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## Fundamenta Mathematicae

2002 | 174 | 1 | 1-21
Tytuł artykułu

### On equivalence relations second order definable over H(κ)

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EN
Abstrakty
EN
Let κ be an uncountable regular cardinal. Call an equivalence relation on functions from κ into 2 second order definable over H(κ) if there exists a second order sentence ϕ and a parameter P ⊆ H(κ) such that functions f and g from κ into 2 are equivalent iff the structure ⟨ H(κ), ∈, P, f, g ⟩ satisfies ϕ. The possible numbers of equivalence classes of second order definable equivalence relations include all the nonzero cardinals at most κ⁺. Additionally, the possibilities are closed under unions and products of at most κ cardinals. We prove that these are the only restrictions: Assuming that GCH holds and λ is a cardinal with $λ^κ = λ$, there exists a generic extension where all the cardinals are preserved, there are no new subsets of cardinality < κ, $2^κ = λ$, and for all cardinals μ, the number of equivalence classes of some second order definable equivalence relation on functions from κ into 2 is μ iff μ is in Ω, where Ω is any prearranged subset of λ such that 0 ∉ Ω, Ω contains all the nonzero cardinals ≤ κ⁺, and Ω is closed under unions and products of at most κ cardinals.
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Rocznik
Tom
Numer
Strony
1-21
Opis fizyczny
Daty
wydano
2002
Twórcy
autor
• Institute of Mathematics, The Hebrew University, Jerusalem, Israel
• Department of Mathematics, Rutgers University, New Brunswick, NJ 08903, U.S.A.
autor
• Department of Mathematics, P.O. Box 4, 00014 University of Helsinki, Finland
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