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CounterexampleConstruction: AI-generatedVerification: AI-generatedprecheck passaudited 2026-08-16
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Boundedness does not replace pointwise relative compactness for an arbitrary metric target

Statement refuted

The pointwise-relative-compactness hypothesis in Ascoli–Arzelà cannot be weakened to pointwise boundedness for an arbitrary metric target.

Facts & Assumptions

Given: The one-point discrete space X={∗} and the infinite set Y=N with d(m,n)=0 for m=n and d(m,n)=1 otherwise.

[L1]

The general Ascoli theorem requires pointwise relative compactness, not merely pointwise boundedness (General Ascoli theorem for locally compact Hausdorff domains and metric targets).

[L3]

A metric on a set X is a function d:X×X→R such that for all x,y,z∈X: (M1) d(x,y)=0 if and only if x=y; (M2) d(x,y)=d(y,x); (M3) d(x,z)≤d(x,y)+d(y,z) (Metric space: d(x,y)=0 iff x=y, symmetry, and the triangle inequality; pseudometric and ultrametric).

[L4]

Compact-open subbasic sets are S(K,V)={f:f[K]⊆V} (The compact-open topology on C(X,Y) for arbitrary topological spaces).

Counterexample

technique · direct
1.1L2L3

The function d satisfies the three axioms of [L3]: (M1) holds because d(m,n)=0 was defined to mean m=n; (M2) holds because the defining cases are symmetric in m,n; and for (M3), if d(m,p)=0 the inequality is trivial, while if d(m,p)=1 then m≠p, so n differs from at least one of m,p and the right side is at least 1. So d is a metric. Its metric topology is discrete because B(n,1)={n}.

1.2given

Every map X→Y is constant. The whole family F=C(X,Y) is equicontinuous, and F(∗)=Y is bounded because it lies in the radius-2 ball about 0.

1.3L4

Evaluation at ∗ is a bijection F→Y. By [L4], the inverse image of each open V⊆Y is S({∗},V), so evaluation is a homeomorphism for the compact-open topology.

2.1L1L2step 1.2step 1.3∎

The open cover {{n}:n∈N} of the infinite discrete space Y has no finite subcover. Thus Y and hence F are not compact, while the family is equicontinuous and pointwise bounded. Moreover F(∗)‾=Y is not compact, displaying exactly the missing hypothesis in [L1].

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