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Discreteness properties of translation numbers in solvable groups. (English) Zbl 0956.20039
Let $$G$$ be a group with a metric $$d$$ which is invariant under left multiplication by $$G$$, let $$\|\;\|\colon G\to\mathbb{Z}$$ be defined by $$\|x\|=d(x,1_G)$$ and let $$\tau(x)=\limsup_{n\to\infty}\tfrac{\|x^n\|}{n}$$. This quantity is called the translation number of $$x$$. A group is called translation proper if it carries a left-invariant metric in which the translation numbers of the non-torsion elements are non-zero and translation discrete if they are bounded away from zero. The main results of this paper are that a translation proper solvable group of finite virtual cohomological dimension is metabelian-by-finite, and that a translation discrete solvable group of finite virtual cohomological dimension $$m$$ is a finite extension of $$\mathbb{Z}^m$$. The author also gives two examples – one of a polycyclic group which is translation proper but not translation discrete, and another of a non-Abelian solvable group of infinite cohomological dimension which is translation discrete.

##### MSC:
 20F65 Geometric group theory 20F16 Solvable groups, supersolvable groups 57M07 Topological methods in group theory
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