Shelah's three cardinal and model-existence claims

From papers

Let α\alpha be an ordinal, and let χ<μ<λ<Dedχ\chi<\mu<\lambda<\operatorname{Ded}^*\chi. For a countable theory TT, let a λ\lambda-like model mean a model of cardinality λ\lambda in the sense used in the source. Let ψLω1,ω\psi\in L_{\omega_1,\omega}.

Shelah's three cardinal and model-existence claims. The following assertions hold:

A)

(α+ω+ω,α+ω,α)(λ,μ,χ).(\aleph_{\alpha+\omega+\omega},\aleph_{\alpha+\omega},\aleph_\alpha)\rightarrow(\lambda,\mu,\chi).

B) If TT has a λ\lambda-like model, λ\lambda is a limit cardinal, and Tμ<λ1Ded(μ)|T|\leq\mu<\lambda_1\operatorname{Ded}^*(\mu) for a singular cardinal λ1\lambda_1, then TT has a λ1\lambda_1-like model. If λ\lambda is an ω\omega-Mahlo weakly inaccessible cardinal, the stated cardinal and singularity restrictions on λ1\lambda_1 can be removed.

C) If ψ\psi has a model of cardinality ω1\aleph_{\omega_1}, then it has a model of cardinality 202^{\aleph_0}.

These are two-cardinal, partition, and model-existence assertions from Shelah's work. The source flags the last assertion with a reference to [Sh:522], so its status and the precise interpretation of the notation should be checked against that reference.

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Sources & referencesView supporting material

Primary source

Saharon Shelah, “A collection of abstracts of Shelah's Papers”, arXiv:2209.01617 (2022).

Additional references

2 papers in this index state this conjecture (2005–2022). The statement above is taken from the most recent of them; the others are arXiv:math/0509707.

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