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Diamond implies clubsuit
Statement
In ZFC, implies .
Facts & Assumptions
Given: A diamond sequence ; assume AC.
Every subset of is guessed stationarily often. Diamond on ω1
The club principle and the explicit thinning of cofinal sets to order-type- ladders are as defined here. The Ostaszewski club principle
The limit points of an unbounded subset of an ordinal of uncountable cofinality form a club. Limit points of an unbounded set form a club
A finite intersection of clubs of uncountable cofinality is club. Intersections of fewer than the cofinality many clubs
No countable subset of is cofinal under countable choice. Assuming countable choice: every at most countable subset of is bounded below , so no at most countable subset of is cofinal in it, and a supremum of at most countably many at most countable ordinals is at most countable
Assume AC. The Axiom of Choice
Proof
Fix the ordinal enumerations in F2 using A1. At a nonzero countable limit , if is cofinal in , apply the explicit minimum recursion in F2 to it and call its range . Otherwise apply the same recursion to itself. In both situations is cofinal of order type ; when is cofinal we also have . This defines the whole ladder sequence from the fixed parameters.
Let be uncountable. It is unbounded, since a bounded subset lies inside a countable ordinal and is countable. F5 and A1 give , so F3 makes club. Let , stationary by F1. For any club , F4 makes club, so it meets . Thus is stationary.
If , then is a nonzero limit and is cofinal in . The first alternative of step 1.1 therefore applies, giving . Hence the containment-guess set contains the stationary set and itself meets every club. This is exactly F2's club principle.
Depends on
- Diamond on ω1
- The Ostaszewski club principle
- Limit points of an unbounded set form a club
- Intersections of fewer than the cofinality many clubs
- Assuming countable choice: every at most countable subset of $\omega_1$ is bounded below $\omega_1$, so no at most countable subset of $\omega_1$ is cofinal in it, and a supremum of at most countably many at most countable ordinals is at most countable
- The Axiom of Choice
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
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