Approximation of Green functions and domains with uniformly rectifiable boundaries of all dimensions

Abstract: The present paper establishes equivalence between uniform rectifiability of the boundary of a domain and the property that the Green function for elliptic operators is well approximated by affine functions (distance to the hyperplanes). The results are novel in a variety of ways, in particular (1) this is the first time the underlying property of the control of the Green function by affine functions, or by the distance to the boundary, in the sense of the Carleson prevalent sets, appears in the literature; the "direct" result established here is new even in the half space; (2) the results are optimal, providing a full characterization of uniform rectifiability under the (standard) mild topological assumptions; (3) to the best of the authors' knowledge, this is the first free boundary result applying to all elliptic operators, without any restriction on the coefficients (the direct one assumes the standard, and necessary, Carleson measure condition); (4) our theorems apply to all domains, with possibly lower dimensional boundaries: this is the first free boundary result in higher co-dimensional setting and as such, the first PDE characterization of uniform rectifiability for a set of dimension $d$, $d<n-1$, in ${\mathbb{R}}^n$. The paper offers a general way to deal with related issues considerably beyond the scope of the aforementioned theorem, including the question of approximability of the gradient of the Green function, and the comparison of the Green function to a certain version of the distance to the original set rather than distance to the hyperplanes.