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Exhaustion of a Locally Compact Separable Metric Space by Compact Sets, with Cut-Off Functions

A locally compact separable metric space is the union of an increasing sequence of compact sets, each inside the interior of the next, such that every compact set lies inside one of them; and there are continuous compactly supported cut-off functions equal to 1 on each set and supported in the next.

Statement

In the setting of Locally Compact Metric Spaces and Compactly Supported Functions: Standing Notation, let (X,d)(X,d) be a locally compact metric space that is separable. There are compact sets Km⊆XK_{m}\subseteq X and functions φm\varphi_{m} in Cc(X)C_{c}(X), for m∈Nm\in\mathbb{N}, with the following properties.

Exhaustion. For every m∈Nm\in\mathbb{N}, Km⊆int⁡Km+1⊆Km+1K_{m}\subseteq\operatorname{int}K_{m+1}\subseteq K_{m+1}; and

⋃m∈NKm=X.\bigcup_{m\in\mathbb{N}}K_{m}=X.

Compact sets. For every compact C⊆XC\subseteq X there is m∈Nm\in\mathbb{N} with C⊆int⁡KmC\subseteq\operatorname{int}K_{m}.

Cut-off functions. For every m∈Nm\in\mathbb{N}, 0≤φm(x)≤10\le\varphi_{m}(x)\le1 for every x∈Xx\in X, φm(x)=1\varphi_{m}(x)=1 for every x∈Kmx\in K_{m}, and the support of φm\varphi_{m} satisfies supp⁡φm⊆Km+1\operatorname{supp}\varphi_{m}\subseteq K_{m+1}.

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