Quantitative differentiation and the medial axis

Abstract

We study the medial axis of a set $K$ in Euclidean space (the set of points in space with more than one closest point in $K$) from a "coarse" and "quantitative" perspective. We show that on "most" balls $B(x,r)$ in the complement of $K$, the set of almost-closest points to $x$ in $K$ takes up a small angle as seen from $x$. In other words, most locations and scales in the complement of $K$ "appear" to fall outside the medial axis if one looks with only a certain finite resolution. The word "most" involves a Carleson packing condition, and our bounds are independent of the set $K$.