Institut de recherche mathématique avancée

Résumé : Une des contributions principales de Tsimerman, d'après sa laudatio est la conjecture d'André-Oort. Le but de cet exposé est d'en donner l'énoncé ainsi qu'une idée de l'outil nouveau dans la preuve : l'o-minimalité, une théorie venant de la logique.

Résumé : We introduce and study harmonic bands through the geometry of holomorphic curves and their associated Frenet–Serret frames. Starting from an ideal band, we construct a family of bands—which we call generalized Landau levels—whose associated maps to projective space are harmonic, and express their quantum metric and Berry curvature in terms of consecutive osculating curves. We then show explicitly how ordinary Landau levels fit into this construction. The lowest Landau level defines a holomorphic curve in projective Hilbert space, and its Frenet–Serret frame represents all higher Landau levels. Thus ordinary Landau levels are generalized Landau levels and provide the canonical examples of harmonic bands.

Résumé : Une nouvelle approche pour l'étude statistique d'objets arithmétiques s'est ouverte grâce notamment aux travaux de R. Darda et T. Yasuda qui traduit ces questions en termes de l'étude des points rationnels de hauteur bornée sur des champs. Cela permet en particulier d'interpréter les contre-exemples à la conjecture de Malle en termes de phénomènes d'accumulation bien connus dans le contexte du programme de Manin. Pour aller plus loin, il faut être en mesure de paramétrer ces points rationnels des champs pour les compter, ce qui amène à la question de généraliser les concepts de torseurs universels ou d'anneaux de Cox à un cadre champêtre. Le travail de N. Bongiorno fournit des pistes en ce sens. L'exposé s'efforcera de présenter ces développements récents sur des exemples concrets montrant les progrès réalisés et les difficultés rencontrées.

Résumé : This work focuses on the mathematical modeling of cellular tissue dynamics. We first propose an individual-based model that provides the dynamics of the positions, velocities, and polarities of the cells, idealized as hard spheres. Cells interact with each other through contact forces, smooth attraction, and polarity alignment. The present work is an extension of the model proposed in [3] and validated by experiments on cellular rings. We will study the well-posedness of a regularized version. We are particularly interested in the congested regime and the impact of apoptosis on the jammed state. Indeed, cell apoptosis corresponds to programmed cell death: when cells leave the tissue, they induce local contractions but also enable cellular rearrangements. To this end, we add to the previous model a microscopic description of apoptotic and proliferation events. Numerical simulations are performed to show the impact of apoptosis on collective cell dynamics. Next, we derive a macroscopic description, following the methodology proposed in [1] and [2]. We start from a mean-field dynamics of the kinetic distribution function in phase space (position, polarity, radius), where contact forces have been replaced with repulsion forces. We then introduce a specific time and space rescaling and identify the equilibrium distribution functions, which are parameterized by two macroscopic quantities: the density and the mean polarity. Based on the Generalized Collision Invariant (GCI) method [2], we are then able to identify their dynamics: the resulting description can be seen as a modified Self-Organized Hydrodynamics (SOH) model. We finally discuss the obtained model and highlight the effect of the apoptotic events on the dynamics. This is a joint work with Laurent Navoret (Université de Strasbourg) and Marcela Szopos (Université Paris Cité). It has also been carried out in collaboration with Romain Levayer (Institut Pasteur) and Daniel Riveline (IGBMC, Université de Strasbourg) in the context of the ANR project MAPEFLU. References [1] Degond, P., Dimarco, G., Mac, T. B. N., and Wang, N. Macroscopic models of collective motion with repulsion. Communications in Mathematical Sciences, 13(6), 1615–1638 (2015). [2] Degond, P., and Motsch, S. Continuum limit of self-driven particles with orientation interaction. Mathematical Models and Methods in Applied Sciences, 18(supp01), 1193–1215 (2008). [3] Vecchio, S. L., Pertz, O., Szopos, M., Navoret, L., and Riveline, D. Spontaneous rotations in epithelia as an interplay between cell polarity and boundaries. Nature Physics (2024).

Résumé : A central theme in Teichmüller theory is understanding how geometric structures on a surface S degenerate as they approach the "edge" of their moduli space. We will talk about the geometric operation of grafting, bridging between the hyperbolic and the flat geometries, used by Thurston to study the moduli space of complex projective structures. Our main goal will be to show that half-translation (flat) surfaces emerge as Gromov-Hausdorff limits, up to rescaling, of surfaces obtained by grafting hyperbolic surfaces. This also allows us to realise two different compactifications of the moduli space of grafted surfaces by embedding it in the space of projective geodesic currents, and give a geometric description of the limit points.