# A remark on two notions of flatness for sets in the Euclidean space

created by violo on 26 Feb 2021
modified on 09 Jun 2022

[BibTeX]

Accepted Paper

Inserted: 26 feb 2021
Last Updated: 9 jun 2022

Journal: J. Reine Angew. Math.
Year: 2021

ArXiv: 2102.12910 PDF

Abstract:

In this note we compare two ways of measuring the $n$-dimensional "flatness" of a set $S\subset \mathbb{R}^d$, where $n\in \mathbb{N}$ and $d>n$. The first one is to consider the classical Reifenberg-flat numbers $\alpha(x,r)$ ($x \in S$, $r>0$), which measure the minimal scaling-invariant Hausdorff distances in $B_r(x)$ between $S$ and $n$-dimensional affine subspaces of $\mathbb{R}^d$. The second is an `intrinsic' approach in which we view the same set $S$ as a metric space (endowed with the induced Euclidean distance). Then we consider numbers ${\sf a}(x,r)$'s, that are the scaling-invariant Gromov-Hausdorff distances between balls centered at $x$ of radius $r$ in $S$ and the $n$-dimensional Euclidean ball of the same radius. As main result of our analysis we make rigorous a phenomenon, first noted by David and Toro, for which the numbers ${\sf a}(x,r)$'s behaves as the square of the numbers $\alpha(x,r)$'s. Moreover we show how this result finds application in extending the Cheeger-Colding intrinsic-Reifenberg theorem to the biLipschitz case. As a by-product of our arguments, we deduce analogous results also for the Jones' numbers $\beta$'s (i.e. the one-sided version of the numbers $\alpha$'s).

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