TheoremBase

Near a Density Point Every Direction Meets the Set Closely

If xx is a density point of a set GG then, for points ww close enough to xx, the set GG comes within a prescribed fraction of the distance from ww to xx of the point ww.

Statement

We work in the setting of Product Measures and Lebesgue Measure on ℝⁿ: Standing Notation and Background, whose notation is fixed for every dimension and is used here with a natural number nn satisfying 1≤n1\le n: the Euclidean norm ∥ ⋅ ∥\lVert\,\cdot\,\rVert, distance dEd_{E} and closed balls Bˉ(x,r)\bar{B}(x,r) of Rn\mathbb{R}^{n}, the Borel σ\sigma-algebra B(Rn)\mathcal{B}(\mathbb{R}^{n}), and Lebesgue measure λn\lambda_{n} are as fixed there. Write λn∗\lambda_{n}^{\ast} for Lebesgue outer measure, and let density point have the meaning fixed in that definition.

Let G⊆RnG\subseteq\mathbb{R}^{n} be an arbitrary subset, let x∈Rnx\in\mathbb{R}^{n} be a density point of GG, and let δ∈R\delta\in\mathbb{R} with 0<δ0<\delta. Then the following holds.

1. (A nearby point of the set) There is ρ∈R\rho\in\mathbb{R} with 0<ρ0<\rho such that for every w∈Rnw\in\mathbb{R}^{n} with 0<∥w−x∥<ρ0<\lVert w-x\rVert<\rho there is g∈Gg\in G with

∥g−w∥≤δ ∥w−x∥.\lVert g-w\rVert\le\delta\,\lVert w-x\rVert .

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