Designed and built with care, filled with creative elements

Top

Vlad Vicol – On self-similar blowup in 3D incompressible fluids

Jussieu -- salle 15-16-309 4 Place Jussieu, Paris, France

We discuss constraints on hypothetical finite-time (backwards) self-similar blow-up solutions for solutions of incompressible Euler and incompressible Navier-Stokes on R^3. In various situations, we show that the very existence of a smooth similarity profile imposes strong restrictions on the similarity exponent. We also discuss new Liouville-type theorems for the viscous problem.

Rémi Coulon – Les géométries de Thurston : un peu d’illustration au service de la recherche

ENS — amphi Galois 45 rue d'Ulm, Paris, France

Le théorème de géométrisation est un achèvement mathématique majeur du début du XXIème siècle. Il stipule, grosso modo, que toute variété topologique de dimension trois, peut être comprise au moyen de huit géométries modèles, appelées géométries de Thurston. Certaines d'entres elles sont familières des mathématiciennes et mathématiciens : la géométrie sphérique, euclidienne, ou hyperbolique. D'autres sont un plus étranges comme Nil ou Sol. Dans cet exposé on présentera un projet de visualisation (dans le prolongement des travaux de Pierre Berger) : que verrait-on si nous vivions dans l'une de ces […]

Quelques modèles de dynamique des populations: limites d’échelles

Salle W

Nous commencerons par introduire la notion de processus de Markov à sauts qui généralise les chaînes de Markov aux temps continus. Lorsque ces processus sont à valeurs vectorielles, ils peuvent être réécrits sous forme d'équation différentielle stochastique. Ensuite, le modèle logistique de Verlhust sera dérivé en tant que limite en grande population de ces modèles stochastiques. Dans le cas de populations évoluant sur un maillage discret, la méthode permet de dériver des EDP de réaction-diffusion en faisant tendre la taille de la maille vers zéro ainsi que le nombre d'individus […]

Automath! Léo Dreyfus-Schmidt: AI tools for the working mathematician

campus de Jussieu (amphi 15)

Recent AI systems are increasingly entering the working environment of mathematicians, offering new ways to explore ideas, search the literature, and formalize arguments. In this talk, we will present a selection of such tools, focusing on their practical use in everyday mathematical work.

ENS-Data Science colloquium – Rebecca Willett : How do simple rotations affect the implicit bias of Adam?

Amphi Jean Jaurès 29 rue d'Ulm, PARIS, France

Adaptive gradient methods such as Adam and Adagrad are widely used in = machine learning, yet their effect on the generalization of learned = models =E2=80=93 relative to methods like gradient descent =E2=80=93 = remains poorly understood. Prior work on binary classification suggests = that Adam exhibits a =E2=80=9Crichness bias,=E2=80=9D which can help it = learn nonlinear decision boundaries closer to the Bayes-optimal decision = boundary relative to gradient descent. However, the coordinate-wise = preconditioning scheme employed by Adam renders the overall method = sensitive to orthogonal transformations of feature […]

Learning Monge Maps with Constrained Drifting Models

Salle W

The estimation of optimal transport maps (a.k.a. Monge maps) is a central problem in optimal transport literature. Recent observations reveal that the flow map of the Wasserstein gradient flow of the relative entropy closely approximates—though does not exactly equal—the Monge map between a given source distribution and a Gaussian target. In this work, we demonstrate how the evolution equation governing this flow map can be corrected to form a constrained gradient flow that provably converges to the true Monge map.When the maps are parametrized as gradients of convex models (e.g. […]

Hélène Mathis – Modélisation d’écoulements diphasiques compressibles

Salle W - ENS PSL 45 rue d'Ulm, Paris, France

Première partie : La modélisation et la simulation d’écoulements diphasiques constituent un sujet de recherche important, notamment pour leurs applications en sûreté nucléaire.Dans certains scenarii d’accidents interviennent des écoulements très hétérogènes, constitués d’eau liquide et de bulles d’air et/ou de vapeur.Afin de modéliser de tels écoulements, on privilégie des modèles moyennés, donnant une description macroscopique des écoulements, la description à l’échelle des interfaces eau-gaz étant hors portée.Cependant connaître les propriétés de l’interface, en particulier l’évolution de l’aire interfaciale et de la tension de surface, demeure important.Dans cette première partie, nous […]

Constant Bourdrez et Hanna Benarroch

Salle W

Constant Bourdrez : "Learning To Sample From Diffusion Models Via Inverse Reinforcement Learning" Hanna Benarroch : "Certified Per-instance Unlearning using Individual Sensitivity Bounds"

Convexity in Whitney Problems

Salle W (ENS)

Suppose E is a compact subset of R^n, and we are given a function f, mapping E to the real numbers. How can we tell if the function lies on a smooth convex function? Can we construct an almost optimal, smooth, convex interpolant of the function? These are examples of Whitney-type extension and trace problems; while theoretical, they are driven by practical questions of interpolation of data, where convexity is a natural constraint. I will begin with an answer to these questions by presenting work of mine proving there is a […]

Information-estimation geometry: a scale space view of prior probability models

Salle W

Solving most image processing tasks, such as denoising, deblurring, inpainting, etc, require explicitly or implicitly a prior probability model of natural images. Classically, both learning (by maximizing likelihood) and using (via Bayes' rule) such prior models are intractable due to the curse of dimensionality. Diffusion models take a different approach, where the prior density is replaced by a family of score vector fields across noise levels. They have led to impressive success in generative modeling, but the learned density is not explicit nor is it readily usable as a prior […]

Deep Learning as Neural Low-Degree Filtering: A Spectral Theory of Hierarchical Feature Learning

Salle W

Understanding how deep neural networks learn useful internal representations from data remains a central open problem in the theory of deep learning. We introduce Neural Low-Degree Filtering (Neural LoFi), a stylized limit of gradient-based training in which hierarchical feature learning becomes an explicit iterative spectral procedure. In this limit, the dynamics at each layer decouple: given the current representation, the next layer selects directions with maximal accessible low-degree correlation to the label. This yields a tractable surrogate mechanism for deep learning, together with a natural kernel-space interpretation. Neural LoFi provides […]