by Ronald G. Douglas
I. Introduction
For me, the year 1978 was a good year! In the winter I delivered the Hermann Weyl Lectures [e3] at the Institute for Advanced Study in Princeton. In the summer I gave an invited talk at the International Congress of Mathematicians held in Helsinki. And in December, I met Paul Baum. I had to navigate the snowy roads between Stony Brook and Princeton, and I lost my luggage on the trip to Helsinki, but the year ended on a good note!
On the plane trip returning from Helsinki, my then-colleague
Jeff Cheeger
told me I should look up Paul Baum in connection with my work on
My work had started with a problem in abstract operator theory seeming
to have little contact with
In the 60’s a topic of considerable interest among operator theorists concerned compact perturbations and properties of operators which held modulo them. In particular, a result of Herman Weyl and John von Neumann showed that self-adjoint operators with the same essential spectrum were unitarily equivalent modulo the compact operators. Here, essential spectrum is defined as limit points of the spectrum or eigenvalues of infinite multiplicity. People were interested in the analog of this result for normal operators. And, in particular, Paul Halmos had asked if one could characterize such operators. David Berg had shown that normal operators with the same essential spectrum were unitarily equivalent modulo compacts. But whereas an operator that is self-adjoint modulo the compacts must be a self-adjoint plus a compact, the analogous statement is no longer true for normal operators. The unilateral shift provides a counterexample. In particular, Paul Halmos raised two specific questions. First, when is an operator that is normal modulo the compacts the sum of a normal and a compact? Second, is the collection of sums of a normal operator and a compact operator closed in the norm topology? These are some of the problems that Larry Brown, Peter Fillmore, and I were investigating.
In tackling these questions, following earlier work of
Lewis Coburn,
we eventually focused on the associated short exact sequence of
One obstruction to triviality is the existence of a
In considering the analogous question of triviality for spaces
Following
Alexander Grothendieck’s
introduction of algebraic
Paul Baum had started out trying to provide a concrete geometric realization
of
Michael Atiyah had shown the existence of
II. The isomorphism of realizations
At the end of the
70’s, an airline flew from Providence, Rhode Island, to Islip,
New York, and back, stopping at New Haven and Bridgeport in Connecticut, before
returning to Providence. Paul and I flew those routes regularly, once
every month or two for the next year and a half. Initially most of our
time was devoted to explaining to each other our realizations of
In Michael Atiyah’s realization of even
Paul and I realized that the same setup with
Recall that for a finite-dimensional Hilbert space
In the setting of the preceding paragraph, but with
By the time Paul and I attended the AMS Summer Institute in Operator Algebras in Kingston, Ontario, in the summer of 1980, we had understood the isomorphism and how to establish it. It is closely related to the Atiyah–Singer index theorem. These results were described in [1].
The Kingston meeting was Paul’s debut as an “operator algebraist”. After that he was no longer able to be anonymous in that community. He drove from Providence, took the ferry to Long Island, and picked me up at Stony Brook on the way to Kingston. He was one of the “geometer-topologists” who got interested in what is now known as noncommutative geometry. Alain Connes was at the Kingston meeting also.
III. Relative -homology
I had planned to visit Alain Connes in Paris for the fall semester. As it
turned out, I could make it for only a week and Paul joined me. During
that time, Paul and Alain started work on what has become known as the
Baum–Connes conjecture. Paul and I, on the other hand, started working on
understanding an analytic realization for the even relative
In defining
There are two parts to the realization of even relative
Showing that this definition yields the relative even group along with the
boundary map was tackled by Paul and myself during the special year 1984–85
at MSRI in Berkeley. Part of the proof rested on matching up these groups
with corresponding
IV. Differential operators and the relative even group
Although our work provides a rather explicit description of relative
even cycles and the boundary map, the description is not terribly useful
in the form given above. The problem is to decide how one recognizes a
relative cycle and the image of the boundary map when the cycle arises
from a differential operator on a smooth manifold. To accomplish this, we
were joined by
Michael Taylor,
who at that time was my colleague at Stony
Brook and who is an expert on
PDEs. We focused on differential operators,
and a generalization of them, called pseudodifferential operators, defined
from smooth sections of a vector bundle
Since the differential operator defines an unbounded operator on the space
of smooth sections, one has to proceed carefully. In particular, one must
consider extensions of this operator that are closed. We showed that some
of them define even relative cycles. Because such operators have more than
one possible
closed extension, the issue of uniqueness arises, but under broad hypotheses
it turns out that the class represented by the relative cycle is independent
of the choice of closed extension. One can also identify the odd boundary
cycle explicitly, now as an elliptic pseudodifferential operator
The fact that different closed extensions of
This realization is particularly interesting in
the case
This work, which was published in [2], was essentially completed for the 1988 AMS Summer Institute held in New Hampshire and organized by William Arveson and myself. Many of the themes of the subject now called noncommutative geometry were discussed there as well as at the meeting in Bowdoin, Maine, that followed. The location of the meetings was chosen to take advantage of the summer weather, but mother nature thwarted us. It was hot and fans were in short supply. However, one consequence of these meetings being held in New England was the opportunity to meet Paul’s father, an artist. I had already met his mother, also an artist, on several occasions because she lived in New York City. This opportunity provided some insight into Paul’s personality.
V. The odd relative group and concluding comments
It is also of interest to consider the odd relative group. Here the
relative cycles would be pairs of
It had been ten years at this point since I met Paul Baum. Although our
research interests diverged subsequently, we remain good friends and often
reminisce about our roles in the development of