Séminaires

Réunion d’équipe des permanents

Statistical methodology for spatial point patterns has traditionally focused on Euclidean data and planar surfaces. However, with recent advances in 3D biological imaging technologies targeting protein molecules on a cell’s plasma membrane, spatial point patterns are now being observed on complex shapes and manifolds whose geometry must be respected for principled inference. Consequently, there is now a demand for tools that can analyse these data for important scientific studies in cellular and micro-biology. Motivated by studying the spatial distribution of LPS proteins on the surface of E-Coli, we develop the fundamental functional summary statistics for the analysis of point patterns to general convex bounded shapes and demonstrate how they can be used to test for complete spatial randomness. We then develop their multi-type extensions, together with a test for independence of the component marginal processes. To support these methods, we introduce a plug-in estimator for the intensity of a spatial point process on a manifold. We conclude with a discussion on how these methods can readily be extended to a class of non-convex shapes. This talk will aim to provide an accessible overview of the references below.

References:

Ward, E.A.K. Cohen, N. M. Adams. Testing for complete spatial randomness on 3-dimensional bounded convex shapes. Spatial Statistics, Vol. 41, 2021.

Ward, H. S. Battey and E. A. K. Cohen. Nonparametric estimation of the intensity function of a spatial point process on a Riemannian manifold. Biometrika, Vol. 110, 2023.

Kumar, P. Inns, S. Ward, V. Lagage, J. Wang, R. Kaminska, S. Uphoff, E. A. K. Cohen, G. Mamou and C. Kleanthous. Immobile lipopolysaccharides and outer membrane proteins differentially segregate in growing Escherichia coli. Proceedings of the National Academy of Sciences, 122 (10), 2025

Ward, E. A. K. Cohen and N. M. Adams. Functional summary statistics and testing for independence in marked point patterns on the surface of three-dimensional convex shapes. Spatial Statistics, Vol. 67, 2025

Deuxième de deux séances.

Un modèle classique de polytope aléatoire proposé par Renyi et Sulanke dans les années 60 consiste à fixer un corps convexe K de R^d, à y choisir n points aléatoires indépendants et uniformément distribués, et à en prendre l’enveloppe convexe K(n). L’asymptotique, pour d fixé et n tendant vers l’infini, du volume de K(n) a été reliée à l’analyse des corps flottants de K par Bárány et Larman dans les années 80. Certaines idées derrière ce lien ont été généralisées dans le « théorème de l’epsilon-net » prouvé par Haussler et Welzl au début des années 90.

Je donnerai une introduction à ces notions, avec l’idée d’aborder lors d’une éventuelle seconde séance, un travail commun avec Imre Bárány, Matthieu Fradelizi, Alfredo Hubard et Günter Rote sur la généralisation du lien polytope aléatoire/corps flottant au cas où la mesure uniforme sur K est remplacée par une mesure plus générale (https://doi.org/10.5802/ahl.44).

What happens if we want to study the convergence of discontinuous real-valued stochastic processes, which is often the case for modelling purposes? For example, think of tracking the evolution of the population size of living species, where deaths are instantaneous negative jumps… In 1956, Skorokhod proposed a topology on the space of discontinuous functions, which is predominant today. The aim of this talk is to explain the simple and intuitive ideas underlying the construction of Skorokhod to facilitate its understanding, without going in the depth of technical proofs. If we have time, we will introduce measure-valued processes, with biological motivations, and explain how the Skorokhod construction can be generalized to more complex spaces such as these measure spaces.

Even if the present talk is self-contained, it can be seen as an introduction to the GdT of May, 22. I will also present my work about measure-valued processes during the GdT SIMBA of April, 24 (14h, Salle de Conférences). You are warmly welcome to attend one of these to discover some of my PhD research!

First, we introduce c`adl`ag measure-valued processes, with biological motivations. We focus on the
construction with Poisson point measures and the useful martingale properties it entails. Then, we
present a general convergence result for these measure-valued processes. We insist on the topological
difficulties encountered, related to Skorokhod spaces. Thus, even if it is self-contained, this talk can
be seen as a natural continuation of the GdT of May, 15.
Note that I also present this work during the GdT SIMBA on April, 24 (14h, Salle de Conf´erences),
with a focus on the new results we obtain compared to the existing literature. This is joint work with
Nicolas Champagnat and Coralie Fritsch

Réunion d’équipe enseignements

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