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Nice refinements exist and are regular but not Lindelof

Statement

Every ordered fundamental space has a nice refinement (ω1,T~) satisfying (⋆α) for every ω≤α<ω1. The refinement is separable, locally compact, locally countable, zero-dimensional, regular, and Hausdorff, but it is not Lindelöf. Here locally countable means that every point has a countable neighbourhood.

Facts & Assumptions

Given: An ordered fundamental space (ω1,T) and ZFC.

[F1]

Ordered fundamental spaces and nice refinements gives the rings, intermediate topologies, model chain, metrics, nice-refinement clauses, the families Γα(S,ν), and (⋆α).

[F2]

A compact topological space is one whose every open cover has a finite subcover, and a compact subset carries the intrinsic subspace topology (Open cover, subcover, and compact topological space; a compact subset is a subspace that is compact in its own right).

[F3]

A compact metric space is totally bounded: at every positive radius it has a finite net (A compact metric space is complete and totally bounded, and neither implication uses any choice principle).

[F4]

A space is locally compact when every point has a compact neighbourhood (Locally compact topological space: every point has a compact neighbourhood; and what this says in a metric space).

[F6]
[F7]

Countable elementary submodels and their collapses supplies a countable elementary hull containing any prescribed countable parameter set.

[F8]

Transfinite recursion constructs the model chain and the later stage-by-stage refinement from their functional successor and limit clauses.

[F9]

The Axiom of Choice supplies the simultaneous recursive choices of hulls, metrics, finite nets, ring points, and compact clopen enlargements.

Proof

technique · transfinite and finite recursion
1.1F1F7F8F9givenbaseih

Construct the required continuous chain ⟨Mα:α<ω1⟩ explicitly. Start with an F7 hull containing the ordered fundamental space and its base assignment. At a successor, apply F7 to the countable set Mα∪{Mα,α} and the fixed parameters; at a countable limit take the union of the earlier increasing elementary chain, which is countable and elementary by the Tarski--Vaught test. F8 performs this recursion, and F9 makes the simultaneous hull choices. Now choose the compatible metrics dα allowed by F1 and F9. For α<ω put Vnα={α}. Inductively suppose the Vnξ and Hℓξ have been constructed for every ξ<α, and that the resulting topology below α is locally compact and zero-dimensional.

2.1F1F7F9step 1.1

Fix ω≤α<ω1. The countable model Mα+1 contains only countably many sets S satisfying ∣S∣≤ℵ0 and S⊆K(α,T~↾α); list them as ⟨Sν:ν<ω⟩, repeating every such S infinitely often. This collection is nonempty because it contains S=∅, and that particular S makes the later star implication vacuous.

3.1F1F2step 1.1step 2.1ih

Recursively suppose Hℓα is fixed for ℓ<ν and put Aν=⋃ℓ<νHℓα. This is a finite union of compact sets, hence compact. Every K∈Γα(Sν,ν) lies in Aν∪(Wνα∩α): the first ν ring intersections lie in their Hℓα, while all remaining points lie in the nested Wνα.

4.1F1F3F9step 3.1

The family Γα(Sν,ν) has a finite Hausdorff 2−ν-net Pν chosen from itself. Indeed, by [F3] choose a finite ε-net of the compact metric set Aν, where 2ε<2−ν. Partition the remaining tail into the outer ring Rν=(Wνα∖Wν+1α)∩(α+1) and the deeper tail Tν=Wν+1α∩(α+1). Record for each K which finitely many ε-cells meet K∩Aν, whether K meets Rν, and whether it meets Tν. There are finitely many records. Two compacta with the same record are within Hausdorff distance at most 2−ν: match their points in Aν through a common recorded cell; points in Rν are within its internal diameter 2−ν−3; and points in Tν are within 2−ν by the triangle inequality, since every such point has distance at most 2−ν−1 from α. The two tail-incidence bits ensure that each required matching part is nonempty. Choosing one member for each realized record gives Pν; if Γα(Sν,ν)=∅, take Pν=∅.

5.1F1F2F9step 1.1step 4.1

Let Qν be the union of J∩(Wνα∖Wν+1α) over J∈⋃μ≤νPμ. It is a compact subset of the relatively clopen nonempty ν-th ring. Local compactness and the clopen base below α cover Qν by finitely many compact clopen sets lying in that ring; their union is compact clopen. If this union is empty, choose a ring point and one compact clopen neighbourhood of it in the ring. Call the resulting nonempty compact clopen set Hνα; it contains Qν.

6.1F1step 4.1step 5.1

For J∈Pν, membership in Γα(Sν,ν) already puts its intersections with rings ℓ<ν inside Hℓα. For ℓ≥ν, the stage-ℓ definition includes Pν in ⋃μ≤ℓPμ, so Hℓα absorbs the ℓ-th ring intersection of J. Hence Pν⊆Γα(Sν,ω).

6.2F1F2step 5.1

Put Vmα={α}∪⋃ℓ≥mHℓα. Its maximum is α and its part below α is open. It is compact in the intermediate topology: an intermediate-open cover has a member containing α, hence contains some WNα∩(α+1) and all Hℓα for ℓ≥N; only finitely many earlier compact Hℓα remain. It is therefore intermediate-closed, and [F1] makes the Vmα a clopen local base in the final refinement. The same argument for a final-open cover, now using a contained VNα, proves that Vmα is compact in the final topology.

7.1F1step 2.1step 4.1step 6.1

Whenever Sν=S, steps 4.1 and 6.1 give ∀K∈Γα(S,ν) ∃J∈Γα(S,ω) dα(K,J)≤2−ν. Every relevant S occurs infinitely often, so (⋆α) holds.

8.1F1step 1.1step 2.1step 3.1step 4.1step 5.1step 6.1step 6.2step 7.1discharge-induction

Steps 2.1--7.1 together with step 6.2 perform the successor construction at every countable α, while limit stages take the accumulated earlier data. Transfinite induction therefore produces a nice refinement satisfying every (⋆α).

9.1F1F4F6step 6.2step 8.1

Every Vmα is compact by step 6.2, so [F4] gives local compactness. It lies in α+1, a countable ordinal, so it is a countable neighbourhood and the space is locally countable. The base is clopen and Hausdorff by [F1], hence zero-dimensional and regular by [F6].

9.2F1step 5.1step 8.1

The set ω remains dense. Inductively, every nonempty relatively open Hℓα⊆α meets the already dense ω, so every basic tail Vmα meets ω; for α<ω the singleton Vmα itself lies in ω. Thus the refinement is separable.

9.3F1F5step 6.2step 8.1

Every initial segment α is open: if ξ<α, then each basic neighbourhood Vmξ is contained in ξ+1⊆α. Consequently U={α:0<α<ω1} is an open cover of ω1. A countable subfamily has countable supremum δ<ω1 and its union is contained in δ, so it misses δ. By [F5] the refined space is not Lindelöf.

10.1F8F9step 2.1step 4.1step 7.1step 8.1step 9.1step 9.2step 9.3discharge-induction∎

Combining steps 7.1 and 8.1--9.3 gives all asserted properties. The empty Γ cases were handled in steps 2.1 and 4.1, ν=0 has A0=∅, the finite ordinals use singleton bases, and F9 records every nonempty simultaneous choice.

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