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PropositionStatement: Literature-sourcedProof: AI-adaptedprecheck passjudge pass (deepseek-v4-pro + gpt-5.6-terra)audited 2026-08-16
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On a nonempty compact metric domain, the compact-open topology is the uniform topology

Statement

Let X be a nonempty compact metric space and let Y be a metric space. On C(X,Y), the published compact-open topology is equal to the topology of uniform convergence.

Facts & Assumptions

Given: A nonempty compact metric space X and a metric space Y.

[L1]

For metric domain and target, the compact-open topology equals the topology of compact convergence (For a metric domain and a metric target the compact-open topology on C(X,Y) is the topology of compact convergence).

[L2]

Compact convergence has basic sets BK(f,ε) requiring d(f(x),g(x))<ε for every x in a compact K; and by its clause (U3), for f,g∈C(X,Y) and a nonempty compact K⊆X the value max⁡x∈Kd(f(x),g(x)) exists (The topology of compact convergence on C(X,Y) for metric X and Y: uniform convergence on each compact subset of X).

[L3]

The uniform topology is induced by ρˉ(f,g)=sup⁡x∈Xmin⁡{d(f(x),g(x)),1} (Uniform convergence, and the topology of uniform convergence: the metric topology of the uniform metric on YX and on C(X,Y)).

Proof

technique · direct
1.1L2L3

A uniform ball of radius 0<δ<min⁡{ε,1} about f is contained in every compact-convergence basic set BK(f,ε), because its inequality holds at every point of X.

2.1L2L3step 1.1

Conversely take 0<ε<1 and g∈BX(f,ε), so d(f(x),g(x))<ε at every x∈X. Since X is nonempty and compact, clause (U3) of [L2] makes M:=max⁡x∈Xd(f(x),g(x)) exist, and M<ε<1 because the maximum is one of the values. Hence ρˉ(f,g)=sup⁡x∈Xmin⁡{d(f(x),g(x)),1}=M<ε, so g lies in the uniform ball of radius ε. For the reverse inclusion note that step 1.1 is stated for a radius strictly below ε and so cannot be instantiated at ε itself; argue directly instead: if ρˉ(f,g)<ε<1 then min⁡{d(f(x),g(x)),1}<ε<1 at every x, so d(f(x),g(x))<ε at every x and g∈BX(f,ε). So BX(f,ε) is exactly that uniform ball.

3.1L1step 1.1step 2.1∎

Steps 1.1 and 2.1 show that compact convergence and uniform convergence induce the same topology. By [L1], that topology is also the compact-open topology.

Depends on

Used by

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Sources