JI was invited for the first time to present my work at an international conference in July 1981. Stage fright, faced with impressive specialists. After my lecture, an old gentleman came to see me (I know now that he was 52 years old). He warmly congratulated me on my presentation, but an important point in my demonstration had escaped him and he asked me for an explanation. Panic: did he want to delicately point out an error in reasoning? I replied that I was using a result that I had read in André Haefliger’s thesis.
He then introduced himself, and I realized that I was facing André Haefliger, one of the founding fathers of the foliation theory that I cherished at the time. He had not forgotten his own thesis, but he tried to convince me (unsuccessfully) that I had gone further than him in the interpretation of his result. It’s always impressive for a young scientist to come face to face with one of his heroes.
André Haefliger died on March 7, at the age of 95, near Geneva, where he was a teacher from 1962 until his retirement in 1995. Since our first meeting, he has been a source of inspiration and a role model. as a mathematician, both as a researcher, as a leader of the scientific community, as a teacher and as a friend. His thesis, defended in Strasbourg in 1958, bore a title that was esoteric to say the least: “Laminated structures and valued cohomology in a sheaf of groupoids”enough to scare away more than one interlocutor as it was expressed in an abstract language, so common at the time.
In the Swiss mountains
A puff structure actually looks a lot like puff pastry. It is indeed a matter of filling the space with leaves, like the pages of a book. This theory had emerged a few years earlier and drew its motivation from understanding the structure of dynamical systems. In 1969, he invented the “Haefliger classifier”, a concept that excited the young community working on these questions. Older surveyors recount today with emotion their memories of the presentation of his discovery, at the top of Mont Aigoual, in the Cévennes, huddled in a weather station which housed a small conference center. It was apparently very cold, but the atmosphere was warm.
André Haefliger knew how to listen to young people and encourage them to work together, which is not so common in this environment.
When he arrived as a professor at the University of Geneva, there was no research department in mathematics. André always said that he didn’t have an office and that he had to go and telephone in a public phone booth. He largely contributed to the foundation of the remarkable section of mathematics of Geneva, which shelters in particular today two Fields medalists. The greatest mathematicians have come from all over the world to visit him, to share their discoveries with him and to receive his advice. He knew how to listen to young people and encourage them to work together, which is not so common in this environment.
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