Yvon Maday

An applied mathematician and member of the Jacques-Louis Lions Laboratory (Sorbonne University/CNRS/Université Paris Cité), Yvon Maday has devoted his career to transforming practical problems into major mathematical advances. This year, he has been awarded the Chair in Computer and Digital Sciences at the Collège de France, and has received the Medal for Mechanical and Computer Sciences from the French Academy of Sciences.

For as long as he can remember, Yvon Maday has always sought to understand …and to help others understand as well. As a child, he loved "explaining and formally defining things" to help his classmates grasp a problem in physics or mathematics. This early talent naturally led him to the competitive classes préparatoires and then to the École normale supérieure de Saint-Cloud. There, he found himself "in his element" in a world of structures, proofs and abstractions.

Alongside his PhD, he took up medical studies in order to “understand the way doctors think” and collaborate with them. Working with cardiologists, he studied the blood flow of infants who had undergone surgery to correct aortic coarctation, looking to identify the factors triggering post-operative complications and distinguish causes and consequences. 

At the same time, he took an interest in the applications of mathematics in mechanics, and began working with the French Alternative Energies and Atomic Energy Commission (CEA), the French Aeronautics, Space and Defense Research Lab (ONERA) and EDF. With ONERA, he took on a major challenge in scientific computing: breaking down a complex domain into independent sub-problems to take full advantage of the rise of parallel architectures. “I am both an applied mathematician and a mathematician. Which means that when a concept is interesting, I have to see it through to the end, explore it fully within the language and framework of mathematics, even if, for a time, this takes me away from practical applications,” he explains. 

In the early 2000s, his work led him to contribute to fundamental advances in parallelisation, a method that involves dividing a large computational task into smaller subtasks to be executed simultaneously.
 

His collaboration with chemists opened another chapter in his career. To tackle the complexity of simulations of molecular behaviour, chemists had long been developing methods that were more or less empirical - effective but not very formalised. Mathematicians then came "running after them, seeking to understand the models underpinning these approaches", explains Yvon Maday. "Do these models accurately describe reality? Are they properly formulated? In other words, do they provide a single solution, or, on the contrary, a multitude of possible solutions?" 

Alongside his colleagues, Yvon Maday is developing methods for domain decomposition applied to solvation, which help to speed up simulations and enable the transition from stationary models to time-dependent models.

This work was awarded an ERC Synergy Grant in 2018, and the results now make it possible to analyse larger systems with controlled error bars.
 

In 2020, when the pandemic struck, Yvon Maday and his colleagues set up a working group to determine what mathematics could reveal about the dynamics of the epidemic. A fortunate series of collaborations then led him to the Obépine team, which studies the presence of the COVID-19 virus in wastewater.

The challenge was daunting: inaccurate measurements, with errors sometimes “by a factor of 10”, yet major policy decisions still had to be made. As an applied mathematician, Yvon Maday worked to reduce and then manage these uncertainties, ultimately producing real-time indicators which were sufficiently reliable as to be discussed with the Health Minister, to determine whether a lockdown was warranted. Later, his work would even make it possible to use sequencing to detect the emergence of variants in wastewater, sometimes before they were clinically detected.

Throughout his career, Yvon Maday has held fast to one conviction: “Mathematics has its place alongside all other disciplines, provided we are willing to understand the problems faced by others”. Whether it be reformulating perfumes at L’Oréal, monitoring pathogens or industrial collaborations with the Institut Carnot (now SUMMIT), which he helped to establish, he has consistently championed the same approach: drawing on reality to inform theory, and vice versa.
 

This year, Yvon Maday was elected to hold the Chair in Computer and Digital Sciences at the Collège de France. His inaugural lecture, scheduled for 19 February, will launch a series of others devoted to methods for reducing complexity, a field he contributed to establish twenty years ago alongside researcher Anthony Patera at MIT.

His career has also been deeply international. Having spent time at MIT and Brown University, he discovered resources – both material and human – on a whole new scale, as well as a rich scientific culture which he would bring back to France. One of the outcomes this inspired was the creation of the Centre d’Été Mathématique de Recherche Avancée en Calcul Scientifique, a summer school where scientists and industry professionals collaborate on practical problems, set against the stunning backdrop of the Calanques of Luminy in Marseille.

Now based at the Roscoff Biological Station (Sorbonne University/CNRS), Yvon Maday follows the progress made by biologists at the marine research centre with great interest. Aside from mathematics, his other passions, which he shared with his wife, include the cinema, music, the performing arts… and above all his grandchildren, whose ever-changing sense of wonder he loves to observe.