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تسجيل مستخدم جديدTuring Degrees of Hyperjumps
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The Posner-Robinson Theorem states that for any reals $Z$ and $A$ such that $Z oplus 0 leq_mathrm{T} A$ and $0 <_mathrm{T} Z$, there exists $B$ such that $A equiv_mathrm{T} B equiv_mathrm{T} B oplus Z equiv_mathrm{T} B oplus 0$. Consequently, any nonzero Turing degree $operatorname{deg}_mathrm{T}(Z)$ is a Turing jump relative to some $B$. Here we prove the hyperarithmetical analog, based on an unpublished proof of Slaman, namely that for any reals $Z$ and $A$ such that $Z oplus mathcal{O} leq_mathrm{T} A$ and $0 <_mathrm{HYP} Z$, there exists $B$ such that $A equiv_mathrm{T} mathcal{O}^B equiv_mathrm{T} B oplus Z equiv_mathrm{T} B oplus mathcal{O}$. As an analogous consequence, any nonhyperarithmetical Turing degree $operatorname{deg}_mathrm{T}(Z)$ is a hyperjump relative to some $B$.
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