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Initial segments of the lattice of Π10 classes

Published online by Cambridge University Press:  12 March 2014

Douglas Cenzer  and
Andre Nies
Douglas Cenzer
Affiliation:
Department of Mathematics, University of Florida, Gainesville, FL 32611, USA, E-Mail: [email protected]
Andre Nies
Affiliation:
Department of Mathematics, University of Chicago, Chicago, IL 60637, USA, E-Mail: [email protected]
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Abstract.

We show that in the lattice of classes there are initial segments [∅, P] = (P) which are not Boolean algebras, but which have a decidable theory. In fact, we will construct for any finite distributive lattice L which satisfies the dual of the usual reduction property a class P such that L is isomorphic to the lattice (P)*, which is (P). modulo finite differences. For the 2-element lattice, we obtain a minimal class, first constructed by Cenzer, Downey, Jockusch and Shore in 1993. For the simplest new class P constructed, P has a single, non-computable limit point and (P)* has three elements, corresponding to ∅, P and a minimal class P0 ⊂ P, The element corresponding to P0 has no complement in the lattice. On the other hand, the theory of (P) is shown to be decidable.

A class P is said to be decidable if it is the set of paths through a computable tree with no dead ends. We show that if P is decidable and has only finitely many limit points, then (P)* is always a Boolean algebra. We show that if P is a decidable class and (P) is not a Boolean algebra, then the theory of (P) interprets the theory of arithmetic and is therefore undecidable.

Type
Research Article
Information
The Journal of Symbolic Logic , Volume 66 , Issue 4 , December 2001 , pp. 1749 - 1765
Copyright
Copyright © Association for Symbolic Logic 2001

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