5 December 2025

10:55 AM – 11:40 AM

Salle Proudhon

Clément Dombry

Clément Dombry is a professor of statistics and applied probability at Marie and Louis Pasteur University, in the Besançon Mathematics Laboratory. After completing his thesis in 2005 at the University of Lyon and a position as a lecturer at the University of Poitiers, he joined Besançon in 2013 as a professor. His research focuses mainly on extreme value theory, probabilistic forecasting, and statistical learning, with applications in various fields such as the environment, meteorology, and health. In particular, he coordinates the ANR T-REX project, dedicated to improving the forecasting of extreme events through the combination of probabilistic models and methods derived from artificial intelligence. In addition to his research activities, Clément Dombry is director of the Besançon Mathematics Laboratory and teaches in the master’s program in statistical modeling. He also contributes to scientific dissemination as associate editor of the international journal Extremes.

Is your hat still waiting for you in the cloakroom?

Imagine a party where each guest leaves their hat in the cloakroom. When it’s time to leave, chaos ensues: everyone grabs a hat at random. Who will be lucky enough to get their own hat back? This scenario, which seems harmless at first glance, is actually a classic mathematical problem known as the hat problem or the meeting problem. Behind this amusing situation lie some surprising results: regardless of the number of guests, the average number of people who leave with their own hat is always equal to 1. Whether there are 10, 100, or 1,000 guests, chance seems to preserve a single lucky guest on average. Even more surprising: the probability that no guest will find their hat is not rare, but is around 37%. This value, linked to the famous number 𝑒, appears here unexpectedly. This little cloakroom game actually opens a window onto some profound concepts: the law of large numbers, which highlights the regularities of chance; Poisson’s law, which describes rare events; and, more broadly, the power of limits in probability. Beneath the appearance of a simple hat, the elegance of mathematics is revealed.