Intrinsic bounds on complexity and definability at limit levels

Journal of Symbolic Logic 74 (3):1047-1060 (2009)
We show that for every computable limit ordinal α, there is a computable structure A that is $\Delta _\alpha ^0 $ categorical, but not relatively $\Delta _\alpha ^0 $ categorical (equivalently. it does not have a formally $\Sigma _\alpha ^0 $ Scott family). We also show that for every computable limit ordinal a, there is a computable structure A with an additional relation R that is intrinsically $\Sigma _\alpha ^0 $ on A. but not relatively intrinsically $\Sigma _\alpha ^0 $ on A (equivalently, it is not definable by a computable $\Sigma _\alpha $ 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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DOI 10.2178/jsl/1245158098
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Ivan N. Soskov (1996). Intrinsically Hyperarithmetical Sets. Mathematical Logic Quarterly 42 (1):469-480.

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