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Effective Categoricity of Automatic Equivalence and Nested Equivalence Structures

Published: 01 August 2020 Publication History

Abstract

We study automatic equivalence and nested equivalence structures. The goal is to compare and contrast these automatic structures with computable equivalence and nested equivalence structures. Equivalence structures A may be characterized by their characters χ(A) which encodes the number of equivalence classes of any given size. The characters of computably categorical, Δ20 categorical but not computably categorical, or Δ30 categorical but not Δ20 categorical have been determined. We show that every computably categorical equivalence structure has an automatic copy, but not every Δ20 categorical structure has an automatic copy. We construct an automatic equivalence structure which is Δ20 categorical but not computably categorical and another automatic equivalence structure which is not Δ20 categorical. We observe that the theory of an automatic equivalence structure is decidable and hence the character of any automatic equivalence structure is computable. On the other hand, there is a computable character which is not the character of any automatic equivalence structure. We show that any two automatic equivalence structures which are isomorphic are in fact computably isomorphic. Moreover, we show that for certain characters, there is always a exponential time isomorphism between two automatic equivalence structures with that character. Finally, we briefly consider nested equivalence structures and construct an automatic nested equivalence structure that is not Δ30 categorical but Δ40 categorical and an automatic nested equivalence structure that is not Δ40 categorical.

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            cover image Theory of Computing Systems
            Theory of Computing Systems  Volume 64, Issue 6
            Aug 2020
            156 pages

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            Springer-Verlag

            Berlin, Heidelberg

            Publication History

            Published: 01 August 2020

            Author Tags

            1. Computable structures
            2. Online structures
            3. Equivalence structures
            4. Nested equivalence structures
            5. Automatic structures
            6. Effective categoricity

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