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arXiv 1901.05046math.GRmath.GT
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相对因子图的有界性与自由积自同构的子群分类

Boundaries of relative factor graphs and subgroup classification for automorphisms of free products

Vincent Guirardel, Camille Horbez

摘要:

给定可数群$G$分裂为自由积$G=G_1\ast\dots\ast G_k\ast F_N$,我们建立了保持每个$G_i$共轭类的$G$的所有外自同构的群$Out(G,\mathcal{F})$的子群的分类结果.我们发现,每一个生成子群$H\subseteq Out(G,\mathcal{F})$要么包含一个相对完全不可约的自同构,否则它几乎保留了共轭类的一个适当的自由因子相对于分解(有限生成的假设$H$可以下降为$G=F_N$,或更一般地当$G$是toral相对双曲)。在第一种情况下,要么$H$实际上保留了$G$中的非周边共轭类,要么$H$包含一个超环面自同构。获得这些分类结果的关键几何工具是自由因子图$\mathrm{FF}$和$\mathcal{Z}$ -因子图$\mathcal{Z}\mathrm{F}$的相对版本的Gromov边界的描述,作为空间的等价类的arational树(分别相对自由arational树)。我们还确定了$\mathrm{FF}$的斜驶等距与$Out(G,\mathcal{F})$的完全不可约元,以及$\mathcal{Z}\mathrm{F}$的斜驶等距与完全不可约的超环面外自同构。

英文摘要:

Given a countable group $G$ splitting as a free product $G=G_1\ast\dots\ast G_k\ast F_N$, we establish classification results for subgroups of the group $Out(G,\mathcal{F})$ of all outer automorphisms of $G$ that preserve the conjugacy classes of each $G_i$. We show that every finitely generated subgroup $H\subseteq Out(G,\mathcal{F})$ either contains a relatively fully irreducible automorphism, or else it virtually preserves the conjugacy class of a proper free factor relative to the decomposition (the finite generation hypothesis on $H$ can be dropped for $G=F_N$, or more generally when $G$ is toral relatively hyperbolic). In the first case, either $H$ virtually preserves a nonperipheral conjugacy class in $G$, or else $H$ contains an atoroidal automorphism. The key geometric tool to obtain these classification results is a description of the Gromov boundaries of relative versions of the free factor graph $\mathrm{FF}$ and the $\mathcal{Z}$-factor graph $\mathcal{Z}\mathrm{F}$, as spaces of equivalence classes of arational trees (respectively relatively free arational trees). We also identify the loxodromic isometries of $\mathrm{FF}$ with the fully irreducible elements of $Out(G,\mathcal{F})$, and loxodromic isometries of $\mathcal{Z}\mathrm{F}$ with the fully irreducible atoroidal outer automorphisms.

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