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Shapes of hyperbolic triangles and once-punctured torus groups
Mathematische Zeitschrift ( IF 1.0 ) Pub Date : 2021-05-03 , DOI: 10.1007/s00209-021-02745-3
Sang-hyun Kim , Thomas Koberda , Jaejeong Lee , Ken’ichi Ohshika , Ser Peow Tan , Xinghua Gao

Let \(\Delta \) be a hyperbolic triangle with a fixed area \(\varphi \). We prove that for all but countably many \(\varphi \), generic choices of \(\Delta \) have the property that the group generated by the \(\pi \)-rotations about the midpoints of the sides of the triangle admits no nontrivial relations. By contrast, we show for all \(\varphi \in (0,\pi ){\setminus }\mathbb {Q}\pi \), a dense set of triangles does afford nontrivial relations, which in the generic case map to hyperbolic translations. To establish this fact, we study the deformation space \(\mathfrak {C}_\theta \) of singular hyperbolic metrics on a torus with a single cone point of angle \(\theta =2(\pi -\varphi )\), and answer an analogous question for the holonomy map \(\rho _\xi \) of such a hyperbolic structure \(\xi \). In an appendix by Gao, concrete examples of \(\theta \) and \(\xi \in \mathfrak {C}_\theta \) are given where the image of each \(\rho _\xi \) is finitely presented, non-free and torsion-free; in fact, those images will be isomorphic to the fundamental groups of closed hyperbolic 3-manifolds.



中文翻译:

双曲三角形和一次穿孔的圆环组的形状

\(\ Delta \)是一个固定面积\(\ varphi \)的双曲三角形。我们证明,除了几乎所有的所有\(\ varphi \)之外\(\ Delta \)的泛型选择都具有以下性质:由\(\ pi \)-旋转围绕三角形边的中点生成的组不承认任何非平凡的关系。相比之下,对于所有\(\ varphi \ in(0,\ pi){\ setminus} \ mathbb {Q} \ pi \),我们显示了一组紧密的三角形确实具有非平凡的关系,这在一般情况下映射为双曲线翻译。为了建立这一事实,我们研究了具有单个圆锥角的圆环上奇异双曲度量的变形空间\(\ mathfrak {C} _ \ theta \)\(\ theta = 2(\ pi-\ varphi)\),并回答类似双曲结构\(\ xi \)的完整\(\ rho _ \ xi \)的类似问题。在高的附录中,给出了\(\ theta \)\(\ xi \ in \ mathfrak {C} _ \ theta \)中的具体示例,其中每个\(\ rho _ \ xi \)的图像都是有限的呈现的,非自由的和无扭转的;实际上,这些图像对于封闭的双曲3型流形的基本群将是同构的。

更新日期:2021-05-03
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