Jean-Christophe Yoccoz died on September 3. With his death, the Collège de France loses one of its most brilliant members, a mathematician of extraordinary brilliance, who quickly established himself as the world leader in the theory of dynamical systems initiated by Henri Poincaré at the end of the 19th century. This theory seeks to describe movement qualitatively over the long term. The aim, for example, is to understand whether the solar system is stable. Could a planet be ejected suddenly? There are two radically different types of behavior. The motion may be stable and "predictable", or it may be chaotic. Yoccoz's work helps us to better understand the boundary between these two types of behavior.
An alumnus of the École normale supérieure, where he graduated top of his class in 1975 at the age of 18, and a gold medalist in the 1974 Mathematics Olympiad, Jean-Christophe Yoccoz began his career as a researcher at the CNRS before taking up a professorship at Paris-Sud in 1988. He obtained his PhD under the supervision of Michael Herman in 1985 and was awarded the Fields Medal in 1994. That same year, he became a member of the Académie des Sciences and the Brazilian Academy of Sciences. In 1996, he was appointed Professor of Differential Equations and Dynamical Systems at the Collège de France.
He obtained many decisive results on the most difficult problems posed by dynamical systems, and his demonstrations combine uncommon analytical power, dazzling geometric vision and prodigious mastery of Combinatorics, illustrated by what are now known as the "Yoccoz puzzles".
A cheerful man, with a quick laugh even in the worst moments of his illness, discreet but warm-hearted and always available for others, he knew how to stand firm when necessary without ever losing his kindness, good humor and simplicity.
Alain Connes, Etienne Ghys, Pierre-Louis Lions
