I just heard from a colleague that Hans Duistermaar passed away yesterday (March 18th). To me, Hans was not only a great mathematician, but also a great friend. This sad news came as a shock for me.

Hans made many important contributions to mathematics: Fourier integral operators (with Hormander), symplectic geometry (equivariant cohomology and stationary phase formula, with Heckman), integrable systems, etc. He was also a very cultivated person in general, and it was always very interesting to talk to him, about any subject.

The first time I met Hans was in november (?) 1992, at a winter school near Utrecht. (The lecturers were excellent people: Guillemin, Heckman, van Moerbecke, and Duistermaat, and may students of this school also became very well-known mathematicians later). He probably didn’t remember seeing me there in 1992, but in 2001 he was kind enough to be a referee for my habilitation thesis. After 2001, I saw him several times in Holland, on various occasions (AlanDay in honour of Alan Weinstein, Cushman’s conference, …). The last time I saw him was last year,in Leiden, in a conference about monodromy and geometric phases. He was still very active and in very good shape at that time, even though he was officially retired. He was planing to go to Rio this summer (i.e. winter in Rio) to give a course there on Poisson geometry. As recently as 2-3 months ago, Hans and I still exchanged emails about Mineur’s proof of the action-angle variables. He was not very happy with the proof and thought that it contained serious holes, while I was of the opposite opinion (that the proof was essentally complete even though written in an archaic language). My point was that the main ideas there were all correct so the proof should be considered as correct, but Hans was really into the details of the proof.

This news (which we also learned from colleagues in Utrecht) is terribly sad for many of us in Berkeley,

where Hans was a frequent visitor. I have known Hans á a colleague and friend since around 1970, and much of

my work has been inspired by his ideas, both written and in conversation. I same him for (alas) the last time

in July, 2009, at a thesis defense in Utrecht, where Marius Crainic took (after detailed instructions from Hans as

to how we should be arranged in the cloister garden) the very nice photo which I have posted at

http://math.berkeley.edu/~alanw/hans.jpg.

Alan Weinstein

Thank you, Alan, for your comment & for the link to a very nice picture of Hans and you.

We’ll miss Hans very much.