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Recursive expansions

C. Ash, J. Knight (1994)

Fundamenta Mathematicae

Let A be a recursive structure, and let ψ be a recursive infinitary Π 2 sentence involving a new relation symbol. The main result of the paper gives syntactical conditions which are necessary and sufficient for every recursive copy of A to have a recursive expansion to a model of ψ, provided A satisfies certain decidability conditions. The decidability conditions involve a notion of rank. The main result is applied to prove some earlier results of Metakides-Nerode and Goncharov. In these applications,...

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