New bounds on the cardinality of Hausdorff spaces and regular spaces

Carlson · Acta Mathematica Hungarica · 2023 · set-theoretic-topology · 10.1007/s10474-023-01331-9 · record carlson2023-new-bounds-on-the-cardinality-of-h
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Where this record's text came from. faceted 2026-08 by reading the locally held text (pdftext); every evidence string verified as an exact substring of that text before merging. The paper also contains explicit negative results (Example 3.11 shows ψ(X) cannot be replaced by dψc(X)wψc(X) in the Arhangel'skii-Šapirovskii bound, and that the bound of [13] fails under that substitution). Those are counterexamples limiting how far the new invariants can be pushed rather than a refutation of prior published claims, so no contradicts relation was recorded and claim type stays existence. Mangled diacritics ("ˇSapirovski˘ı", "Arhangel′ski˘ı") are extraction artefacts preserved verbatim in the quotes. faceted 2026-08 by reading the locally held text (pdftext); every evidence string verified as an exact substring of that text before merging. The paper also contains explicit negative results (Example 3.11 shows ψ(X) cannot be replaced 2026-08-14 schema-0.3 pass: every quote verified as an exact substring of the staged text and anchored at merge time; concept terms verified present; every edge endpoint an existing node.
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Tags on this paper

Every tag the record carries, each linking to every other paper in the corpus that carries it. A tag appears here whether or not its words occur in the article — the spans below are only the ones a sentence in this paper licenses. Of the 5 concept tags on this paper, all 5 are grounded in a quoted sentence.
separation axiom closing-off argument difficult pseudocharacter cardinal function bounding |X| by the family of regular open sets and then bounding that family by a closing-off argument on density, so the pseudocharacter can be weakened without loss constructing an injection of the space into a family of small subsets indexed by weak closed pseudobases, which transfers existing bounds from ψc(X) to wψc(X) weak closed pseudocharacter wψc(X) dense closed pseudocharacter dψc(X) quasiregular space homogeneous space C-saturated subset weak tightness wt(X) regular open set mathematical-proof

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relation to other workclaim_relationshow this paper stands to another: agrees, contradicts, extends, qualifies
claim typeclaim_typethe shape of the claim: existence, mechanism, method, critique, prediction, null
concept edgeconcept_edgesan asserted relation between two concepts, with the sentence asserting it
conceptconceptsan idea the paper works with, quoted where the paper uses it
content tagcontent_tagsa finding, definition, quantity, statistic or variable named in the paper
limitationlimitationswhat the paper says its evidence cannot support
mechanismmechanismshow the reported effect is said to come about
methodmethodhow the paper produced its evidence
phenomenonphenomenonthe effect or regularity under study
sensesensesa distinct use of a term, quoted where the paper fixes it

The paper

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facettagfieldconf.whose words
claim_typeexistence
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claim_typethe paper's words
phenomenoncardinality bounds for Hausdorff and regular spaces
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phenomenonthe paper's words
methodmathematical-proof
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methodthe paper's words
mechanismsmechanisms
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mechanisms[0]the tagger's words
mechanismsmechanisms
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mechanisms[1]the tagger's words
limitationsWhether the wψc-based bound holds for compact homogeneous Hausdorff spaces is left unresolved by the paper's own examples.
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limitations[0]the tagger's words
limitationsThe corresponding bound for general (non-quasiregular) Hausdorff spaces is posed as an open question rather than proved.
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limitations[1]the tagger's words
limitationsNo separation axiom is assumed anywhere, so every result's hypotheses have to be read locally rather than inherited from the word 'space'.
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what this limits: any theorem quoted from this paper as being about 'a space': the author makes no global assumption of any separation axiom, so the hypotheses stated in each individual result are the only ones in force
limitations[2]1.0the tagger's words
limitationsThe novelty claim for the central new bound is hedged, and the hedge is the author's.
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what this limits: the priority of the bound on cardinality by density and weak closed pseudocharacter: the author says it appears to be new in the literature rather than asserting it is, and notes a related result was already known for regular spaces
limitations[3]1.0the tagger's words
limitationsTwo theorems reach the same kind of conclusion by different sophisticated arguments and the author cannot unify them.
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what this limits: treating the two density bounds here as one result: the author states the proofs are different sophisticated closing-off arguments and asks, as an open question, whether a common proof exists
limitations[4]1.0the tagger's words
limitationsThe substitution the paper's new functions permit is shown not to extend to the compact case.
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what this limits: generalising the replacement of pseudocharacter by the new weaker functions: compact examples are given showing the substitution fails in the classical bound for the cardinality of a compact Hausdorff space
limitations[5]1.0the tagger's words
conceptsseparation axiom
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concepts[0]the paper's words
conceptsclosing-off argument
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concepts[1]the paper's words
conceptsdifficult
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also in: difficult
concepts[2]the paper's words
conceptspseudocharacter
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concepts[3]the paper's words
conceptscardinal function
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concepts[4]the paper's words
senses'space' - WITH NO SEPARATION AXIOM ATTACHED: in much of the topology literature the bare word silently carries Hausdorff or regular or T1 depending on the author, so this paper states that it assumes none of them globally and every hypothesis is local to its theorem
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senses[0]1.0the tagger's words
senses'difficult' - A CLASSIFICATION OF PROOF TECHNIQUE, ATTRIBUTED: the word is scare-quoted and referred to a particular author and section, and what earns it is that the proofs rely on sophisticated closing-off arguments, not that the statements are hard to understand
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senses[1]1.0the tagger's words
sensespseudocharacter and closed pseudocharacter - THE SAME NUMBER ONLY UNDER A HYPOTHESIS: they are ordered for any Hausdorff space and become equal when the space is regular, so whether a bound stated with one is a bound stated with the other depends on a separation axiom
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senses[2]1.0the tagger's words
claim_relationsextends → the cardinality bounds of Šapirovskiĭ [22] and Sun [23]
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claim_relations[0]the tagger's words
claim_relationsextends → Gotchev, Tkachenko and Tkachuk's bound πw(X)ot(X)ψc(X) [13]
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claim_relations[1]the tagger's words
claim_relationsextends → the Bella-Cammaroto bound d(X)t(X)ψc(X) [1] as improved by the author in [6]
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claim_relations[2]the tagger's words
concept_edgescardinal function —contrasted-with→ pseudocharacter
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strength of the relation: 1
concept_edges[1]the tagger's words
content_tagsFor a quasiregular Hausdorff space, |X| ≤ πχ(X)^(c(X)wψc(X)).
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content_tags.findings[0]the tagger's words
content_tagsFor any Hausdorff space, |X| ≤ πχ(X)^(c(X)dψc(X)wψc(X)).
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content_tags.findings[1]the tagger's words
content_tagsEvery Hausdorff space satisfies |X| ≤ 2^(d(X)wψc(X)).
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content_tags.findings[2]the tagger's words
content_tagsThe density of any Hausdorff space is at most πχ(X)^(c(X)dψc(X)).
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content_tags.findings[3]the tagger's words
content_tagsThe cardinality of a Hausdorff space is at most |RO(X)| raised to wψc(X).
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content_tags.findings[4]the tagger's words
content_tagsCompact examples show that ψ(X) cannot be replaced by dψc(X)wψc(X) in the bound 2^ψ(X) for compact Hausdorff spaces.
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content_tags.findings[5]the tagger's words
content_tagsA quasiregular Hausdorff space whose cellularity is at most the continuum and whose π-character is countable has at most 2^c points.
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content_tags.findings[6]the tagger's words
content_tagsFor a Hausdorff space, |X| ≤ d(X)^(wt(X)wψc(X)).
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content_tags.findings[7]the tagger's words
content_tagsIn a homogeneous Hausdorff space the new dense closed pseudocharacter collapses to the ordinary closed pseudocharacter, so it yields no improvement there.
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content_tags.findings[8]the tagger's words
content_tags2^wψc(X) is not a cardinality bound for all σ-compact homogeneous spaces.
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content_tags.findings[9]the tagger's words
content_tagsweak closed pseudocharacter wψc(X)
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content_tags.definitions[0]the tagger's words
content_tagsdense closed pseudocharacter dψc(X)
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content_tags.definitions[1]the tagger's words
content_tagsquasiregular space
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content_tags.definitions[2]the tagger's words
content_tagshomogeneous space
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content_tags.definitions[3]the tagger's words
content_tagsC-saturated subset
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content_tags.definitions[4]the tagger's words
content_tagsweak tightness wt(X)
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content_tags.definitions[5]the tagger's words
content_tagsregular open set
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content_tags.definitions[6]the tagger's words

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