ASSIGNING AN ISOMORPHISM TYPE TO A HYPERDEGREE
HOWARD BECKER · Journal of Symbolic Logic · 2019
Abstract Let L be a computable vocabulary, let XL be the space of L-structures with universe ω and let $f:{2^\omega } \to {X_L}$ be a hyperarithmetic function such that for all $x,y \in {2^\omega }$ , if $x{ \equiv _h}y$ then $f\left( x \right) \cong f\left( y \right)$ . One of the following two properties must hold. (1) The Scott rank of f (0) is $\omega _1^{CK} + 1$ . (2) For all $x \in {2^\omega },f\left( x \right) \cong f\left( 0 \right)$ .