Search for: All records

Abstract For every$$n\geq 2$$, Bourgain’s constant$$b_n$$is the largest number such that the (upper) Hausdorff dimension of harmonic measure is at most$$n-b_n$$for every domain in$$\mathbb {R}^n$$on which harmonic measure is defined. Jones and Wolff (1988,Acta Mathematica161, 131–144) proved that$$b_2=1$$. When$$n\geq 3$$, Bourgain (1987,Inventiones Mathematicae87, 477–483) proved that$$b_n>0$$and Wolff (1995,Essays on Fourier analysis in honor of Elias M. Stein (Princeton, NJ, 1991), Princeton University Press, Princeton, NJ, 321–384) produced examples showing$$b_n<1$$. Refining Bourgain’s original outline, we prove that$$\begin{align*}b_n\geq c\,n^{-2n(n-1)}/\ln(n),\end{align*}$$for all$$n\geq 3$$, where$$c>0$$is a constant that is independent ofn. We further estimate$$b_3\geq 1\times 10^{-15}$$and$$b_4\geq 2\times 10^{-26}$$. 

We continue to develop a program in geometric measure theory that seeks to identify how measures in a space interact with canonical families of sets in the space. In particular, extending a theorem of the first author and R. Schul in Euclidean space, for an arbitrary locally finite Borel measure in an arbitrary Carnot group, we develop tests that identify the part of the measure that is carried by rectifiable curves and the part of the measure that is singular to rectifiable curves. Our main result is entwined with an extension of the Analyst's Traveling Salesman Theorem, which characterizes subsets of rectifiable curves in R^2 (P. Jones, 1990), in R^n (K. Okikolu, 1992), or in an arbitrary Carnot group (the second author) in terms of local geometric least squares data called Jones' beta numbers. In a secondary result, we implement the Garnett-Killip-Schul construction of a doubling measure in R^n that charges a rectifiable curve in an arbitrary complete, doubling, locally quasiconvex metric space.