2019
DOI: 10.1002/malq.201800015
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Jump inversions of algebraic structures and Σ‐definability

Abstract: It is proved that for every countable structure A and a computable successor ordinal α there is a countable structure A −α which is ≤ -least among all countable structures C such that A is -definable in the αth jump C (α) . We also show that this result does not hold for the limit ordinal α = ω. Moreover, we prove that there is no countable structure A with the degree spectrum {d : a ≤ d (ω) } for a > 0 (ω) .

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Cited by 2 publications
(1 citation statement)
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“…Soskov gave an example of an isomorphism spectrum of a structure scriptA such that prefixDgSpfalse(scriptAfalse)false{boldd:boldd0false(ωfalse)false} and showed that no structure has {d:boldd(ω)prefixDgSpfalse(scriptAfalse)} as its isomorphism spectrum . Faizrahmanov, Kach, Kalimullin, and Montalbán recently showed that no structure realizes the family {d:boldd(ω)bolda(ω)} for abold0(ω) as its isomorphism spectrum .…”
Section: Be Trivialitymentioning
confidence: 99%
“…Soskov gave an example of an isomorphism spectrum of a structure scriptA such that prefixDgSpfalse(scriptAfalse)false{boldd:boldd0false(ωfalse)false} and showed that no structure has {d:boldd(ω)prefixDgSpfalse(scriptAfalse)} as its isomorphism spectrum . Faizrahmanov, Kach, Kalimullin, and Montalbán recently showed that no structure realizes the family {d:boldd(ω)bolda(ω)} for abold0(ω) as its isomorphism spectrum .…”
Section: Be Trivialitymentioning
confidence: 99%