# Estimating the Reach of a Manifold

## TL;DR

This paper introduces the first methods for estimating the reach of a manifold, providing geometric insights, an estimator with efficiency bounds, and minimax rate bounds for the problem.

## Contribution

It develops new geometric results on reach, proposes an estimator with theoretical bounds, and establishes minimax rates for reach estimation.

## Key findings

- Estimator achieves uniform expected loss bounds
- New geometric results on reach for boundaryless submanifolds
- Upper and lower bounds on minimax estimation rates

## Abstract

Various problems in manifold estimation make use of a quantity called the reach, denoted by $\tau\_M$, which is a measure of the regularity of the manifold. This paper is the first investigation into the problem of how to estimate the reach. First, we study the geometry of the reach through an approximation perspective. We derive new geometric results on the reach for submanifolds without boundary. An estimator $\hat{\tau}$ of $\tau\_{M}$ is proposed in a framework where tangent spaces are known, and bounds assessing its efficiency are derived. In the case of i.i.d. random point cloud $\mathbb{X}\_{n}$, $\hat{\tau}(\mathbb{X}\_{n})$ is showed to achieve uniform expected loss bounds over a $\mathcal{C}^3$-like model. Finally, we obtain upper and lower bounds on the minimax rate for estimating the reach.