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PropositionStatement: Literature-sourcedProof: AI-adaptedprecheck passjudge pass (deepseek-v4-pro + gpt-5.6-terra)audited 2026-08-21
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A local homeomorphism from a nonempty compact Hausdorff space to a connected Hausdorff space is a finite-sheeted covering

Statement

Let f:X→Y be a local homeomorphism. If X is nonempty, compact, and Hausdorff and Y is connected and Hausdorff, then f is a finite-sheeted covering map.

Facts & Assumptions

Given: A local homeomorphism f:X→Y satisfying the hypotheses in the Statement, and a point y∈Y.

[L1]

Under these hypotheses, f is surjective and the fibre over every point is finite and nonempty (A local homeomorphism from a nonempty compact space to a connected Hausdorff space is surjective with finite fibres).

[F2]

A finite natural-number-indexed family of nonempty sets has a choice function (Every natural-number-indexed list of nonempty sets has a choice function on its family of values).

[F6]

A covering map is a continuous surjection for which every target point has an open neighbourhood whose full preimage is a disjoint union of open sheets mapped homeomorphically onto that neighbourhood (Covering maps, evenly covered neighbourhoods, fibres, sheets, and trivial coverings).

Proof

technique · direct
1.1L1F1F2

List the finite fibre as f−1(y)={x1,…,xm} with m≥1 by [L1]. Using [F1] finitely many times and [F2] for the finite selections, choose pairwise disjoint open neighbourhoods Ni of the xi. Intersect each Ni with a local-homeomorphism chart at xi; its image is still an open neighbourhood of y. Let W be the finite intersection of these images, and replace each chart by its inverse image of W. We obtain pairwise disjoint open sets Ui with f∣Ui:Ui→W a homeomorphism.

2.1step 1.1F3F4F5

The set K=X∖⋃iUi is closed and therefore compact by [F3]. Its image f[K] is compact by [F4] and closed in Y by [F5]. No point of the fibre over y lies in K, so y∉f[K]. Hence V:=W∖f[K] is an open neighbourhood of y.

3.1step 1.1step 2.1L1F6∎

Put Vi=Ui∩f−1(V). Each Vi is open and f∣Vi:Vi→V is a homeomorphism. If x∈f−1(V) then x∉K, so x lies in exactly one Ui and hence in exactly one Vi. Thus f−1(V) is the disjoint union of the finitely many Vi. Since y was arbitrary and f is surjective by [L1], [F6] makes f a finite-sheeted covering.

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