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An atom-free symmetric model has countable pairs without choice
Statement
The atom-free socks symmetric extension is a ZF model containing the countable family of pairs of sets of reals but no choice function; fails directly in pure ZF.
Facts & Assumptions
Given: The hypotheses, objects, and conventions in the Statement.
The atom-free socks symmetric system gives the pair sequence and coordinate group.
Symmetry lemma for forcing automorphisms transports a choice decision under a swap.
Choice for pairs and countable finite choice identifies the failed principle.
Proof
F1 makes every and the indexed sequence HS, so the symmetric ZF model regards its range as countable pairs. Distinct-coordinate dense sets ensure .
Suppose for a supported choice name . Choose outside the finitely many supported pair indices and strengthen to deciding, say, . Choose a block swap at that fixes the support. By additionally permuting unused -coordinates in that block, arrange that and its image have disjoint moved domains and hence are compatible.
F2 says the image condition forces the same to equal . A common extension then forces the two distinct mates equal, contradiction. Therefore no choice function exists and F3 fails. The construction is in pure ZF and uses neither the Recorded transfer result nor AC.
Depends on
Used by
- A coordinate swap defeats an atom-free sock choice Example
- Every symmetric submodel satisfies Choice False statement
Dependency tree · two levels
10 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Jech, The Axiom of Choice, Lemmas 5.17–5.19 and Theorem 5.20, pp. 69–71 (standard reference, not scraped)