In memoriam André Haefliger

Our colleague and friend André Haefliger passed away on 7 March 2023, shortly before his 94th birthday. He played a major role in founding the University of Geneva’s Section of Mathematics and making it renowned, as much by the scientific quality as by the social and friendly generosity that characterised him. The best mathematicians from around the world as well as students who were just starting out would come to Geneva to discuss with him, be encouraged, and share many good moments with André and Minouche Haefliger.

André Haefliger attended primary and secondary school in Nyon and Geneva, then obtained a degree in mathematics from the University of Lausanne in 1952, in addition to a violin certificate and an apprenticeship on the viola. After that, he was a student and assistant at the University of Lausanne (1952-1954), in Strasbourg (1954) and in Paris (1955-1958), where he defended his thesis in May 1958, entitled "Structures feuilletées et cohomologie à valeur dans un faisceau de groupoïdes", before a jury consisting of Henri Cartan, Charles Ehresmann, and Pierre Lelong. He was then a substitute at the University of Geneva (1958-1959) and a temporary member at the Institute of Advanded Study in Princeton (1959-1961). He was appointed at the University of Geneva, first as an associate professor, and then as a full professor (1962).

During the 1950s, he established a long-lasting mathematical and personal relationship with Georges de Rham, Charles Ehresmann, Georges Reeb, Stephen Smale, René Thom, Hasler Whitney, Solomon Lefschetz, Shiing-When Chern, and many others. Later, together with Michel Kervaire and Claude Weber, every year they would gather about forty experts and students for mathematical weeks in a rustic chalet in Les Plans-sur-Bex. The former primary school of this small mountain village hosted seminars on a variety of subjects; local mathematicians learned the latest results and rubbed shoulders with future celebrities from international congresses.

His early mathematical work was on foliations and their classifying spaces; it was very pioneering and its importance was not recognised until many years after its publication, notably by Bill Thurston. Subsequent works included differential geometry, algebraic topology, singularity theory, analytic spaces, and group theory. André Haefliger's influence is profound. One cannot count the number of papers in gestation that matured in his office, where experts and beginners alike came to his blackboard with their ideas and questions - André was a very good listener, had an unparalleled intuition, and often gave profound insights even on subjects that were unfamiliar to him. He had a significant mark on the early work of many young mathematicians, such as Etienne Ghys and Vaughan Jones, and his writings have a deep and lasting influence on many subjects, as for example his book with Martin Bridson which has more than 2500 citations in Mathscinet. André Haefliger received honorary doctorates from the University of Burgundy and the Zurich Polytechnic. In 2003 he was awarded the Prix de la Ville de Genève, proving his fame had gone beyond the mathematical world.

At home, in his office, in his courses and seminars, and during the many trips he loved so much, André left the mark of a warm-hearted man whom we loved to see, listen to, and hear him laugh.

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