Triangulating stratified manifolds I: a reach comparison theorem
Résumé
In this paper, we define the reach for submanifolds of Riemannian manifolds, in a way that is similar to the Euclidean case. Given a d-dimensional submanifold S of a smooth Riemannian manifold M and a point p ∈ M that is not too far from S we want to give bounds on local feature size of exp −1 p (S). Here exp −1 p is the inverse exponential map, a canonical map from the manifold to the tangent space. Bounds on the local feature size of exp −1 p (S) can be reduced to giving bounds on the reach of exp −1 p (B), where B is a geodesic ball, centred at c with radius equal to the reach of S. Equivalently we can give bounds on the reach of exp −1 p • exp c (B c), where now B c is a ball in the tangent space T c M, with the same radius. To establish bounds on the reach of exp −1 p • exp c (B c) we use bounds on the metric and on its derivative in Riemann normal coordinates. This result is a first step towards answering the important question of how to triangulate stratified manifolds.
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