Compact families of noncompact subgroups and clopen partial orders

For a locally compact Hausdorff group \\(G\\), let \\(\\mathcal N(G)\\) be the space of closed noncompact subgroups with the full Vietoris topology. We prove that a compact Hausdorff space \\(X\\) embeds in some \\(\\mathcal N(G)\\) if and only if \\(X\\) admits a partial order with clopen principal ideals. In the affirmative case, \\(G\\) may be chosen to be the discrete abelian group \\(\\mathbb Z\\oplus\\bigoplus_X\\mathbb Z/2\\mathbb Z\\). The quotient of \\([0,\\omega_1]^2\\) obtained by collapsing its closed lower triangle is a scattered compact space that admits no such order. It therefore gives a negative answer to Kourovka Problem 9.47.

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Publication Details

Journal
Zenodo (CERN European Organization for Nuclear Research)
Published
2026-09-19
DOI
https://doi.org/10.5281/zenodo.22838025
Primary Topic
Advanced Topology and Set Theory
Type
preprint
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preprint

Compact families of noncompact subgroups and clopen partial orders

Achyuth Jayadevan
Zenodo (CERN European Organization for Nuclear Research)
Advanced Topology and Set Theory
preprint

Compact families of noncompact subgroups and clopen partial orders

Achyuth Jayadevan
preprint en

Abstract

For a locally compact Hausdorff group \(G\), let \(\mathcal N(G)\) be the space of closed noncompact subgroups with the full Vietoris topology. We prove that a compact Hausdorff space \(X\) embeds in some \(\mathcal N(G)\) if and only if \(X\) admits a partial order with clopen principal ideals. In the affirmative case, \(G\) may be chosen to be the discrete abelian group \(\mathbb Z\oplus\bigoplus_X\mathbb Z/2\mathbb Z\). The quotient of \([0,\omega_1]^2\) obtained by collapsing its closed lower triangle is a scattered compact space that admits no such order. It therefore gives a negative answer to Kourovka Problem 9.47.

Zenodo (CERN European Organization for Nuclear Research)
Manipal Academy of Higher Education (IN)
Advanced Topology and Set Theory
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