Abstract
The main result is as follows: Fix an arbitrary prime \( q \). A \( q \)-divisible torsion-free (discrete, countable) abelian group \( G \) has a \( \Delta^{0}_{2} \)-presentation if, and only if, its connected Pontryagin–van Kampen Polish dual \( \widehat{G} \) admits a computable complete metrization (in which we do not require the operations to be computable). We use this jump-inversion/duality theorem to transfer the results on the degree spectra of torsion-free abelian groups to the results about the degree spectra of Polish spaces up to homeomorphism. For instance, it follows that for every computable ordinal \( \alpha>1 \) and each \( {\mathbf{a}}>0^{(\alpha)} \) there is a connected compact Polish space having proper \( \alpha^{th} \) jump degree \( {\mathbf{a}} \) (up to homeomorphism). Also, for every computable ordinal \( \beta \) of the form \( 1+\delta+2n+1 \), where \( \delta \) is zero or is a limit ordinal and \( n\in\omega \), there is a connected Polish space having an \( X \)-computable copy if and only if \( X \) is \( non \)-\( low_{\beta} \). In particular, there is a connected Polish space having exactly the \( non \)-\( low_{2} \) complete metrizations. The case when \( \beta=2 \) is an unexpected consequence of the main result of the author’s M.Sc. Thesis written under the supervision of Sergey S. Goncharov.
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Notes
By this we mean the metric completion of a countable metric space upon the domain of \( \omega \) in which the predicates \( D_{<}(i,j,k)\iff d(i,j)<2^{-k} \) and \( D_{>}(i,j,k)\iff d(i,j)>2^{-k} \) are uniformly primitive recursive.
Recall that an oracle \( X \) is low if the halting problem \( X^{\prime} \) for Turing machines with oracle \( X \) can be computed using \( \varnothing^{\prime} \), the usual halting problem (with no oracles). The class low\( {}_{4} \) is defined similarly but on using \( X^{(4)} \) and \( \varnothing^{(4)} \). This is a degree-invariant property. We cite [23] for further background.
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Funding
A. G. Melnikov was supported by the Mathematical Center in Akademgorodok under Agreement No. 075–15–2019–1613 with the Ministry of Science and Higher Education of the Russian Federation, and Rutherford Discovery Fellowship (Wellington) RDF-MAU1905, Royal Society Te Ap\( \bar{\mathrm{a}} \)rangi.
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Translated from Sibirskii Matematicheskii Zhurnal, 2021, Vol. 62, No. 5, pp. 1091–1108. https://doi.org/10.33048/smzh.2021.62.511
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Melnikov, A.G. New Degree Spectra of Polish Spaces. Sib Math J 62, 882–894 (2021). https://doi.org/10.1134/S0037446621050116
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DOI: https://doi.org/10.1134/S0037446621050116