by Rufus Willett
I first met Paul when I was a graduate student at Penn State University,
and my first serious interaction with him was as a graduate student in his
introductory
As one would expect from someone who has a large personal stake in the
subject, Paul’s course was quite different from what one might find elsewhere,
or in the standard texts, on
Outside of his formal course, as a starting graduate student I had
almost zero sense of how mathematicians interacted with each other, and
was intimidated by pretty much everyone. Initially, Paul was certainly
amongst the intimidators, but through no fault of his own: he has the
sort of ebullient and nonjudgmental personality that breaks through such
barriers quickly. Paul was always approachable (when not traveling, as he
was often elsewhere, doing research and as a highly in-demand lecturer).
I distinctly remember asking Paul a technical question about the
I should say a little more about the Baum–Connes conjecture, in order to
explain what happened next in my interactions with Paul. This is a deep
conjecture connecting algebra, analysis, and topology. It was originally
formulated by Paul and
Alain Connes
in around 1980 using the geometric picture
of
After graduating from Penn State in 2009, my next substantive involvement with Paul was at the Oberwolfach meeting on Noncommutative Geometry in 2011. My postdoctoral mentor Guoliang Yu and I had been revisiting the examples of Higson, Lafforgue, and Skandalis, and we were attempting to see “how bad” the problems really were, in some sense. We were able to show that if one changed the analytic ingredients involved slightly, then the previous counterexamples actually become confirming examples; however, the analytic changes we made were clearly incompatible with known results on the original Baum–Connes conjecture, so our results were in some sense just a curiosity at this stage.
Nonetheless, Paul was very enthusiastic! His support at this stage in my career was very much appreciated, and certainly a boost to my, at the time, rather fragile sense of mathematical self. Paul quickly saw how to make the next step, which was to change the analytic ingredients involved in a way intermediate to the original Baum–Connes conjecture and the results of Yu and myself. He traveled the world asking various mathematicians whether this intermediate step was indeed possible (amongst other things!), eventually getting a positive answer from Eberhard Kirchberg. Having also joined forces with Erik Guentner, we were up and running. Over the course of several enjoyable visits and excellent dinners (particularly at the JMM in Baltimore in 2014, around the completion of the project), we were able to give a natural reformulation of the Baum–Connes conjecture which agrees with the original conjecture in all previously known cases, to which there are no known counterexamples, and for which the previously known counterexamples become confirming examples. This remains the current state of the reformulated conjecture.
On a more personal note, throughout my interactions with him, Paul has been a pleasure. He has been supportive and generous with his time, knowledge, and ideas. He was, and remains, most enjoyable company.