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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 ; for the compactness clause, also the Axiom of Choice.
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).
Assuming the Axiom of Choice, arbitrary products of compact spaces are compact (Tychonoff's theorem: an arbitrary product of compact spaces is compact in the product topology, assuming the Axiom of Choice).
Compactness and Hausdorffness are the usual topological notions (Open cover, subcover, and compact topological space; a compact subset is a subspace that is compact in its own right, Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not).
Arbitrary products of Hausdorff spaces are Hausdorff (Arbitrary products preserve , , and Hausdorffness).
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).
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).
Finite choice is available in ZF (Every natural-number-indexed list of nonempty sets has a choice function on its family of values).
Proof
Every finite discrete group 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.
By [L2] and step 1.1, the full product is Hausdorff. Intersecting two disjoint ambient neighbourhoods with shows directly that the subspace is Hausdorff. Under the Axiom of Choice, [F1] and step 1.1 make the product compact; since is closed by [L1], [L3] makes compact.
By [L4], fixing finitely many coordinates gives a neighbourhood basis in . Because every factor is discrete, each such cylinder trace is clopen. In particular, for and an index , the set is clopen.
If a connected subset contained distinct points , they would differ in some coordinate . Then and would be disjoint nonempty subsets that are both open and closed in , a contradiction. Hence every nonempty connected subset of is a singleton, so is totally disconnected.
Depends on
- The inverse limit of finite discrete groups is a closed topological subgroup of the full product
- Tychonoff's theorem: an arbitrary product of compact spaces is compact in the product topology, assuming the Axiom of Choice
- Open cover, subcover, and compact topological space; a compact subset is a subspace that is compact in its own right
- Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not
- Arbitrary products preserve $T_0$, $T_1$, and Hausdorffness
- A closed subspace of a compact space is compact, and a finite union of compact subspaces is compact
- The inverse limit of finite groups carries the subspace topology from the product of discrete factors
- Every natural-number-indexed list of nonempty sets has a choice function on its family of values
Used by
- A profinite group is a topological group isomorphic to an inverse limit of finite discrete groups Definition
- Assuming Choice, a topological group is profinite exactly when it is compact, Hausdorff, and totally disconnected Theorem
- The profinite completion is initial among continuous homomorphisms from G to profinite groups Theorem
Dependency tree · two levels
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Sources
- Brian Osserman, Math 6112 notes on inverse limits and profinite groups (standard reference, not scraped)
- H. W. Lenstra, Profinite groups and Galois groups (standard reference, not scraped)