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arXiv · 1210.4220

Degrees that are not degrees of categoricity

Bernard A. Anderson·Barbara F. Csima
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Abstract

A computable structure A is x-computably categorical for some Turing degree x, if for every computable structure B isomorphic to A there is an isomorphism f:B -> A with f computable in x. A degree x is a degree of categoricity if there is a computable A such that A is x-computably categorical, and for all y, if A is y-computably categorical then y computes x. We construct a Sigma_2 set whose degree is not a degree of categoricity. We also demonstrate a large class of degrees that are not degrees of categoricity by showing that every degree of a set which is 2-generic relative to some perfect tree is not a degree of categoricity. Finally, we prove that every noncomputable hyperimmune-free degree is not a degree of categoricity.

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Bernard A. Anderson, Barbara F. Csima. 2012-10-15. Degrees that are not degrees of categoricity. https://doi.org/10.1215/00294527-3496154

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