Alphabeta Math
TheoremStatement: Literature-sourcedProof: AI-adaptedPipeline-generatedjudge pass (gpt-5.6-terra)audited 2026-09-14
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  • Literature-sourced: the exact statement appears in a cited source; only wording and notation differ.
  • AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
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The basic Cohen set has no countably infinite subset

Statement

Every map from ω into A in the basic Cohen model has finite range; therefore no injection ωA, no countably infinite subset, and no enumeration of A exists.

Facts & Assumptions

Given: The hypotheses, objects, and conventions in the Statement.

[F1]

The Cohen reals form a symmetric set but their enumeration is not symmetric states the exact coordinate action πa˙n=a˙πn, pairwise distinctness, and infinitude of A.

[F2]

Symmetry lemma for forcing automorphisms transports decisions under swaps.

Proof

1.1

Let pf˙:ωˇA˙, and enlarge a finite support E of f˙ to support p. If some qp forces f˙(i)=a˙n with nE, choose m outside E{n} and outside the first-coordinate support of q. Let π swap n,m. Then πf˙=f˙, πp=p, and F2 gives πqf˙(i)=a˙m.

F1F2
2.1

The conditions q and πq agree wherever both are defined: the m coordinate was fresh and all other moved coordinates are fixed. Their union is a common extension forcing a˙n=a˙m, contradicting F1. Hence no extension of p can put a value outside {an:nE}, and density of decisions yields pranf˙{a˙n:nE}.

F1step 1.1
3.1

Thus every such map has finite range, so none is injective or enumerates the infinite set A. A countably infinite subset would, by its meaning in ZF, carry a bijection from ω and hence an injection into A, also impossible. Fresh indices were chosen only from explicit complements of finite sets; no AC is used.

step 2.1

Depends on

Used by

Dependency tree · two levels

7 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