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TheoremStatement: Literature-sourcedProof: AI-generatedprecheck passaudited 2026-09-01
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Inverse limits of finite discrete groups are Hausdorff and totally disconnected, and compact assuming Choice

Statement

An inverse limit of finite discrete groups is Hausdorff and totally disconnected. Assuming the Axiom of Choice, it is also compact.

Facts & Assumptions

Given: An inverse system of finite discrete groups and its inverse limit L; for the compactness clause, also the Axiom of Choice.

[L1]

The inverse limit is a closed topological subgroup of the ambient product (The inverse limit of finite discrete groups is a closed topological subgroup of the full product).

[F1]
[L2]

Arbitrary products of Hausdorff spaces are Hausdorff (Arbitrary products preserve T0, T1, and Hausdorffness).

[L4]

Traces of finite-coordinate cylinders form a basis for the inverse-limit topology (The inverse limit of finite groups carries the subspace topology from the product of discrete factors).

Proof

technique · direct
1.1

Every finite discrete group Gi is Hausdorff because distinct points have disjoint singleton neighbourhoods. It is compact because an open cover has one member containing each point and [F3] makes those finitely many choices; the selected family is a finite subcover.

F2F3given
2.1

By [L2] and step 1.1, the full product is Hausdorff. Intersecting two disjoint ambient neighbourhoods with L shows directly that the subspace L is Hausdorff. Under the Axiom of Choice, [F1] and step 1.1 make the product compact; since L is closed by [L1], [L3] makes L compact.

F1L1L2L3step 1.1given
3.1

By [L4], fixing finitely many coordinates gives a neighbourhood basis in L. Because every factor is discrete, each such cylinder trace is clopen. In particular, for xL and an index i, the set Ci(x):={zL:zi=xi} is clopen.

L4step 2.1given
4.1

If a connected subset CL contained distinct points xy, they would differ in some coordinate i. Then CCi(x) and CCi(x) would be disjoint nonempty subsets that are both open and closed in C, a contradiction. Hence every nonempty connected subset of L is a singleton, so L is totally disconnected.

step 3.1contradiction: connected sets cannot be split by a clopen separation

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