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Eventual access is uniform on the trimmed compact inside core

Statement

Let Σ be a manifold with nonempty compact boundary S, equipped with a metric d inducing its manifold topology, and write Σ∘=Σ∖S. Let B⊂Σ be a compact inside region containing an open neighborhood of S, and denote its outer boundary by Γ=∂ΣB, the topological boundary relative to Σ. Set ρ(x)=d(x,S)=inf⁡s∈Sd(x,s). Suppose Σ∘⊂M, where M is an ambient manifold and the inclusion Σ∘↪M is continuous for the subspace topology inherited from Σ. Let (Pt)t∈R be open subsets of M with Ps⊆Pt for s≤t, and suppose that for every x∈Σ∘ there is Tx∈R such that x∈Pt for all t≥Tx. Then for every sufficiently small η>0 there is tη∈R such that Lη=B∩{ρ≥η/2}⊂Pt for all t≥tη. Moreover Γ is compact and disjoint from S, and there is an open section neighborhood Nη of the whole outer boundary Γ with Nη⊂Pt for every t≥tη.

Statement references

Facts & Assumptions

[F1]

The interior of a manifold with boundary S is Σ∘=Σ∖S. A compatible metric has the manifold open sets as its metric open sets. The relative topological boundary of B is ∂ΣB=B‾∖int⁡ΣB, and int⁡ΣB is open. Interior and boundary of a manifold with boundary Metric space: d(x,y) = 0 iff x = y, symmetry, and the triangle inequality; pseudometric and ultrametric The metric topology: a set is open when every one of its points has a ball around it inside the set; closed means open complement Interior, closure, boundary, limit point, isolated point and dense subset of a metric space

[F2]

For a nonempty subset S of a metric space, d(x,S)=inf⁡s∈Sd(x,s) is a finite real number and ∣d(x,S)−d(y,S)∣≤d(x,y). Bounded subset, diameter, distance from a point to a set, and distance between two sets in a metric space |d(x,A) - d(y,A)| ≤ d(x,y), so the distance to a fixed nonempty set is 1-Lipschitz

[F4]

Subspace open sets are traces of ambient open sets, and subspace topologies are transitive. For a continuous inclusion, inverse images of ambient open sets are open. An indexed family of open sets of a topological space covering a compact subset has finitely many indices whose sets cover it, unless the subset is empty. Subspace topology: the traces of the open sets, its closed sets and its bases, the continuity of the inclusion, and the characteristic property of a map into a subspace Continuity of a map of topological spaces at a point and globally A subspace is compact exactly when every family of open subsets of the ambient space covering it, indexed or not, has finitely many members covering it

[F5]

A finite nonempty list of real numbers has a largest member. A continuous real function on a nonempty compact metric space attains a minimum. Every nonempty finite set of reals has a maximum and a minimum A continuous real-valued function on a nonempty compact metric space is bounded and attains a greatest and a least value

Proof

Given: The hypotheses in the Statement.

Goal: Establish the trimmed-core containment and the persistent open neighborhood of the outer boundary.

1.1givenF1F3

By compatibility of d and [F3], B and S are compact metric subsets of Σ, hence closed. Since B contains an open neighborhood of S, we have S⊂int⁡ΣB. Closedness of B implies B‾=B: a point outside B has a metric ball in its open complement and so is not adherent to B. Thus Γ=B∖int⁡ΣB is closed in B, compact by [F3], and contained in B∖S⊂Σ∘. Also Σ∘ is open in Σ because S is closed.

2.1givenF1F2F3F4step 1.1

By [F2] and S≠∅, ρ is finite and 1-Lipschitz, hence continuous: d(x,y)<ϵ gives ∣ρ(x)−ρ(y)∣<ϵ. It vanishes on S. If x∉S, closedness of S gives a ball of radius r>0 about x disjoint from S, so d(x,s)≥r for every s∈S and ρ(x)≥r>0. For any η>0, Lη is closed in B: at a point with ρ(x)<η/2, the Lipschitz bound keeps ρ<η/2 throughout a sufficiently small ball. By [F3], Lη is compact, and it avoids S. Its topology inherited from Σ∘ equals that inherited from Σ, by transitivity of subspace topologies in [F4]; hence it remains intrinsically compact as a subset of Σ∘.

3.1givenF4F5step 2.1

Fix η>0 and let Ut=Σ∘∩Pt. The continuity of the inclusion makes Ut open in Σ∘. Eventual entry applies to every point of Lη⊂Σ∘, so the indexed family (Ut)t∈R covers Lη. By its compactness and the indexed ambient-cover criterion in [F4], if Lη≠∅ there are finitely many indices t0,…,tm whose sets cover it. Put tη=max⁡{t0,…,tm}, which exists by [F5]. Increasingness gives Lη⊂Pt for every t≥tη. If Lη=∅, take tη=0 and the same containment is automatic.

4.1givenF1F4F5step 1.1step 2.1step 3.1∎

If Γ≠∅, its compactness and the continuity of ρ allow [F5] to give δ=min⁡Γρ attained at a point of Γ. Step 2.1 and Γ∩S=∅ give δ>0. For 0<η<δ, we have Γ⊂Lη. Consequently Nη=Σ∘∩Ptη contains Γ, is open in Σ∘ by [F4], and is open in Σ because Σ∘ is open. Increasingness gives Nη⊂Pt for all t≥tη. If Γ=∅, take 0<η<1 and Nη=∅; the neighborhood assertion is vacuous and the trimmed-core containment still follows from step 3.1. This proves both conclusions, with a single neighborhood valid at all later times.

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