## A characterization of compactly generated metric groups

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- by Hiroshi Fujita and Dmitri Shakhmatov PDF
- Proc. Amer. Math. Soc.
**131**(2003), 953-961 Request permission

## Abstract:

Recall that a topological group $G$ is: (a)*$\sigma$-compact*if $G=\bigcup \{K_n:n\in \mathbf N\}$ where each $K_n$ is compact, and (b)

*compactly generated*if $G$ is algebraically generated by some compact subset of $G$. Compactly generated groups are $\sigma$-compact, but the converse is not true: every countable non-finitely generated discrete group (for example, the group of rational numbers or the free (Abelian) group with a countable infinite set of generators) is a counterexample. We prove that a metric group $G$ is compactly generated if and only if $G$ is $\sigma$-compact and for every open subgroup $H$ of $G$ there exists a finite set $F$ such that $F\cup H$ algebraically generates $G$. As a corollary, we obtain that a $\sigma$-compact metric group $G$ is compactly generated provided that one of the following conditions holds: (i) $G$ has no proper open subgroups, (ii) $G$ is dense in some connected group (in particular, if $G$ is connected itself), (iii) $G$ is totally bounded (= subgroup of a compact group). Our second major result states that a countable metric group is compactly generated if and only if it can be generated by a sequence converging to its identity element (eventually constant sequences are not excluded here). Therefore, a countable metric group $G$ can be generated by a (possibly eventually constant) sequence converging to its identity element in each of the cases (i), (ii) and (iii) above. Examples demonstrating that various conditions cannot be omitted or relaxed are constructed. In particular, we exhibit a countable totally bounded group which is not compactly generated.

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## Additional Information

**Hiroshi Fujita**- Affiliation: Department of Mathematical Sciences, Faculty of Science, Ehime University, Matsuyama 790-8577, Japan
- Email: fujita@math.sci.ehime-u.ac.jp
**Dmitri Shakhmatov**- Affiliation: Department of Mathematical Sciences, Faculty of Science, Ehime University, Matsuyama 790-8577, Japan
- MR Author ID: 196690
- Email: dmitri@dpc.ehime-u.ac.jp
- Received by editor(s): May 5, 1997
- Received by editor(s) in revised form: August 17, 2001, and October 17, 2001
- Published electronically: July 17, 2002
- Communicated by: Alan Dow
- © Copyright 2002 American Mathematical Society
- Journal: Proc. Amer. Math. Soc.
**131**(2003), 953-961 - MSC (2000): Primary 54H11, 22A05
- DOI: https://doi.org/10.1090/S0002-9939-02-06736-9
- MathSciNet review: 1937434