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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.
Cantor and Baire sequence spaces and coordinate codings gives Cantor space its clopen cylinder topology; it is separable, has no isolated points, and the finite words form a countable cylinder base.
Over ZFC, The continuum hypothesis, and what this page does not prove identifies the cardinality of , and hence of its characteristic-function copy , with .
Ordered fundamental spaces and nice refinements defines ordered fundamental spaces, their strict clopen local bases, nice refinements, and the condition .
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.
Under CH, CH makes the nice refinement strongly hereditarily separable makes every nonempty finite power of such a refinement hereditarily separable.
Product projections are continuous for the product topology (The product set of functions choosing a point in each factor, the projections, the box topology, and the product topology as the initial topology of the projections; the empty product is a one-point space); products of regular spaces are regular (Arbitrary products of regular spaces are regular), and products of Hausdorff spaces are Hausdorff (Arbitrary products preserve , , and Hausdorffness).
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.
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
Let consist of the binary sequences of finite support. Sending a finite support to enumerates bijectively by , and meets every cylinder: extend the prescribed finite word by zeros. The map , where and , injects into . Inclusion gives the reverse injection, so Cantor--Bernstein and [F2] give .
Choose a bijection with : use the enumeration from step 1.1 on and a bijection on the complements. Pull the cylinder topology back along . It is Hausdorff, zero-dimensional, separable, and second countable, with dense. Every nonempty open set contains a cylinder. For a word , prefixing defines a bijection from onto its cylinder , so every nonempty open set has size .
For and , put . Then , each is clopen, and these sets form a local base at . They are strictly decreasing because the next unrestricted bit can be changed, and their intersection is . Consequently step 2.1 with these bases is a second-countable ordered fundamental space in the exact sense of [F3].
Apply [F4] to obtain a nice refinement satisfying every . The space is regular and Hausdorff and is not Lindelöf.
Fix . By [F5], is hereditarily separable. By [F6], is regular and Hausdorff.
The power is not Lindelöf. Otherwise let be an open cover of with no countable subcover, supplied by step 4.1. The inverse images form an open cover of . Lindelöfness would give countably many of them covering . The projection is onto: fill all coordinates other than with the fixed point . Hence the corresponding countable members of would cover , a contradiction.
Steps 5.1 and 5.2 show that every positive finite power of is regular, Hausdorff, hereditarily separable, and not Lindelöf. Thus every such power is an S-space, and [F7] says exactly that is a strong S-space. The case is included, while is deliberately excluded. The construction is nonempty because its underlying set is . Step 2.1 is the only new choice in this assembly; [F8] also propagates the ZFC choices in the two refinement suppliers.
Depends on
- L-spaces, S-spaces, and strong S-spaces
- Ordered fundamental spaces and nice refinements
- Cantor and Baire sequence spaces and coordinate codings
- The continuum hypothesis, and what this page does not prove
- Nice refinements exist and are regular but not Lindelof
- CH makes the nice refinement strongly hereditarily separable
- The product set $\prod_{i \in I} X_i$ of functions choosing a point in each factor, the projections, the box topology, and the product topology as the initial topology of the projections; the empty product is a one-point space
- Arbitrary products of regular spaces are regular
- Arbitrary products preserve $T_0$, $T_1$, and Hausdorffness
- The Axiom of Choice
Used by
Dependency tree · two levels
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Sources
- Hart–Kunen, Ultra Strong S-Spaces, Definition 4.1 discussion and Corollary 4.18, printed pp. 95 and 103 (standard reference, not scraped)