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TheoremStatement: Literature-sourcedProof: AI-adaptedPipeline-generatedjudge pass (gpt-5.6-terra)audited 2026-09-14
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CH implies that an S-space exists

Statement

ZFC plus the continuum hypothesis proves that there is a strong S-space.

Facts & Assumptions

Given: ZFC and CH.

[F1]

Cantor and Baire sequence spaces and coordinate codings gives Cantor space C=2ω its clopen cylinder topology; it is separable, has no isolated points, and the finite words form a countable cylinder base.

[F2]

Over ZFC, The continuum hypothesis, and what this page does not prove identifies the cardinality of P(ω), and hence of its characteristic-function copy 2ω, with 1.

[F3]

Ordered fundamental spaces and nice refinements defines ordered fundamental spaces, their strict clopen local bases, nice refinements, and the condition (α).

[F4]

Nice refinements exist and are regular but not Lindelof gives every ordered fundamental space a star-satisfying nice refinement that is regular and Hausdorff but not Lindelöf.

[F5]

Under CH, CH makes the nice refinement strongly hereditarily separable makes every nonempty finite power of such a refinement hereditarily separable.

[F7]

L-spaces, S-spaces, and strong S-spaces defines an S-space and a strong S-space and excludes the zeroth power from the latter definition.

[F8]

The Axiom of Choice supplies the ZFC well-orderings and bijections in the initial reindexing and propagates all choices made in [F4] and [F5].

Proof

technique · direct construction and assembly
1.1

Let DC consist of the binary sequences of finite support. Sending a finite support a to ka2k enumerates D bijectively by ω, and D meets every cylinder: extend the prescribed finite word by zeros. The map xyx, where yx(2k)=x(k) and yx(2k+1)=1, injects C into CD. Inclusion gives the reverse injection, so Cantor--Bernstein and [F2] give CD=C=1.

F1F2
2.1

Choose a bijection b:ω1C with b[ω]=D: use the enumeration from step 1.1 on ω and a bijection ω1ωCD on the complements. Pull the cylinder topology back along b. It is Hausdorff, zero-dimensional, separable, and second countable, with ω dense. Every nonempty open set contains a cylinder. For a word s, prefixing s defines a bijection from C onto its cylinder Ns, so every nonempty open set has size 1.

F1F2F8step 1.1
3.1

For α<ω1 and n<ω, put Wnα=b1[Nb(α)n]. Then W0α=ω1, each Wnα is clopen, and these sets form a local base at α. They are strictly decreasing because the next unrestricted bit can be changed, and their intersection is {α}. Consequently step 2.1 with these bases is a second-countable ordered fundamental space in the exact sense of [F3].

F1F3step 2.1
4.1

Apply [F4] to obtain a nice refinement X=(ω1,T~) satisfying every (α). The space X is regular and Hausdorff and is not Lindelöf.

F4step 3.1
5.1

Fix 0<n<ω. By [F5], Xn is hereditarily separable. By [F6], Xn is regular and Hausdorff.

F5F6step 4.1
5.2

The power Xn is not Lindelöf. Otherwise let U be an open cover of X with no countable subcover, supplied by step 4.1. The inverse images {π01[U]:UU} form an open cover of Xn. Lindelöfness would give countably many of them covering Xn. The projection π0:XnX is onto: fill all coordinates other than 0 with the fixed point 0ω1. Hence the corresponding countable members of U would cover X, a contradiction.

F4F6step 4.1contradiction
6.1

Steps 5.1 and 5.2 show that every positive finite power of X is regular, Hausdorff, hereditarily separable, and not Lindelöf. Thus every such power is an S-space, and [F7] says exactly that X is a strong S-space. The case n=1 is included, while n=0 is deliberately excluded. The construction is nonempty because its underlying set is ω1. Step 2.1 is the only new choice in this assembly; [F8] also propagates the ZFC choices in the two refinement suppliers.

F7F8step 2.1step 5.1step 5.2

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