A bound for the density of any Hausdorff space

Carlson · Topology and its Applications · 2024 · set-theoretic-topology · 10.1016/j.topol.2024.108838 · record carlson2024-a-bound-for-the-density-of-any-hau
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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. Replication status left "untested" even though the paper re-proves two earlier theorems: that is a unified re-derivation of published results, not an independent empirical replication, and I recorded it instead as a replicates claim_relation. Diacritics in "ˇSapirovski˘ı" are PDF-extraction artefacts and are quoted exactly as they appear. The paper closes with three open questions (3.6-3.8) that I treated as future work rather than limitations, keeping the limitation slots for statements about what the result cannot cover. faceted 2026-08 by reading the locally held text (pdftext); every evidence string verified as an exact substring of that text before merging. Replication status left "untested" even though the paper re-proves two earlier theorems 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.
density quasiregular nonquasiregularity degree nq-space Hausdorff space The nonquasiregularity degree simultaneously generalizes quasiregularity and the closed pseudocharacter, so a single inequality in nq(X) specialises to both Šapirovskiĭ's regular-space bound and Sun's Hausdorff bound. quasiregular space nonquasiregularity degree nq(X) weak closed pseudobase weak closed pseudocharacter wψc(X) dense closed pseudocharacter dψc(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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Every tag on this page

claim_relations 3 · claim_type 1 · concept_edges 2 · concepts 5 · content_tags 19 · limitations 5 · mechanisms 1 · method 1 · phenomenon 1 · senses 3

Relations on this page carry a quotation for the claim. Where a relation was separately evidenced — and so tested against the vocabulary for its type — the row says so; none of 2 relations here carry that mark. The rest are the tagger’s reading of the sentence quoted beside them.

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facettagfieldconf.whose words
claim_typeexistence
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claim_typethe paper's words
phenomenondensity bounds for Hausdorff 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
limitationsThe new cardinal function nq(X) is not defined for every topological space, so the main bound does not apply universally.
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limitations[0]the tagger's words
limitationsA compact T1 space is exhibited for which the invariant fails to exist.
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limitations[1]the tagger's words
limitationsThe nq(X) factor cannot simply be dropped: examples are given where the plain bound fails.
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limitations[2]the tagger's words
limitationsThe two classical bounds this paper unifies had complicated proofs that were quite different from one another, which is the gap the paper exists to close.
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what this limits: the relation between Sapirovskii's density bound and Sun's cardinality bound before this paper: Sun's proof is described as a complicated closing-off argument quite different from the proof of the density bound, so the two results were not visibly connected
limitations[3]1.0the tagger's words
limitationsThe class of spaces the theorem covers is named after the invariant being defined on them, so membership is settled by whether the quantity exists rather than by a topological property.
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what this limits: the scope of the main theorem: it applies to nq-spaces, and a space is an nq-space precisely when nq(X) is defined, which is why a compact T1 example without the invariant falls outside it
limitations[4]1.0the tagger's words
conceptsdensity
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also in: density
concepts[0]the paper's words
conceptsquasiregular
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also in: quasiregular
concepts[1]the paper's words
conceptsnonquasiregularity degree
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concepts[2]the paper's words
conceptsnq-space
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also in: nq-space
concepts[3]the paper's words
conceptsHausdorff space
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concepts[4]the paper's words
sensesnonregularity - AS A DEGREE RATHER THAN A DICHOTOMY: instead of a space being regular or not, the paper introduces a cardinal measuring HOW nonquasiregular it is, and the classical bound then holds for every space with that degree as a factor
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senses[0]1.0the tagger's words
sensesquasiregular - THE HYPOTHESIS WEAKENED ONCE BEFORE IT WAS MEASURED: the classical result was stated for regular spaces, then observed to need only quasiregularity and not even Hausdorff, and only afterwards replaced by a degree
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senses[1]1.0the tagger's words
sensesthe degree - SMALL FOR THE SPACES THAT MATTER: nq(X) is countable when the space is quasiregular and bounded by the closed pseudocharacter when it is Hausdorff, which is what stops the new factor from making the bound vacuous
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senses[2]1.0the tagger's words
claim_relationsextends → Šapirovskiĭ's density bound d(X) ≤ πχ(X)c(X) for regular spaces
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claim_relations[0]the tagger's words
claim_relationsextends → Sun's bound πχ(X)c(X)ψc(X) for the cardinality of a Hausdorff space
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claim_relations[1]the tagger's words
claim_relationsreplicates → Šapirovskiĭ's and Sun's theorems (re-derived from one argument)
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claim_relations[2]the tagger's words
concept_edgesHausdorff space —is-a→ nq-space
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strength of the relation: 1
concept_edges[0]the tagger's words
concept_edgesquasiregular —is-a→ nq-space
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strength of the relation: 1
concept_edges[1]the tagger's words
content_tagsThe density of any space for which the nonquasiregularity degree is defined — in particular every Hausdorff space and every quasiregular space — is at most πχ(X) times c(X) times nq(X).
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content_tags.findings[0]the tagger's words
content_tagsIn a Hausdorff space the new cardinal invariant stays small, being bounded above by the closed pseudocharacter.
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content_tags.findings[1]the tagger's words
content_tagsThe bound yields a unified proof of Šapirovskiĭ's density bound for regular spaces and Sun's cardinality bound πχ(X)c(X)ψc(X) for Hausdorff spaces.
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content_tags.findings[2]the tagger's words
content_tagsIt follows that the cardinality of any Hausdorff space is at most πχ(X)c(X)nq(X)wψc(X), which improves Sun's bound because wψc(X) never exceeds ψc(X) in Hausdorff spaces.
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content_tags.findings[3]the tagger's words
content_tagsThe number of regular open sets of an nq-space is likewise bounded by πχ(X)c(X)nq(X).
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content_tags.findings[4]the tagger's words
content_tagsIn a Hausdorff space the nonquasiregularity degree is bounded above by the Lindelöf number.
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content_tags.findings[5]the tagger's words
content_tagsEvery Lindelöf Hausdorff space satisfies d(X) ≤ πχ(X)c(X).
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content_tags.findings[6]the tagger's words
content_tagsTwo spaces are exhibited, one of them Urysohn, whose density strictly exceeds πχ(X)c(X), showing the extra factor nq(X) cannot be dropped from the bound.
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content_tags.findings[7]the tagger's words
content_tagsThe nonquasiregularity degree is not defined for every space: there is a compact T1 space with no nq(X).
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content_tags.findings[8]the tagger's words
content_tagsdensity
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also in: density
content_tags.definitions[0]the tagger's words
content_tagsquasiregular space
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content_tags.definitions[1]the tagger's words
content_tagsnonquasiregularity degree nq(X)
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about this quote: THE QUOTE STOPS BEFORE A FORMULA: the set-theoretic condition that follows 'contains a' is garbled in the extraction ('V ∈V V ⊆ nonempty Gκ-set G = U …'), so the quote ends on the word that introduces it.
content_tags.definitions[2]the tagger's words
content_tagsnq-space
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also in: nq-space
content_tags.definitions[3]the tagger's words
content_tagsweak closed pseudobase
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content_tags.definitions[4]the tagger's words
content_tagsweak closed pseudocharacter wψc(X)
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content_tags.definitions[5]the tagger's words
content_tagsdense closed pseudocharacter dψc(X)
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content_tags.definitions[6]the tagger's words
content_tagsregular open set
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content_tags.definitions[7]the tagger's words
content_tagsnq(X) = ℵ0 — For a quasiregular space the nonquasiregularity degree takes its least possible value, the first infinite cardinal — i.e. quasiregular spaces are exactly the 'countably nonquasiregular' ones.
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content_tags.quantities[0]the tagger's words
content_tagsχ(X) = 2κ and d(X) > cf (22κ ) > 2κ — In van Douwen's example the character equals 2^κ while the density strictly exceeds 2^κ, so density outruns πχ(X)c(X) and the nq factor is unavoidable.
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content_tags.quantities[1]the tagger's words

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