On the jump classes of noncuppable enumeration degrees. (English) Zbl 1215.03056

The upper semilattice of enumeration degrees (e-degrees) is generated by the relation ‘\(A\) is enumeration reducible to \(B\)’ \((A \leq _{\text{e}} B)\), which holds iff there is an effective procedure that takes any enumeration of \(B\) to an enumeration of \(A\). The jump of the e-degree of \(A\) is defined as the e-degree of the characteristic function of the set \(\{e\mid e \in\Phi^A_e\}\). The Turing degrees are embedded in the e-degrees by a map that takes any set’s Turing degree to the e-degree of its characteristic function. This preserves jump. Thus ‘high\(_m\)’, ‘low\(_n\)’, and ‘cuppable’ may be ascribed in the natural way to e-degrees.
A main theorem shows that for every \(\Sigma^0_2\) e-degree b there is a noncuppable \(\Sigma^0_2\) e-degree \({\mathbf a}>{\mathbf{0}}_{\text{e}}\) with \({\mathbf b}'\leq_{\text{e}}{\mathbf a}'\) and \({\mathbf a}''\leq_{\text{e}}{\mathbf b}''\). It follows that for every \(m \geq 0\) and \(n \geq 1\) there are noncuppable e-degrees \({\mathbf x},{\mathbf y}< {\mathbf{0}}'_{\text{e}}\) such that \({\mathbf x}\) is high\(_{m+1}\) but not high\(_m\) and \({\mathbf y}\) is low\(_{n+1}\) but not low\(_n\). Moreover, there is a noncuppable \({\mathbf z}<{\mathbf{0}}'_{\text{e}}\) with the property that \({\mathbf{0}}^n _{\text{e}}< {\mathbf z}^n< {\mathbf{0}}^{n+1}_{\text{e}}\) for all \(n\).


03D25 Recursively (computably) enumerable sets and degrees
03D30 Other degrees and reducibilities in computability and recursion theory
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