Fanny Kassel, Laureate of the Mathematics Medal of the French Academy of Sciences

Fanny Kassel, CNRS Directrice de recherche at IHES, has been awarded the mathematics Medal of the French Academy of Sciences during a formal prize ceremony held at the Palace of the Institut de France on October 15th, 2024.

Founded by Colbert in 1666, the French Academy of Sciences is an assembly of scientists selected among the most prominent French and foreign experts. The Academy’s discussions and debates provide a framework of expertise and advice on political, ethical, and societal issues raised by the latest advances in science.

The prize recognizes Fanny Kassel’s work at the intersection of geometry, group theory, Lie theory, and dynamics. Her research focuses more specifically on discrete subgroups of Lie groups, especially in higher rank, in connection with higher Teichmüller theory.

During the prize citation, Étienne Ghys, Perpetual Secretary of the French Academy of Sciences, emphasized the ubiquity of Lie groups in mathematics:

“Lie groups are wonderful mathematical objects which have played a central role in our field for 150 years. As Henri Poincaré once said, mathematics is nothing but a story of groups.”

He ended the citation by warmly congratulating Fanny Kassel, recalling that he was once a member of her PhD-thesis committee.

“In the name of the whole IHES community, I wish to warmly congratulate Fanny for receiving the mathematics Medal of the French Academy of Sciences. Her impressive body of work follows in the footsteps of illustrious predecessors such as Armand Borel, Jacques Tits, and Gregory Margulis, all of whom have also spent time at the Institute. I am delighted that this field of research is still represented at such a high level at IHES.”
— Emmanuel Ullmo, Director of IHES.

The following statements are from some of Fanny Kassel’s colleagues.

“Working with Fanny Kassel over the past decade has been a truly wonderful and inspiring experience. I consider our collaboration, together with François Guéritaud, to be one of the highlights of my career. Fanny is absolutely brilliant, in both her creative vision and her meticulous attention to detail. Our collaboration began during an IHP program in the winter of 2012, a period I fondly remember as one of the most stimulating times of my early career. I vividly recall how excited the three of us were, working fervently on our first paper in the Jardin du Luxembourg after realizing how our perspectives on anti-de Sitter (AdS) three-manifolds could fit together. That initial spark led to a long and fruitful collaboration, one I am incredibly proud to be a part of, and I truly hope we continue working together for many years to come. I am overjoyed to see her receive this well-deserved recognition.”
— Jeffrey Danciger, Associate Professor of Mathematics at the University of Texas at Austin

“Creating a new theory of spectral analysis beyond the Riemannian context has provided insights that advance the geometric theory of discontinuous groups. Among these, the concept of sharp action was introduced in our joint paper Poincaré series for non-Riemannian locally symmetric spaces as one of the key tools for proving the existence of a “universal spectrum”. In a recent preprint, Fanny, along with Nicolas Tholozan, has made yet another application of the sharpness condition, remarkably finding a new obstruction to the existence of cocompact discontinuous groups in non-Riemannian homogeneous spaces.”
— Toshiyuki Kobayashi, Professor of Mathematics at the University of Tokyo

“Fanny is a brilliant mathematician, and her body of work is exceptional. Most recently, she proved — with her collaborators — some major results, for instance on the amalgamation of linear hyperbolic groups — giving a negative answer to a difficult question of Nori — and also on compact quotients of homogeneous spaces. Each time, she has produced unexpected results.”
— François Labourie, Professor of Mathematics at Université Côte d’Azur

Photo credit: Chris Peus / IHES