A proper permutation group transitive on countable sections

Let \\(\\Omega\\) be a countably infinite set. We prove that a proper transitive subgroup of \\(\\mathrm{Sym}(\\Omega)\\) acts transitively on partitions of \\(\\Omega\\) into countably many infinite parts, answering Kourovka Problem 9.41(c) without an additional set-theoretic hypothesis. After identifying \\(\\Omega\\) with \\(\\mathbb{N}\\), we use the group of permutations that are increasing on the parts of a finite partition. Every prescribed matching of two countable partitions is realized by a permutation increasing on two complementary subsets. The proof applies the relational Schröder–Bernstein theorem to increasing embeddings that preserve the part labels.

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Publication Details

Journal
Zenodo (CERN European Organization for Nuclear Research)
Published
2026-09-18
DOI
https://doi.org/10.5281/zenodo.22820555
Primary Topic
Advanced Topology and Set Theory
Type
preprint
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preprint

A proper permutation group transitive on countable sections

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

A proper permutation group transitive on countable sections

Achyuth Jayadevan
preprint en

Abstract

Let \(\Omega\) be a countably infinite set. We prove that a proper transitive subgroup of \(\mathrm{Sym}(\Omega)\) acts transitively on partitions of \(\Omega\) into countably many infinite parts, answering Kourovka Problem 9.41(c) without an additional set-theoretic hypothesis. After identifying \(\Omega\) with \(\mathbb{N}\), we use the group of permutations that are increasing on the parts of a finite partition. Every prescribed matching of two countable partitions is realized by a permutation increasing on two complementary subsets. The proof applies the relational Schröder–Bernstein theorem to increasing embeddings that preserve the part labels.

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