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Asymptotic geometry of lamplighters over one-ended groups

  • Anthony Genevois,
  • Romain Tessera

摘要

This article is dedicated to the asymptotic geometry of wreath products F H : = ( H F ) H $F\wr H := \left ( \bigoplus _{H} F \right ) \rtimes H$ where F $F$ is a finite group and H $H$ is a finitely generated group. Our first main result says that a coarse map from a finitely presented one-ended group to F H $F\wr H$ must land at bounded distance from a left coset of H $H$ . Our second main result, building on the later, is a very restrictive description of quasi-isometries between two lamplighter groups on finitely presented one-ended groups. Third, we obtain a complete classification of these groups up to quasi-isometry. More precisely, given two finite groups F 1 $F_{1}$ , F 2 $F_{2}$ and two finitely presented one-ended groups H 1 $H_{1}$ , H 2 $H_{2}$ , we show that F 1 H 1 $F_{1} \wr H_{1}$ and F 2 H 2 $F_{2} \wr H_{2}$ are quasi-isometric if and only if either (i) H 1 $H_{1}$ , H 2 $H_{2}$ are non-amenable quasi-isometric groups and | F 1 | $|F_{1}|$ , | F 2 | $|F_{2}|$ have the same prime divisors, or (ii) H 1 $H_{1}$ , H 2 $H_{2}$ are amenable, | F 1 | = k n 1 $|F_{1}|=k^{n_{1}}$ and | F 2 | = k n 2 $|F_{2}|=k^{n_{2}}$ for some k , n 1 , n 2 1 $k,n_{1},n_{2} \geq 1$ , and there exists a quasi- ( n 2 / n 1 ) $(n_{2}/n_{1})$ -to-one quasi-isometry H 1 H 2 $H_{1} \to H_{2}$ . This can be seen as far reaching extension of a celebrated work of Eskin-Fisher-Whyte who treated the case of H = Z $H=\mathbb{Z}$ . Our approach is however fundamentally different, as it crucially exploits the assumption that H $H$ is one-ended. Our central tool is a new geometric interpretation of lamplighter groups involving natural families of quasi-median spaces.