# Transactions of the American Mathematical Society

Published by the American Mathematical Society since 1900, Transactions of the American Mathematical Society is devoted to longer research articles in all areas of pure and applied mathematics.

ISSN 1088-6850 (online) ISSN 0002-9947 (print)

The 2020 MCQ for Transactions of the American Mathematical Society is 1.48.

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## T-degrees, jump classes, and strong reducibilitiesHTML articles powered by AMS MathViewer

by R. G. Downey and C. G. Jockusch
Trans. Amer. Math. Soc. 301 (1987), 103-136 Request permission

## Abstract:

It is shown that there exist r.e. degrees other than 0 and $\mathbf {0}β$ which have a greatest r.e. $1$-degree. This solves an old question of Rogers and Jockusch. We call such degrees $1$-topped. We show that there exist incomplete $1$-topped degrees above any low r.e. degree, but also show that no nonzero low degree is $1$-topped. It then follows by known results that all incomplete $1$-topped degrees are low$_{2}$ but not low. We also construct cappable nonzero $1$-topped r.e. degrees and examine the relationships between $1$-topped r.e. degrees and high r.e. degrees. Finally, we give an analysis of the βlocalβ relationships of r.e. sets under various strong reducibilities. In particular, we analyze the structure of r.e. ${\text {wtt-}}$ and ${\text {tt}}$-degrees within a single r.e. ${\text {T}}$-degree. We show, for instance, that there is an r.e. degree which contains a greatest r.e. ${\text {wtt-}}$-degree and a least r.e. ${\text {tt}}$-degree yet does not consist of a single r.e. ${\text {wtt}}$-degree. This depends on a new construction of a nonzero r.e. ${\text {T}}$-degree with a least ${\text {tt}}$-degree, which proves to have several further applications.
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