Thomas H. Wolff (1954–2000) Lennart Carleson, Sun-Yung Alice Chang, Peter W
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fea-wolff.qxp 3/20/01 10:26 AM Page 482 Thomas H. Wolff (1954–2000) Lennart Carleson, Sun-Yung Alice Chang, Peter W. Jones, Markus Keel, Peter D. Lax, Nikolai Makarov, Donald Sarason, Wilhelm Schlag, and Barry Simon homas H. Wolff, a leading analyst and a win- Lennart Carleson ner of the Salem and Bôcher Prizes, was killed Tin an automobile accident on July 31, 2000, Harmonic analysis has a position in mathe- when he was forty-six years old. matics comparable to that of the theory of the Tom was raised in a mathematical environment. atom in physics. By understanding what goes on His uncle, Clifford Gardner, was a professor at at the micro level, we can understand large-scale NYU’s Courant Institute of Mathematics for many phenomena (even such as meteorology). By Fourier years, and Tom’s mother, Lucile, was a technical expansions we can analyze and understand global editor of volume 1 of the English translation of the functions, arithmetic problems, or differential celebrated book Methods of Mathematical Physics equations. As physics has a miracle method in by Courant and Hilbert. quantum mechanics, we use the theory of analytic Tom was an undergraduate at Harvard, where, functions, and of course, in a deeper sense, all he once told me, he regularly played poker with a this comes together as one piece. These methods fellow student named Bill Gates. After graduating are efficient when the number of variables n is from Harvard in 1975, Tom went to Berkeley, where either small or very large (where the theories be- he got his Ph.D. under Don Sarason in 1979. Tom come probability). In the intermediate range, say 10 <n<100, physicists and chemists have been then spent one year at the University of Washing- more successful than we mathematicians, and it ton and two at the University of Chicago before remains an important challenge for the future to coming to Caltech in the fall of 1982 as an assis- develop relevant harmonic analysis in this range. tant professor. The problems in harmonic analysis (of few vari- Tom spent most of the rest of his career at Cal- ables) today, after two hundred years of research, tech, although, for personal reasons, he resigned are very combinatorial and very complicated. Tom twice, spending two years (1986–88) at Courant and Wolff had a unique talent and a profound knowl- three (1992–95) at Berkeley. His promotion or edge in the area. The fundamental problems that appointment to a professorship at Caltech three he considered required long preparation, deep times is a record for our institution. concentration, and an ability to keep a very com- Tom is survived by his widow, Carol Shubin, a plicated set of arguments simultaneously active mathematics professor at California State Univer- and available in his mind. Two quotations come to sity, Northridge; two sons (aged three and five at mind. The first is Newton’s answer to how he his death); his parents; and two sisters. —Barry Simon Lennart Carleson is professor emeritus of mathematics at the Royal Institute of Technology, Stockholm, Sweden and Photographs included in this article are courtesy of Carol at the University of California, Los Angeles. His e-mail ad- Shubin. dress is [email protected]. 482 NOTICES OF THE AMS VOLUME 48, NUMBER 5 fea-wolff.qxp 3/20/01 10:26 AM Page 483 found the law of gravitation: “By constantly think- ing about it,” and the second is Tom’s remark Ph.D. Students of Tom Wolff when he was awarded the Bôcher Prize in 1999: “It’s never been easy for me.” Nobody else could have Ivo Klemes, Caltech, 1985 done what he did. Peter Holden, Caltech, 1987 Let me highlight some of Tom’s most striking Gregory Hungerford, Caltech, 1988 results, all coming from combinatorial harmonic Stewart Gleason, NYU, 1990 analysis. Tom’s thesis and early work were in the Dean Evasius, Caltech, 1992 direction of complex analysis. (His thesis is de- Wensheng Wang, Caltech, 1993 scribed below by D. Sarason.) While working on his Lawrence Kolasa, Caltech, 1994 thesis Tom found a new proof of the corona the- Wilhelm Schlag, Caltech, 1996 orem for H∞ of the unit disk. Here he managed to David Alvarez, UC Berkeley, 1997 summarize the combinatorics in a beautiful lemma Themistoklis Mitsis, Caltech, 1998 on bounded solutions, u of ∂u = f. As first pointed Oleg Kovrijkine, Caltech, 2000 out by Hörmander, one should first solve the rel- Burak Erdogan, Caltech, 2001 evant equations in a nonanalytic way and then modify the solution to make it analytic bounded, which leads to the above equation. To get the ω>0, but barely so. Consider, for example, = boundedness of the solutions, the old combina- complement of disjoint disks torics came back, and this is where Tom’s lemma works; see [4]. D : |z a|,a = + i , This lemma has had a large impact, although N N Tom never published it. He did not return much 0 < ||, ||N, to function algebras. I can mention a joint paper 1 with Alan Noell (1989) which gives an almost com- and choose so that ω(D)= 4N2 for all and plete description of sets E on |z| =1where an an- . The radii in the outer part of the square must alytic function f (z) in |z| < 1, satisfying be very small, but a proof is not easy and the |f (z) f (w)|C|z w| , |f (z)|1, f (0) = 0 , can precise size of is not known (to me). It was a be =1 (so-called peak sets). This is surprisingly tour de force when Tom and Peter Jones in 1988 hard! proved the statement about dim(E) 1 in full From function algebras Tom turned to potential generality [6], and Tom even proved (published theory, where his most important work in the 1980s in 1993 but from the same time) that E can be can be found. The first fundamental result [5] is chosen of -finite length.P In the above example, from 1983. L. I. Hedberg had proved that if is an Tom’s method proves that C, |a|1/2. Rn ∞ open set in , then C0 () is dense in all higher- There is a natural conjecture for the analogous m,p m order Sobolev spaces W0 () when p>2 n . problem in higher dimension n (Øksendal 1981) m The exceptions 1 <p 2 n had their origin in that E ∂ can be chosen of dimension n 1. In potential theory and more precisely in the miss- 1987 Tom produced a counterexample to this ing Kellogg lemma on the size of the set of thin conjecture for n =3. This was a sensation not boundary points outside that range. Tom closed only because of its complexity but also because it this gap by a clever construction. Today more simultaneously disproved two other conjectures. streamlined approaches exist. It was published only in 1991 [11]. Tom next turned to harmonic measure. Let The construction is based on the following be a domain in the plane containing ∞, and sup- lemma: For each unit vector e in R3 , there is a har- pose that we can solve the Dirichlet problem in monic function u in x3 > 0, vanishing at ∞, so for arbitrary continuous f on ∂. The value of the that Z solution at ∞ is given by integrating f against a (*) log |e + ∇u| dx1dx2 < 0. positive probability measure ω in the plane sup- R2 ported on ∂. This is called harmonic measure and is also the hitting probability on ∂ of a Brown- This fails in two dimensions by subharmonicity. ian particle in , starting at ∞. From this inter- This entropy-type of integral is relevant because pretation, it is natural to surmise that most mea- the counterexample domain is obtained in a sure lies in the exposed parts of ∂. The harmonic dynamic way by a snowflake construction where measure should therefore be 1-dimensional; more almost independent products of gradient vectors precisely: ∃?E of dimension 1 on ∂ with as above occur. (A result by Bourgain tells us that ω(E)=1. By using the Riemann map, N. G. dim(E) n n is possible, but the correct n is Makarov proved this in 1985 in a very precise not known.) Tom’s construction also solves (par- sense for simply connected domains. For general tially) a problem of Bers: There is u harmonic in 1+ domains it is much more complicated. The main x3 > 0, C up to x3 =0, such that u = |∇u| =0 difficulty comes from those parts of ∂ where on a set of positive measure on x3 =0. Here is MAY 2001 NOTICES OF THE AMS 483 fea-wolff.qxp 3/20/01 10:26 AM Page 484 Charles Fefferman’s solution of the “ball-multi- plier-problem” (1971). If ˆf (), ∈ Rd , d 2, is the Fourier transform of f ∈ Lp, p>1, is ˆf , restricted to ||1, the Fourier transform of some g ∈ Lp ? This is true for d =1(Hilbert transform) and for p =2 and all d, but Fefferman gave a counterex- ample for all other p and d. The obstruction comes from thin layers 1 ||1 which contain long ( 1/2) rectangles of width of all directions. In this way the counterexample comes from Besi- covitch’s 1928 construction (as a solution of a problem of Kakeya’s concerning “sets where you can turn a needle”) of a compact set in R2 of zero measure that contains a line segment of length 1 in every direction. (For this classical theory see [3].) It is rather easy to see that the Hausdorff di- mension of a Kakeya set is 2.