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Every limit ordinal has cofinality omega in Gitik's model
Statement
In , every nonzero limit ordinal has a cofinal map . Consequently for every such .
Facts & Assumptions
Given: The completed Gitik symmetric model .
Gitik's symmetric submodel satisfies ZF: is a transitive ZF model containing the ground model.
Gitik's filter system and proper-class forcing: At every regular coordinate , trunks have finite one-to-one -sections, support extension is available, and successors are selected from uniform filters on .
Gitik's finite-support symmetric submodel: A name fixed by the pointwise stabilizer of one coordinate and having HS subnames belongs to .
Cofinality , and regular and singular cardinals and ; and ; for a limit ordinal the value is an infinite cardinal with , so it is regular; and every cofinal subset of has cardinality at least , a value that is attained: The ground cofinality of a limit ordinal is an infinite regular cardinal and has a strictly increasing cofinal witness; no finite subset is cofinal in a limit ordinal.
Proof
Fix an infinite regular ground cardinal . Use the canonical name whose value is the union of the -sections of conditions in : a pair enters the named graph exactly under conditions with . Directedness makes this union a one-to-one partial map. For every , support extension and finitely many legal successors give a dense class of conditions whose -section contains . For every , uniformity makes each relevant successor set unbounded, so pruning above and taking the next -successor is dense. Thus the value is total and cofinal, but need not be onto. Every automorphism in fixes the -section and permutes the witnessing conditions only outside that coordinate. Its graph entries use only check names, so the name is hereditarily symmetric with support . F3 therefore puts in .
Let be a nonzero limit ordinal. In the ground model put and take the strictly increasing cofinal map supplied by F4. Its check name is fixed by every automorphism and hereditarily symmetric, so F3 puts in . If , it is already the required witness. If , then is an infinite regular ground cardinal, so step 1.1 gives and ZF in F1 forms . Given , choose with and then with . Monotonicity of gives , so is cofinal. This proves the claimed omega upper bound in both cases without any surjectivity assertion.
By F4, the cofinality of a limit ordinal is infinite. Equivalently, the empty range is not cofinal and every nonempty finite set of ordinals has a maximum still below , so no finite map is cofinal in . Step 2.1 gives a cofinal map of length ; since is the least infinite ordinal, the least cofinal length is exactly . The construction and this verification take place inside the transitive ZF model .
Depends on
- Gitik's symmetric submodel satisfies ZF
- Gitik's filter system and proper-class forcing
- Gitik's finite-support symmetric submodel
- Cofinality $\operatorname{cf}(\alpha)$, and regular and singular cardinals
- $\operatorname{cf}(\alpha) \le \alpha$; $\operatorname{cf}(0) = 0$ and $\operatorname{cf}(\alpha + 1) = 1$; for a limit ordinal $\lambda$ the value $\operatorname{cf}(\lambda)$ is an infinite cardinal with $\operatorname{cf}(\operatorname{cf}(\lambda)) = \operatorname{cf}(\lambda)$, so it is regular; and every cofinal subset of $\lambda$ has cardinality at least $\operatorname{cf}(\lambda)$, a value that is attained
Used by
Dependency tree · two levels
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Sources
- Schürz, Gitik's model, abstract, coordinate forcing on pages 3–7 and final theorem (standard reference, not scraped)
- Dimitriou, Symmetric Models, Lemma 2.38, pages 69–70 (standard reference, not scraped)