Journal of Symbolic Logic

Intrinsic bounds on complexity and definability at limit levels

John Chisholm, Ekaterina B. Fokina, Sergey S. Goncharov, Valentina S. Harizanov, Julia F. Knight, and Sara Quinn
Source: J. Symbolic Logic Volume 74, Issue 3 (2009), 1047-1060.

Abstract

We show that for every computable limit ordinal α, there is a computable structure 𝒜 that is Δα⁰ categorical, but not relatively Δα⁰ categorical (equivalently, it does not have a formally Σα⁰ Scott family). We also show that for every computable limit ordinal α, there is a computable structure 𝒜 with an additional relation R that is intrinsically Σα⁰ on 𝒜, but not relatively intrinsically Σα⁰ on 𝒜 (equivalently, it is not definable by a computable Σα formula with finitely many parameters). Earlier results in [7], [10], and [8] establish the same facts for computable successor ordinals α.

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Permanent link to this document: http://projecteuclid.org/euclid.jsl/1245158098
Digital Object Identifier: doi:10.2178/jsl/1245158098
Zentralblatt MATH identifier: 05609403
Mathematical Reviews number (MathSciNet): MR2548479

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