Geometry and topology of wild translation surfaces Zur Erlangung des akademischen Grades eines DOKTORS DER NATURWISSENSCHAFTEN von der Fakultät für Mathematik des Karlsruher Instituts für Technologie (KIT) genehmigte DISSERTATION von Dipl.-Math. oec. Anja Randecker aus Horb am Neckar Tag der mündlichen Prüfung: 17. Juni 2015 Referentin: JProf. Dr. Gabriela Weitze-Schmithüsen Korreferenten: Prof. Dr. Frank Herrlich JProf. Dr. Ferrán Valdez Contents Introduction 1 1 Basics on translation surfaces 9 1.1 Translation surfaces . .9 1.2 Singularities of translation surfaces . 16 1.3 Ends of topological spaces . 26 1.4 Veech groups of translation surfaces . 34 1.5 Dynamics on translation surfaces . 39 2 Examples that are worth knowing 43 2.1 Chamanara surface . 43 2.2 Exponential surface . 50 2.3 Stack of boxes . 54 2.4 Slit constructions . 57 3 Ends of translation surfaces 71 3.1 Different types of ends . 71 3.2 Spaces of ends that can occur . 74 3.3 Relations between ends and singularities . 80 4 Around wild singularities 87 4.1 Topological and geometric properties of a neighborhood of a singularity . 87 4.2 Saddle connections in a dense set of directions . 91 4.3 Short saddle connections and xossiness . 94 4.4 Conditions for singularities to fulfill xossiness . 99 5 The influence of wild singularities on the genus 109 5.1 Saddle connections as a tool for detecting infinite genus . 109 5.2 Conditions for wild singularities to imply infinite genus . 118 6 Spaces of rotational components 127 6.1 Topological properties of rotational components . 127 6.2 Coarsening from L(σ) to Le(σ) ................ 133 6.3 Every finite space occurs as space of rotational components 137 Bibliography 143 Index 149 Introduction The subject of this thesis, wild translation surfaces, is a very recent top- ic in mathematics. However, the first results in the classical theory of translation surfaces go back to the early 20th century. The objects were not called translation surfaces then but had different names over the years. This is due to the fact that translation surfaces arise in very different contexts which makes them particularly interesting. There are relations to dynamical systems, Teichmüller theory, algebraic geometry, topology, geometric group theory, and many other fields. The most visual way to define a classical translation surface isby considering finitely many polygons in the plane. If every edge ofthe polygons can be identified with a parallel edge of the same length so that we obtain a connected, orientable surface then the resulting object is a translation surface. It is locally flat at all points with the possible exception of the former vertices of the polygons. These exceptional points are called singularities and they are cone points of the resulting surface with cone angle 2πk for some k ≥ 2. Classical translation surfaces arise for example in the study of billiards in a polygon with rational angles, i.e. in a polygon with angles that are rational multiples of π. Fox and Kershner study billiard paths in such polygons in [FK36] by “unfolding” the polygon. This means they reflect the polygon across all edges, then reflect the images of the polygon across all images of the edges, and so on. The original, perhaps complicated billiard path in the polygon can then be considered as a straight line flow in this space of copies. To obtain a translation surface, Fox and Kershner choose a finite subset of these copies that constitute a fundamental domain under the group of reflections. The edges of the reflected polygons are identified via the product of the corresponding reflections which isinfact a translation. The condition on the rationality of the angles is important to guarantee that the fundamental domain consists of finitely many polygons. In Introduction particular, it ensures that the singularities of the resulting translation surface are cone points as discussed above. The unfolding construction of Fox and Kershner is described again by Katok and Zemlyakov in [ZK76] which leads to a number of articles, for instance [KMS86] by Kerckhoff, Masur, and Smillie which provides an ergodicity result on billiard paths. This is also where abelian differentials come into the theory. One obtains a translation structure on a Riemann surface by integrating over an abelian differential in neighborhoods of all points except for the zeros of the differential. The name “translation surface”, however, goes back to Thurston’s lecture notes [Thu81, Chapter 3] where he defines (G, X)-manifolds and G-manifolds without mentioning translation surfaces explicitly. In the related language of F-structures, the translation surfaces occur again in the work of Veech. His article [Vee89] associates a group, now called Veech group, to a translation surface and relates the group to properties of the dynamics on the Teichmüller space. In [Vee90], Veech introduces the idea to use translation surfaces to estimate the volume of moduli spaces which opens up the field to algebraic geometry. Since then, the theory of classical translation surfaces has grown to a research field on its own with numerous mathematicians, including several Fields medalists, from algebraic geometry, dynamical systems, and geometric group theory working on it. Having spoken about the classical translation surfaces (which are called finite translation surfaces from now on) so far, a more general concept shines through along the way. In the literature, it is often called infinite translation surface but as the more general object it should have the more general name – translation surface – in the author’s view. The definition of a general translation surface is that it is a connected surface with a translation structure (see Definition 1.1). If we do not insist, for example, on rational angles then the unfolding of a polygon provides an infinite translation surface, studied by Valdez in [Val09] and [Val12]. As we have indicated in the discussion before, the singularities of the resulting translation surface do not need to be cone points. Another possibility to produce examples is to consider infinite coverings of finite translation surfaces as was done by Hubert and Schmithüsen in [HS10], by Hooper, Hubert, and Weiss in [HW12], [HHW13], and [HW13], 2 Introduction and by Frączek and Ulcigrai in [FU14], for example. A special example of this class of translation surfaces is the wind-tree model which goes back to a model of Ehrenfest and Ehrenfest that describes the movement of gaseous particles. Many different aspects of the wind-tree model are studied, for instance, by Avila, Delecroix, Hubert, Lelièvre, and Troubetzkoy in [DHL14], [HLT11], and [AHar]. In the two mentioned constructions, the singularities of the translation surfaces are not cone points in the strict sense any more but they can be considered as infinite angle analogs of cone points. For both kinds of cone points it is true that there exists a punctured neighborhood of the singularity which is a finite or infinite cyclic translation covering ofthe once-punctured flat disk. In [Cha04], Chamanara describes a first example of a family of transla- tion surfaces so that each translation surface in that family has a singularity which has no punctured neighborhood that is a translation covering of a once-punctured disk. In particular, every neighborhood of a singularity in that family has infinite genus and so the topology of the corresponding translation surface is more complicated but also raises more interesting questions. Singularities of this type are called wild and translation surfaces with wild singularities are called wild translation surfaces. Different examples of wild translation surfaces have then been studied, for instance arising as a geometric limit of the Arnoux-Yoccoz surfaces defined by Bowman in [Bow13] and arising from a generalization of Thurston’s construction (see [Thu88]) by Hooper in [Hoo15]. However, the systematic study of wild singularities started quite recently with the work of Bowman and Valdez. In [BV13], they define linear approaches and rotational components (see Section 1.2 for definitions) which makes it possible to study a wild singularity in terms of geodesic rays emanating from the singularity. In the theory of finite translation surfaces, an easy-to-prove but fasci- nating equation is true: the Gauß-Bonnet formula provides a correlation between the topology of translation surfaces and the geometry of their sin- gularities. In fact, the equation relates the genus and the cone angle of the singularities, without using any other information (see Proposition 1.14). A similar result on general translation surfaces would not only be nice in itself but could help to classify translation surfaces. For finite translation surfaces, there exists a well-established theory on moduli spaces and strata 3 Introduction of translation surfaces which has recently made a big leap with the results of Eskin, Mirzakhani, and Mohammadi in [EMM15] and of Eskin and Mirzakhani in [EM13]. Such a theory is desirable for general translation surfaces but there is still a lack of a good notion of moduli spaces and even more of strata of translation surfaces. To remedy this lack it seems natural to first study singularities (especially wild singularities) in more detail and establish relations between the singularities as a geometric object of a translation surface and the topology in terms of genus and ends. The purpose of this thesis is to contribute to these new aspects of the theory. We do this under three different points of view. As one aspect, we study the local neighborhood of a singularity, in terms of topological properties and their spaces of rotational components. As a second aspect we consider the global topology of wild translation surfaces, in particular the spaces of ends that can occur. The third aspect is to bring together these two points of view and show under which conditions the existence of a singularity with given properties can imply topological properties of the translation surface.