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AD gives the perfect-set property in sequence spaces and the real line
Statement
In ZF+AD every subset of , or is at most countable (admits an injection into ) or contains a compact subspace homeomorphic to . In particular every uncountable such set has a nonempty perfect closed subset. No DC is assumed.
Facts & Assumptions
Perfect-set game strategy dichotomy on Cantor space gives a Cantor copy from an I winning block-game strategy and an injection into from a II winning one.
Cantor and Baire sequence spaces and coordinate codings gives the homeomorphism and sequence coding.
Only the ZF clauses of Dyadic coding supplies coin measure and its completed Lebesgue transfer are used: the injection b and the compact-copy transfer using continuous with .
AD implies countable choice for subsets of Baire space supplies countable selection from sets of Baire codes under AD.
Every complete ordered field is Archimedean puts every real in an integer unit interval.
Proof
Given: ZF+AD and a subset of one of the stated spaces.
For , F1's explicit finite-block codes and illegal-bit payoff make its game a natural-number game, determined by A1. If I wins, F1 gives a compact Cantor copy in A; if II wins, F1 gives an injection . These cover every case, including empty A.
For , apply step 1.1 to h[A]. If it injects into , compose that injection with h restricted to A. If it contains a compact Cantor copy K, F2's continuous inverse on D restricts to K, giving a homeomorphic copy in A that is compact by the open-cover definition. In a metric space compact sets are closed: for an exterior x finitely many balls cover the compact set, leaving a sufficiently small ball about x disjoint. Homeomorphism with Cantor space gives no isolated point by F2. Thus this is also a nonempty perfect closed subset of Baire space.
For , enumerate integers as and set . By F5 these pieces cover A after translation. Apply step 1.1 to each b[A_i]. If one contains a compact Cantor copy, F3's ZF clause transfers it into A_i, and translation, with its continuous inverse, transfers it into A. It is compact and therefore closed by the separation argument in step 2.1, and has no isolated point.
Otherwise each b[A_i] injects into . Each nonempty such set has a surjective enumeration: invert an injection on its range and fill unused indices with the value at the least occupied index. A sequence of binary reals is coded as one Baire real by F2's pairing. For each nonempty b[A_i] let C_i be the nonempty set of all codes of its enumerations; for an empty piece use the singleton all-zero code and retain that it was empty. F4 with A1 selects c_i for every i. Decode them, apply F3's , translate back by m_i and interleave the two indices. If A is nonempty, fill slots from empty pieces with one fixed element a of A. This gives a surjection because every point belongs to a piece and appears in that piece's enumeration. Assign each point its least enumeration index to inject A into . For empty A use the empty injection. No unrestricted countable choice has been used. QED.
Depends on
- Axiom of determinacy for natural-number games
- Perfect-set game strategy dichotomy on Cantor space
- Cantor and Baire sequence spaces and coordinate codings
- Dyadic coding supplies coin measure and its completed Lebesgue transfer
- AD implies countable choice for subsets of Baire space
- Every complete ordered field is Archimedean
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
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Sources
- Theorem 10.10(i) and Claims 10.11–10.12, printed pp100–101 (standard reference, not scraped)