## Notre Dame Journal of Formal Logic

### Automorphisms of Models of True Arithmetic: Recognizing Some Basic Open Subgroups

#### Abstract

Let M be a countable recursively saturated model of Th($\mathbb{N}$), and let G$=$Aut(M), considered as a topological group. We examine connections between initial segments of M and subgroups of G. In particular, for each of the following classes of subgroups H$<$G, we give characterizations of the class of terms of the topological group structure of H as a subgroup of G.

(a) $\{ H : H =G_{(K)}$ for some $K\prec_{\rm e} M \}$

(b) $\{ H : H =G_{\{K\}}$ for some $K\prec_{\rm e} M \}$

(c) $\{ H : H =G_{\{M(a)\}}$ for some $a\in M \}$

(d) $\{ H : H =G_{\{M(a)\}}=G_a$ for some $a\in M \}$

(Here, M(a) denotes the smallest $I\prec_{\rm e}$M containing a, $G_{\{A\}}=\{g \in G: A=\{gx: x\in A\} \}$, $G_{(A)}=\{g\in G: \forall a\in A\:ga=a\}$, and $G_a=\{g\in G: ga=a\}$.)

#### Article information

Source
Notre Dame J. Formal Logic, Volume 35, Number 1 (1994), 1-14.

Dates
First available in Project Euclid: 22 December 2002

Permanent link to this document
https://projecteuclid.org/euclid.ndjfl/1040609291

Digital Object Identifier
doi:10.1305/ndjfl/1040609291

Mathematical Reviews number (MathSciNet)
MR1271695

Zentralblatt MATH identifier
0824.03016

#### Citation

Kotlarski, Henryk; Kaye, Richard. Automorphisms of Models of True Arithmetic: Recognizing Some Basic Open Subgroups. Notre Dame J. Formal Logic 35 (1994), no. 1, 1--14. doi:10.1305/ndjfl/1040609291. https://projecteuclid.org/euclid.ndjfl/1040609291

#### References

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