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Let $G$ be an amenable group of exponential growth and let $S$ be a finite symmetric generating set. For each $k$, let $B_{k}$ be the closed ball of radius $k$ about the identity element in the corresponding Cayley graph of $G$ and let $b_{k} = |B_{k}|$. If $\lim b_{k+1}/b_{k}$ exists, then $\lim b_{k+1}/b_{k} = \lim b_{k}^{1/k} > 1$ and this easily implies that no subsequence of the $B_{k}$ forms a Folner sequence for $G$. But is this also true for those amenable groups of exponential growth for which $\lim b_{k+1}/b_{k}$ does not exist?

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@Simon: I do not know any group for which the limit $\lim b_{k+1}/b_{k}$ does not exist. –  Mark Sapir Oct 28 '10 at 15:21
    
@Mark: see mathoverflow.net/questions/36126/… –  Andreas Thom Oct 28 '10 at 15:56
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