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UNCOUNTABLE COFINALITIES OF PERMUTATION GROUPS

Published online by Cambridge University Press:  06 April 2005

MANFRED DROSTE
Affiliation:
Institut für Informatik, Universität Leipzig, 04009 Leipzig, Germanydroste@informatik.uni-leipzig.de
RÜDIGER GÖBEL
Affiliation:
Fachbereich 6, Mathematik und Informatik, Universität Duisburg Essen, 45117 Essen, Germanyr.goebel@uni-essen.de
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Abstract

A sufficient criterion is found for certain permutation groups $G$ to have uncountable strong cofinality, that is, they cannot be expressed as the union of a countable, ascending chain $(H_i)_{i\in\o}$ of proper subsets $H_i$ such that $H_iH_i \subseteq H_{i+1}$ and $H_i\,{=}\,H_i^{-1}$. This is a strong form of uncountable cofinality for $G$, where each $H_i$ is a subgroup of $G$. This basic tool comes from a recent paper by Bergman on generating systems of the infinite symmetric groups, which is discussed in the introduction. The main result is a theorem which can be applied to various classical groups including the symmetric groups and homeomorphism groups of Cantor's discontinuum, the rationals, and the irrationals, respectively. They all have uncountable strong cofinality. Thus the result also unifies various known results about cofinalities. A notable example is the group BSym ($\Q$) of all bounded permutations of the rationals $\Q$ which has uncountable cofinality but countable strong cofinality.

Type
Notes and Papers
Copyright
The London Mathematical Society 2005

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Footnotes

This work is supported by project I-706-54.6/2001 of the German–Israeli Foundation for Scientific Research & Development.