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Transactions of the American Mathematical Society
Transactions of the American Mathematical Society
ISSN 1088-6850(online) ISSN 0002-9947(print)


Decidability and invariant classes for degree structures

Authors: Manuel Lerman and Richard A. Shore
Journal: Trans. Amer. Math. Soc. 310 (1988), 669-692
MSC: Primary 03D25; Secondary 03B25, 03D30, 03D55
MathSciNet review: 973174
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Abstract: We present a decision procedure for the $ \forall \exists $-theory of $ \mathcal{D}[{\mathbf{0}},\,{\mathbf{0}}' ]$, the Turing degrees below $ {\mathbf{0}}' $. The two main ingredients are a new extension of embeddings result and a strengthening of the initial segments results below $ {\mathbf{0}}' $ of [Le1]. First, given any finite subuppersemilattice $ U$ of $ \mathcal{D}[{\mathbf{0}},\,{\mathbf{0}}' ]$ with top element $ {\mathbf{0}}' $ and an isomorphism type $ V$ of a poset extending $ U$ consistently with its structure as an usl such that $ V$ and $ U$ have the same top element and $ V$ is an end extension of $ U - \{ {\mathbf{0}}' \} $, we construct an extension of $ U$ inside $ \mathcal{D}[{\mathbf{0}},\,{\mathbf{0}}' ]$ isomorphic to $ V$. Second, we obtain an initial segment $ W$ of $ \mathcal{D}[{\mathbf{0}},\,{\mathbf{0}}' ]$ which is isomorphic to $ U - \{ {\mathbf{0}}' \} $ such that $ W \cup \{ {\mathbf{0}}' \} $ is a subusl of $ \mathcal{D}$. The decision procedure follows easily from these results.

As a corollary to the $ \forall \exists $-decision procedure for $ \mathcal{D}$, we show that no degree $ {\mathbf{a}} > {\mathbf{0}}$ is definable by any $ \exists \forall $-formula of degree theory. As a start on restricting the formulas which could possibly define the various jump classes we classify the generalized jump classes which are invariant for any $ \forall $ or $ \exists $-formula. The analysis again uses the decision procedure for the $ \forall \exists $-theory of $ \mathcal{D}$. A similar analysis is carried out for the high/low hierarchy using the decision procedure for the $ \forall \exists $-theory of $ \mathcal{D}[{\mathbf{0}},\,{\mathbf{0}}' ]$. (A jump class $ \mathcal{C}$ is $ \sigma $-invariant if $ \sigma ({\mathbf{a}})$ holds for every $ {\mathbf{a}}$ in $ \mathcal{C}$.)

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Additional Information

PII: S 0002-9947(1988)0973174-0
Keywords: Degrees below $ {\mathbf{0}}' $, extension of embeddings, initial segments, $ \forall \exists $-theory, invariant jump classes, definability
Article copyright: © Copyright 1988 American Mathematical Society

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