It is with immense sadness that we announce the passing of our colleague and friend Gérard Laumon on Saturday morning, October 4th.

Gérard learned he was ill in July 2024, and his health deteriorated considerably over the past summer. Since then, physically very weak, he had not returned to the department. Gérard wished to continue living at home, remaining independent, for as long as possible, and only agreed to be hospitalized last Tuesday. Gérard had been in critical condition since Thursday—but remained fully lucid until the end—and on Friday evening, after the final visits from his friends, he fell asleep and never regained consciousness.

This is not the place to discuss the importance of Gérard Laumon's mathematical work. Let us simply recall that, through his direct contributions and those of the mathematicians he trained, which were repeatedly recognized with prestigious awards, Gérard was a major figure in algebraic geometry and number theory over the last half-century, particularly through the developments he and his group made in the theory of automorphic forms and their geometric analogues.

Beyond his mathematical work itself and all his "ancillary" (but considerable) activities, notably as an editor and member of the French Academy of Sciences, Gérard Laumon played a central role in the life of our department, where he spent his entire career within the Arithmetic and Algebraic Geometry team. Gérard was recruited here as an assistant in 1976, before becoming a research fellow and then a research director at the CNRS in 1985 and 1989.

He played a central role in the major scientific developments of the department and the team. Gérard Laumon's research and research leadership made an essential contribution to the "automorphic turn" successfully adopted by this team in the mid-1980s. Furthermore, Gérard was always committed to bringing mathematicians working on new topics to Orsay, topics that were not yet represented there. This concern always guided his work as a member of the specialist committee—where he played a decisive role in recruitment, extending well beyond the AGA team—and as head of the AGA team. Successive expansion over the past few decades of themes studied in this department, while pushed by the entire team, were largely initiated by Gérard.

He also played a central role in the department's daily life. With his constant ironic nature, Gérard projected an image quite different from the conventional image of a mathematician "in the clouds." This irony, which could sometimes border on sarcasm, reflected his profound aversion to "taking himself too seriously." It could, at first glance, conceal the exceptional level of his professionalism in all his mathematical activities. He also possessed a deep concern for all his colleagues in the department, regardless of their age or qualifications. This was demonstrated by his actions, through practical assistance—Gérard had near-professional IT skills and was always willing to share them with anyone who needed them—or through sound advice on more personal matters, particularly career-related ones. Many young colleagues (and not only in the AGA team, far from it) owe it to him that they didn't become discouraged during job applications, but instead benefited from his valuable guidance to succeed.

Gérard's passing leaves a great void among his colleagues and friends, and especially within our department, to which he was deeply attached.

The above text was written by Jean-Benoît Bost, with the assistance and proofreading of Luc Illusie (French) and Kevin Bleakley (English).