The horofunction compactification of Teichmüller metric

A commentary on Teichmüller's paper "Extremale quasikonforme Abbildungen und quadratische Differentiale"

We provide a commentary on Teichm{ü}ller's paper "Extremale quasikonforme Abbildungen und quadratische Differentiale" (Extremal quasiconformal mappings of closed oriented Riemann surfaces), Abh. Preuss. Akad. Wiss., Math.-Naturw. Kl. 1940, No.22, 1-197 (1940). The paper is quoted in several works, although it was read by very few people. Some of the results it contains were rediscovered later on and published without any reference to Teichm{ü}ller.In this commentary, we highlight the main results and the main ideas contained in that paper and we describe some of the important developments they…

Teichmüller空间上的水平度量

The horocyclic metric on Teichm{ü}ller spaces

本文从共形视角研究Teichmüller空间上的水平度量，将Thurston的理论扩展到任意亏格的Riemann面，并通过复Legendre变换建立Weil-Petersson symplectic形式的对偶关系。

On the early history of moduli and Teichm{ü}ller spaces

We survey some major contributions to Riemann's moduli space and Teichm{ü}ller space. Our report has a historical character, but the stress is on the chain of mathematical ideas. We start with the introduction of Riemann surfaces, and we end with the discovery of some of the basic structures of Riemann's moduli space and Teichm{ü}ller space. We point out several facts which seem to be unknown to many algebraic geometers and analysts working in the theory. The period we are interested in starts with Riemann, in 1851, and ends in the early 1960s, when Ahlfors and Bers confirmed that Teichm{ü}lle…

A Commentary on Teichm{ü}ller's paper ``Vollst{ä}ndige L{ö}sung einer Extremalaufgabe der quasikonformen Abbildung'' (Complete solution of an extremal problem of the quasiconformal mapping)

We comment on Teichm{ü}ller 's paper ''Vollst{ä}ndige L{ö}sung einer Extremalaufgabe der quasikonformen Abbildung'' (Complete solution of an ex-tremal problem of the quasiconformal mapping),, published in 1941. In this paper, Teichm{ü}ller gives a proof of the existence of extremal quasiconformal mappings in the case of the pentagon (disc with five distinguished points on the boundary). The final version of this paper will appear as a chapter in Volume VI of the Handbook of Teichm{ü}ller theory.

A Commentary on Teichm{ü}ller's paper "Untersuchungen über konforme und quasikonforme Abbildungen" (Investigations on conformal and quasiconformal mappings) (to appear in Vol. VII of the \emph{Handbook of Teichmüller theory}

This is a commentary on Teichm{ü}ller's paper Unter-suchungen{ü}ber konforme und quasikonforme Abbildungen (Inves-tigations on conformal and quasiconformal mappings) published in 1938. The paper contains fundamental results in conformal geometry , in particular a lemma, known as the Modulsatz, which insures the almost circularity of certain loci defined as complementary components of simply connected regions in the Riemann sphere, and another lemma, which we call the Main Lemma, which insures the circularity near infinity of the image of circles by a qua-siconformal map. The two results find w…

Infinite-dimensional Teichm{ü}ller spaces

In this paper, the Teichm{ü}ller spaces of surfaces appear from two points of views: the conformal category and the hyperbolic category. In contrast to the case of surfaces of topologically finite type, the Teichm{ü}ller spaces associated to surfaces of topologically infinite type depend on the choice of a base structure. In the setting of surfaces of infinite type, the Teichm{ü}ller spaces can be endowed with different distance functions such as the length-spectrum distance, the bi-Lipschitz distance, the Fenchel-Nielsen distance, the Teichm{ü}ller distance and there are other distance functi…

TeichmUller's work on the type problem (To appear in Vol. VII of the Handbook of Teichm{ü}ller theory, 2020)

The type problem is the problem of deciding, for a simply connected Riemann surface, whether it is conformally equivalent to the complex plane or to the unit dic in the complex plane. We report on Teichm{ü}ller's results on the type problem from his two papers Eine Anwendung quasikonformer Abbildungen auf das Typenproblem (An application of quasiconformal map-pings to the type problem) (1937) and Untersuchungen{ü}ber kon-forme und quasikonforme Abbildungen (Investigations on con-formal and quasiconformal mappings) (1938). They concern simply connected Riemann surfaces defined as branched cover…

Deforming hyperbolic hexagons with applications to the arc and the Thurston metrics on Teichm{ü}ller spaces

For each right-angled hexagon in the hyperbolic plane, we construct a one-parameter family of right-angled hexagons with a Lipschitz map between any two elements in this family, realizing the smallest Lipschitz constant in the homotopy class of this map relative to the boundary. As a consequence of this construction, we exhibit new geodesics for the arc metric on the Teichm{ü}ller space of an arbitrary surface of negative Euler characteristic with nonempty boundary. We also obtain new geodesics for Thurston's metric on Teichm{ü}ller spaces of hyperbolic surfaces without boundary.

Ideal triangles, hyperbolic surfaces and the Thurston metric on Teichm{ü}ller space

These are notes on the hyperbolic geometry of surfaces, Teichm{ü}ller spaces and Thurston's metric on these spaces. They are associated with lectures I gave at the Morningside Center of Mathematics of the Chinese Academy of Sciences in March 2019 and at the Chebyshev Laboratory of the Saint Petersburg State University in May 2019. In particular, I survey several results on the behavior of stretch lines, a distinguished class of geodesics for Thurston's metric and I point out several analogies between this metric and Teichm{ü}ller's metric. Several open questions are addressed. The final versio…

A commentary on Teichmüller's paper "Verschiebungssatz der quasikonformen Abbildung" (A displacement theorem of quasiconformal mapping)

This is a commentary on Teichm{ü}ller's paper Ein Verschiebungssatz der quasikonformen Abbildung (A displacement theorem of quasiconformal mapping), published in 1944. We explain in detail how Teichm{ü}ller solves the problem of finding the quasiconformal mapping from the unit disc to itself, sending 0 to a strictly negative point on the real line, holding the boundary of the disc pointwise fixed and with the smallest quasiconformal dilatation. We mention also some consequences of this extremal problem and we ask a question.

关于锐角三角形的Teichmüller空间

On the Teichm{ü}ller space of acute triangles

该研究延续前人工作，通过直接计算给出锐角三角形子空间上类Thurston度量的距离函数与Finsler结构显式表达式，推导其无穷小刚性，并描述极端Lipschitz映射的最大拉伸轨迹。

Derivatives of length functions and shearing coordinates on teichm{ü}ller spaces

Let $S$ be a closed oriented surface of genus at least $2$, and denote by $\mathcal{T}(S)$ its Teichm{ü}ller space. For any isotopy class of closed curves $γ$, we compute the first three derivatives of the length function $\ell\_γ:\mathcal{T}(S)\rightarrow\mathbf{R}\_+$ in the shearing coordinates associated to a maximal geodesic lamination $λ$. We show that if $γ$ intersects every leaf of $λ$, then the Hessian of $\ell\_γ$ is positive-definite. We extend this result to length functions of measured laminations. We also provide a method to compute higher derivatives of length functions of geode…

Teichm{ü}ller空间的无穷小与全局Thurston几何

The infinitesimal and global Thurston geometry of Teichm{ü}ller space

本文系统研究了Thurston度量的无穷小几何，证明切球与余切球结构可恢复带标记双曲曲面的标记与几何，并由此导出全局几何刚性陈述。

Teichmüller度量的horoboundary的定性描述

A qualitative description of the horoboundary of the Teichmüller metric

本文通过证明horofunction紧化比视觉紧化更细，揭示了Teichmüller空间两种紧化的关系，并由此得出horoboundary的道路连通性、Busemann点不稠密等拓扑性质。

具有Thurston Finsler度量的Teichmüller空间的单位切球和余切球的组合结构

The combinatorial structure of the unit tangent spheres and cotangent spheres of Teichm{ü}ller space with Thurston's Finsler metric

本文研究了负欧拉示性数闭曲面的Teichmüller空间上Thurston度量诱导的切空间和余切空间范数的单位球面的组合结构，给出了切空间单位球面各面维数的公式，证明了该组合结构独立于Teichmüller空间中的基点，并建立了扩展映射类群与单位球面组合自同构群之间的自然同构（亏格2时为满同态），同时得到了余切空间单位球面面的余维数公式以及面暴露和对应投影加权多曲线的充要条件。

Teichmüller 空间上的地震度量

The earthquake metric on Teichm{ü}ller space

本文首次系统研究 Thurston 在 Teichmüller 空间上引入的非对称 Finsler 地震度量，证明其若干性质，提出 FD-完备化概念，并将其与 Weil–Petersson 完备化及全局最小化问题联系起来。

Higher spin mapping class groups and strata of Abelian differentials over Teichm{ü}ller space

For $g \ge 5$, we give a complete classification of the connected components of strata of abelian differentials over Teichmüller space, establishing an analogue of Kontsevich and Zorich's classification of their components over moduli space. Building off of work of the first author (arXiv:1901.05482), we find that the non-hyperelliptic components are classified by an invariant known as an $r$-spin structure. This is accomplished by computing a certain monodromy group valued in the mapping class group. To do this, we determine explicit finite generating sets for all $r$-spin stabilizer subgroup…

泰希米勒空间中单位切球的凸结构

Convex structures of the unit tangent spheres in Teichm{ü}ller space

研究泰希米勒空间中单位切球的凸结构，通过分析瑟斯顿度量的芬斯勒无穷小球，得到面、暴露面和极点的特征，包括面与链回归测地线层叠的关系等，为相关拓扑部分提供了新方法并肯定了一个猜想。

Gardiner-Masur紧化边界点的最优测地线

Optimal geodesics for boundary points of the Gardiner-Masur compactification

本文证明了Gardiner-Masur紧化中填满曲面的边界点对存在唯一最优的Teichmüller测地线，并给出了测地线序列收敛性的应用。

Teichmüller 空间中极值长度的渐进行为

The asymptoticity of extremal length in Teichmüller space

研究Teichmüller空间中极值长度的渐进行为，推导极限表达式及Teichmüller距离公式，并确定渐近射线的条件。

The asymptotic geometry of the Teichmüller metric

We determine the asymptotic behaviour of extremal length along arbitrary Teichmüller rays. This allows us to calculate the endpoint in the Gardiner-Masur boundary of any Teichmüller ray. We give a proof that this compactification is the same as the horofunction compactification. An important subset of the latter is the set of Busemann points. We show that the Busemann points are exactly the limits of the Teichmüller rays, and we give a necessary and sufficient condition for a sequence of Busemann points to converge to a Busemann point. Finally, we determine the detour metric on the boundary.

On Grothendieck's tame topology

Grothendieck's Esquisse d'un programme is often referred to for the ideas it contains on dessins d'enfants, the Teichm{ü}ller tower, and the actions of the absolute Galois group on these objects or their etale fundamental groups. But this program contains several other important ideas. In particular, motivated by surface topology and moduli spaces of Riemann surfaces, Grothendieck calls there for a recasting of topology, in order to make it fit to the objects of semialgebraic and semianalytic geometry, and in particular to the study of the Mumford-Deligne compactifications of moduli spaces. A …

The horoboundary and isometry group of Thurston's Lipschitz metric

We show that the horofunction boundary of Teichmüller space with Thurston's Lipschitz metric is the same as the Thurston boundary. We use this to determine the isometry group of the Lipschitz metric, apart from in some exceptional cases. We also show that the Teichmüller spaces of different surfaces, when endowed with this metric, are not isometric, again with some possible exceptions of low genus.

通过测地流实现的Thurston紧致化：非紧有限面积曲面的情形

Thurston's compactification via geodesic currents: The case of non-compact finite area surfaces

本文提出了一种适用于有限面积曲面的变体方法，解决了Bonahon使用测地流构造Teichm{ü}ller空间Thurston紧致化时仅适用于闭曲面的问题。

Gromov双曲空间与Teichmüller空间上随机等距变换平移长度的线性增长

Linear growth of translation lengths of random isometries on Gromov hyperbolic spaces and Teichmüller spaces

该研究针对Gromov双曲空间等距群上的随机游走，证明无矩条件下非初等随机游走平移长度至少线性增长，有限一阶矩下增长率等于漂移，还将技术推广到Teichmüller空间，在有限一阶矩下证明谱定理，拓展了已有结果。

极限边界与填充测地流的刚性

Horoboundary and rigidity of filling geodesic currents

该研究通过构建双曲面上测地流的非对称度量，揭示了不同亏格表面的测地流空间在度量上的刚性差异。

Minimal stretch maps between Euclidean triangle?

Given two triangles whose angles are all acute, we find a homeomorphism with the smallest Lipschitz constant between them and we give a formula for the Lipschitz constant of this map. We show that on the set of pairs of acute triangles with fixed area, the function which assigns the logarithm of the smallest Lipschitz constant of Lipschitz maps between them is a symmetric metric. We show that this metric is Finsler, we give a necessary and sufficient condition for a path in this metric space to be geodesic and we determine the isometry group of this metric space. This study is motivated by Thu…