TheoremBase

A Continuous Function on a Compact Set Admits a Nondecreasing Modulus of Continuity

A real-valued continuous function on a nonempty compact subset of a metric space admits a modulus of continuity that is nondecreasing and dominates the oscillation of the function at every scale.

Statement

Let (M,d)(M,d) be a metric space, equipped with the collection of all subsets that are open in (M,d)(M,d), which is a topology by The Open Subsets of a Metric Space Form a Topology. Let R\mathbb{R} be the ordered field of real numbers, let ∣⋅∣|\cdot| be the absolute value on R\mathbb{R}, let (R,dR)(\mathbb{R},d_{\mathbb{R}}) be the real line, so that dR(s,t)=∣s−t∣d_{\mathbb{R}}(s,t)=|s-t|, and let T={t∈R:0≤t}T=\{t\in\mathbb{R}:0\le t\}.

Let K⊆MK\subseteq M be nonempty and compact in MM, and let f:K→Rf:K\to\mathbb{R} be continuous on KK, as a map into (R,dR)(\mathbb{R},d_{\mathbb{R}}).

Then there exists a modulus of continuity ω:T→R\omega:T\to\mathbb{R} with the following two properties.

1. (Monotonicity) ω(s)≤ω(t)\omega(s)\le\omega(t) for all s,t∈Ts,t\in T with s≤ts\le t.

2. (Domination) For all x,y∈Kx,y\in K and every t∈Tt\in T with d(x,y)≤td(x,y)\le t,

∣f(x)−f(y)∣≤ω(t).|f(x)-f(y)|\le\omega(t).

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