can significantly reduce urban and periurban heat island effects and enhance overall resilience. Practical and policy-oriented recommendations are provided to strengthen community resilience, including agroforestry practices, water resource management, climate-sensitive urban planning, and early warning systems. The evidence further suggests that the intensity and frequency of recent heat extremes would not have occurred without anthropogenic climate forcing, and that substantial gains in local heat mitigation can be achieved through vegetationbased strategies and strengthened local governance.</div
Three-dimensional imaging of hadrons with hard exclusive reactions: advances in experiment, theory, phenomenology, and lattice QCD
No full textInternational audience113 pages, 68 figures, white paper of the workshops "3D Structure of the Nucleon via Generalized Parton Distributions'', Incheon, Republic of Korea, June 25-28, 2024, and "Towards improved hadron tomography with hard exclusive reactions'', ECT*-Trento, Italy, August 5-9, 202
Optimisation of space-time periodic eigenvalues
No full textInternational audienceThe goal of this paper is to provide a qualitative analysis of the optimisation of space-time periodic principal eigenvalues. Namely, considering a fixed time horizon T and the d-dimensional torus T d , let, for any m ∈ L ∞ ((0, T ) × T d ), λ(m) be the principal eigenvalue of the operator ∂t -∆ -m endowed with (time-space) periodic boundary conditions. The main question we set out to answer is the following: how to choose m so as to minimise λ(m)? This question stems from population dynamics. We prove that in several cases it is always beneficial to rearrange m with respect to time in a symmetric way, which is the first comparison result for the rearrangement in time of parabolic equations. Furthermore, we investigate the validity (or lack thereof) of Talenti inequalities for the rearrangement in time of parabolic equations. The numerical simulations which illustrate our results were obtained by developing a framework within which it is possible to optimise criteria with respect to functions having a prescribed rearrangement (or distribution function).</div
Convergence locale de mariages et d'autres problèmes d'optimisations sur des graphes à travers des méthodes de propagation de rumeurs
No full textThis thesis consists of several coherent works on the geometry of maximal size and maximal weight matchings on random graphs converging locally to Bienaymé-Galton-Watson trees, along with an independent work on arithmetic models of percolation.The first résult is about the local geometry of maximal weight matchings on random graphs that converge locally to a unimodular Bienaymé-Galton-Watson tree decorated with independent and identically distributed weights. We show that, in this case, the distribution of the graph and any maximal weight matching converge locally to the tree and an optimal matching which law is explcit. This optimal limit law is defined through the stationary distribution of a belief propagation algorithm.The second result of this thesis follows the first and studies the maximal size matchings in the same setting. We break the ties between the maximal matchings by choosing the one with the highest weight. We then show analagous results as the case of maximal weight matchings with no size constraint.The third result of this thesis, in the continuation of the precedent two, seeks to understand some objets which appear in a renormalisation of the messages that was necessary for maximal size matchings. We show a link between these messages and the Karp-Sipser core, and show simultaneously a convergence criterion for the local geometry of the Karp-Sipser core of random graphs that converge locally.Finally, the last independent result of this thesis studies an arithmetic percolation model. This model can be described by the limiting distribution of the visible point of the hypercube lattice of dimension d from a uniformly chosen point. We show that the set of visible points has a unique infinite connected component, and the set of invisible points have no infinite connected components. We also extend these results to other lattices than the hypercubic one.Cette thèse consiste en plusieurs travaux cohérents sur la géométrie de mariage à poids et tailles maximaux dans des graphes ayant pour limite locale des arbres de Bienaymé-Galton-Watson ainsi qu'un aparté sur la percolation sur des modèles arithmétiques.Le premier résultat de la thèse porte sur la géométrie locale des mariages à poids maximaux sur des arbres convergeant localement vers un arbre de Bienaymé-Galton-Watson unimodulaire décoré de poids indépendants et identiquement distribués. Nous démontrons que dans ce cas, le graphe muni d'un mariage à poids maximal converge localement vers l'arbre muni d'un mariage optimal dont la loi est explicite. Cette loi limite optimale est construite à partir de la loi stationnaire d'un algorithme dit de propagation de rumeurs sur l'arbre limite.Le deuxième résultat de cette thèse consiste à étudier les mariages de taille maximale dans le même contexte. Pour départager les nombreux mariages de tailles maximales, on introduit un modèle de mariages à poids maximale sous contrainte de taille maximale. Nous démontrons à travers une procédure de renormalisation des messages survenant dans le paragraphe précédent qu'il existe une unique loi de mariage à poids maximal sous contrainte de taille maximale dans l'arbre de Bienaymé-Galton-Watson unimodulaire. Ce mariage est explicitement construit à partir d'une famille de messages qui sont les renormalisations du cas sans contrainte de taille. On montre aussi que si un graphe pondéré converge localement vers cet arbre, alors il existe une suite de mariages sur ces graphes qui converge vers le mariage limite en question.Le troisième résultat de la thèse, dans la continuation des deux précédents, cherche à comprendre Cette thèse consiste en plusieurs travaux cohérents sur la géométrie de mariage à poids et tailles maximaux dans des graphes aléatoires ayant pour limite locale des arbres de Bienaymé-Galton-Watson ainsi qu'un aparté sur la percolation sur des modèles arithmétiques.Le premier résultat de la thèse porte sur la géométrie locale des mariages à poids maximaux sur des arbres convergeant localement vers un arbre de Bienaymé-Galton-Watson unimodulaire décoré de poids indépendants et identiquement distribués. Nous démontrons que dans ce cas, le graphe muni d'un mariage à poids maximal converge localement vers l'arbre muni d'un mariage optimal dont la loi est explicite. Cette loi limite optimale est construite à partir de la loi stationnaire d'un algorithme dit de propagation de rumeurs sur l'arbre limite.Le deuxième résultat de la thèse suit le premier et étudie les mariages de taille maximale à la place de mariages de poids maximal. On départage alors l'ensemble des mariages de taille maximale en choisissant celui qui maximise le poids total. Nous démontrons alors des résultats similaires que dans le cas des mariages à poids maximal.Le troisième résultat de la thèse, dans la continuation des deux précédents, cherche à comprendre certains objets qui sont apparus lors d'une renormalisation de messages qui a été nécessaire pour les mariages à taille maximale. Nous démontrons alors un lien entre ces messages et le core dit de Karp et Sipser, et prouvons en même temps un critère de convergence pour la géométrie locale du core de Karp-Sipser pour des graphes convergeant localement.Enfin, le dernier résultat indépendant de la thèse concerne l'étude d'un modèle de percolation arithmétique. Ce modèle peut être décrit comme la loi limite des points visibles du réseau hypercubique en dimension d depuis un point uniforme du réseau. Nous démontrons que l'ensemble des points visibles possède une unique composante connexe infinie, et que l'ensemble des points invisibles n'est constitué que de composantes connexes finies. Nous étendons enfin ces résultats à quelques réseaux autres que le réseau hypercubique
Machine Learning Model for Predicting Visual Acuity Improvement After Intrastromal Corneal Ring Surgery in Patients With Keratoconus
No full textInternational audienceBackground: Keratoconus is a progressive, degenerative corneal disease that can lead to significant visual impairment. The intrastromal ring segment implantation procedure is effective in reshaping the cornea and improving vision. However, vision does not improve postoperatively in all operated eyes, and the results vary widely among patients, making it challenging to predict postoperative visual gain. Purpose: This study investigated the potential of machine learning in predicting postoperative visual acuity in keratoconus patients undergoing intrastromal ring segment implantation with the aim of enhancing surgical decision-making. Methods: This retrospective study analyzed 120 eyes of 102 patients with keratoconus who underwent ring segment implantation (1 symmetric or asymmetric segment, 150-300 mm thick, 150 degrees, or 160 degrees-arc). Preoperative and postoperative refraction, corneal topography, and tomographic data were collected. Various models were trained to predict postoperative visual acuity improvements. Results: The models demonstrated excellent performance, with XGBoost achieving perfect results in predicting whether vision will improve after surgery (R 2 = 1.0, Youden Index = 1.0; all test observations being correctly classified). The CatBoost model achieved an R 2 of 0.59 [0.7-line mean absolute error (MAE)] for predicting postoperative visual acuity, an R 2 of 0.76 (MAE, 1.08 D) for predicting keratometry, and an R 2 of 0.54 (MAE, 0.29) for predicting corneal asphericity. Key features for accurate predictions included preoperative keratometry values (K1, K2, Kmax), corneal asphericity, and visual acuity, whereas segment characteristics featured low importance. Conclusions: This study shows the strong potential of machine learning for selecting candidates for surgery and predicting postoperative visual improvements after ring segment implantation in keratoconus eyes
WAGE MARKDOWNS AND THE SORTING OF WORKERS TO FIRMS WHEN SKILLS ARE BUNDLED
No full textWe study a competitive labor market where heterogeneous workers supply multidimensional skills to heterogeneous firms. Firms produce output by aggregating employees' skill bundles, using them as inputs in a concave production function. A single friction—the bundling of workers' skills—generates an equilibrium with workers-to-firms sorting based on comparative advantage, where each firm has a unique optimal size, and where the wage schedule is convex. Skill prices vary across firms despite competitive wage-setting, with a markdown for workers endowed with balanced skills. We illustrate the empirical relevance of key assumptions and predictions using matched worker-firm data on workers' skill profiles
Debiasing piecewise deterministic Markov process samplers using couplings
No full textInternational audienceAbstract Monte Carlo methods—such as Markov chain Monte Carlo (MCMC) and piecewise deterministic Markov process (PDMP) samplers—provide asymptotically exact estimators of expectations under a target distribution. There is growing interest in alternatives to this asymptotic regime, in particular in constructing estimators that are exact in the limit of an infinite number of computing processors, rather than in the limit of an infinite number of Markov iterations. In particular, coupled MCMC estimators remove the non‐asymptotic bias, resulting in MCMC estimators that can be embarrassingly parallelized. In this work, we extend these estimators to the continuous‐time context and derive couplings for the bouncy, the boomerang, and the coordinate samplers. Some preliminary empirical results are included that demonstrate the reasonable scaling of our method with the dimension of the target
A Sheaf-Theoretic Characterization of Tasks in Distributed Systems
No full textWe introduce a sheaf-theoretic characterization of task solvability in general distributed computing models, unifying distinct approaches to message-passing models. Extending our brief announcement at SIROCCO'25, we establish cellular sheaves as a natural mathematical framework for analyzing the global consistency requirements of local computations. Our main contribution is a task sheaf construction that explicitly relates both the distributed system and task, in which terminating solutions are precisely its global sections. We prove that a task can only be solved by a system when such sections exist in a task sheaf obtained from an execution cut, the frontier in which processes have enough information to decide. Our characterization is model-independent, working under varying synchronicity, failures and message adversaries, as long as the model produces runs composed of global states of the system. Furthermore, we show that the cohomology of the task sheaf provides a linear algebraic description of the decision space of the processes, and encodes the obstructions to find solutions. This opens way to a computational approach to protocol synthesis, which we illustrated by deriving a protocol for approximate agreement. This work bridges distributed computing and sheaf theory, providing both theoretical foundations for analyzing task solvability and tools for protocol design leveraging computational topology