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Scott sentences for certain groups

Scott sentences for certain groups We give Scott sentences for certain computable groups, and we use index set calculations as a way of checking that our Scott sentences are as simple as possible. We consider finitely generated groups and torsion-free abelian groups of finite rank. For both kinds of groups, the computable ones all have computable $$\varSigma _3$$ Σ 3 Scott sentences. Sometimes we can do better. In fact, the computable finitely generated groups that we have studied all have Scott sentences that are “computable d- $$\varSigma _2$$ Σ 2 ” (the conjunction of a computable $$\varSigma _2$$ Σ 2 sentence and a computable $$\varPi _2$$ Π 2 sentence). In [9], this was shown for the finitely generated free groups. Here we show it for all finitely generated abelian groups, and for the infinite dihedral group. Among the computable torsion-free abelian groups of finite rank, we focus on those of rank 1. These are exactly the additive subgroups of $$\mathbb {Q}$$ Q . We show that for some of these groups, the computable $$\varSigma _3$$ Σ 3 Scott sentence is best possible, while for others, there is a computable d- $$\varSigma _2$$ Σ 2 Scott sentence. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Archive for Mathematical Logic Springer Journals

Scott sentences for certain groups

Archive for Mathematical Logic , Volume 57 (4) – Aug 19, 2017

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References (34)

Publisher
Springer Journals
Copyright
Copyright © 2017 by Springer-Verlag GmbH Germany
Subject
Mathematics; Mathematical Logic and Foundations; Mathematics, general; Algebra
ISSN
0933-5846
eISSN
1432-0665
DOI
10.1007/s00153-017-0578-z
Publisher site
See Article on Publisher Site

Abstract

We give Scott sentences for certain computable groups, and we use index set calculations as a way of checking that our Scott sentences are as simple as possible. We consider finitely generated groups and torsion-free abelian groups of finite rank. For both kinds of groups, the computable ones all have computable $$\varSigma _3$$ Σ 3 Scott sentences. Sometimes we can do better. In fact, the computable finitely generated groups that we have studied all have Scott sentences that are “computable d- $$\varSigma _2$$ Σ 2 ” (the conjunction of a computable $$\varSigma _2$$ Σ 2 sentence and a computable $$\varPi _2$$ Π 2 sentence). In [9], this was shown for the finitely generated free groups. Here we show it for all finitely generated abelian groups, and for the infinite dihedral group. Among the computable torsion-free abelian groups of finite rank, we focus on those of rank 1. These are exactly the additive subgroups of $$\mathbb {Q}$$ Q . We show that for some of these groups, the computable $$\varSigma _3$$ Σ 3 Scott sentence is best possible, while for others, there is a computable d- $$\varSigma _2$$ Σ 2 Scott sentence.

Journal

Archive for Mathematical LogicSpringer Journals

Published: Aug 19, 2017

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