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2 Fundamenta Mathematicae

2 2002

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Potential isomorphism and semi-proper trees

100%

Hellsten A. , Hyttinen T. , Shelah S.

tom 175

nr 2

127-142

We study a notion of potential isomorphism, where two structures are said to be potentially isomorphic if they are isomorphic in some generic extension that preserves stationary sets and does not add new sets of cardinality less than the cardinality of the models. We introduce the notion of weakly semi-proper trees, and note that there is a strong connection between the existence of potentially isomorphic models for a given complete theory and the existence of weakly semi-proper trees. We show that the existence of weakly semi-proper trees is consistent relative to ZFC by proving the existence of weakly semi-proper trees under certain cardinal arithmetic assumptions. We also prove the consistency of the non-existence of weakly semi-proper trees assuming the consistency of some large cardinals.

More on the Ehrenfeucht-Fraisse game of length ω₁

100%

Hyttinen T. , Shelah S. , Vaananen J.

tom 175

nr 1

79-96

By results of [9] there are models 𝔄 and 𝔅 for which the Ehrenfeucht-Fraïssé game of length ω₁, $EFG_{ω₁}(𝔄,𝔅)$, is non-determined, but it is consistent relative to the consistency of a measurable cardinal that no such models have cardinality ≤ ℵ₂. We now improve the work of [9] in two ways. Firstly, we prove that the consistency strength of the statement "CH and $EFG_{ω₁}(𝔄,𝔅)$ is determined for all models 𝔄 and 𝔅 of cardinality ℵ₂" is that of a weakly compact cardinal. On the other hand, we show that if $2^{ℵ₀} < 2^{ℵ₃}$, T is a countable complete first order theory, and one of (i) T is unstable, (ii) T is superstable with DOP or OTOP, (iii) T is stable and unsuperstable and $2^{ℵ₀} ≤ ℵ₃$, holds, then there are 𝓐,ℬ ⊨ T of power ℵ₃ such that $EFG_{ω₁}(𝓐,ℬ)$ is non-determined.