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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 be a local homeomorphism. If is nonempty, compact, and Hausdorff and is connected and Hausdorff, then is a finite-sheeted covering map.
Facts & Assumptions
Given: A local homeomorphism satisfying the hypotheses in the Statement, and a point .
Under these hypotheses, 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).
Distinct points in a Hausdorff space have disjoint open neighbourhoods (Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not).
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).
A closed subspace of a compact space is compact (A closed subspace of a compact space is compact, and a finite union of compact subspaces is compact).
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
List the finite fibre as with by [L1]. Using [F1] finitely many times and [F2] for the finite selections, choose pairwise disjoint open neighbourhoods of the . Intersect each with a local-homeomorphism chart at ; its image is still an open neighbourhood of . Let be the finite intersection of these images, and replace each chart by its inverse image of . We obtain pairwise disjoint open sets with a homeomorphism.
The set is closed and therefore compact by [F3]. Its image is compact by [F4] and closed in by [F5]. No point of the fibre over lies in , so . Hence is an open neighbourhood of .
Put . Each is open and is a homeomorphism. If then , so lies in exactly one and hence in exactly one . Thus is the disjoint union of the finitely many . Since was arbitrary and is surjective by [L1], [F6] makes a finite-sheeted covering.
Depends on
- A local homeomorphism from a nonempty compact space to a connected Hausdorff space is surjective with finite fibres
- Local homeomorphisms
- Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not
- A continuous image of a compact space is compact; a continuous real-valued map on a nonempty compact space attains a maximum and a minimum; and a continuous bijection from a compact space to a Hausdorff space is a homeomorphism
- In a Hausdorff space a point and a disjoint compact set, and two disjoint compact sets, have disjoint open neighbourhoods; hence every compact subset is closed, and in a compact Hausdorff space the compact subsets are exactly the closed ones
- A closed subspace of a compact space is compact, and a finite union of compact subspaces is compact
- Every natural-number-indexed list of nonempty sets has a choice function on its family of values
- Covering maps, evenly covered neighbourhoods, fibres, sheets, and trivial coverings
Used by
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Sources
- J. Peter May, A Concise Course in Algebraic Topology, Chapter 3, Problem 4 (standard reference, not scraped)