DOCSLIB.ORG

Fuchsian Groups, Coverings of Riemann Surfaces, Subgroup Growth, Random Quotients and Random Walks

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Fuchsian Groups, Coverings of Riemann Surfaces, Subgroup Growth, Random Quotients and Random Walks Fuchsian groups, coverings of Riemann surfaces, subgroup growth, random quotients and random walks Martin W. Liebeck Aner Shalev Department of Mathematics Institute of Mathematics Imperial College Hebrew University London SW7 2BZ Jerusalem 91904 England Israel Abstract Fuchsian groups (acting as isometries of the hyperbolic plane) oc- cur naturally in geometry, combinatorial group theory, and other con- texts. We use character-theoretic and probabilistic methods to study the spaces of homomorphisms from Fuchsian groups to symmetric groups. We obtain a wide variety of applications, ranging from count- ing branched coverings of Riemann surfaces, to subgroup growth and random finite quotients of Fuchsian groups, as well as random walks on symmetric groups. In particular we show that, in some sense, almost all homomor- phisms from a Fuchsian group to alternating groups An are surjective, and this implies Higman’s conjecture that every Fuchsian group sur- jects onto all large enough alternating groups. As a very special case we obtain a random Hurwitz generation of An, namely random gen- eration by two elements of orders 2 and 3 whose product has order 7. We also establish the analogue of Higman’s conjecture for symmetric groups. We apply these results to branched coverings of Riemann sur- faces, showing that under some assumptions on the ramification types, their monodromy group is almost always Sn or An. Another application concerns subgroup growth. We show that a Fuchsian group Γ has (n!)µ+o(1) index n subgroups, where µ is the mea- sure of Γ, and derive similar estimates for so-called Eisenstein numbers of coverings of Riemann surfaces. A final application concerns random walks on alternating and sym- metric groups. We give necessary and sufficient conditions for a collec- tion of ‘almost homogeneous’ conjugacy classes in An to have product equal to An almost uniformly pointwise. Our methods involve some new asymptotic results for degrees and values of irreducible characters of symmetric groups. 1 Contents 1. Introduction 2. Characters of symmetric groups: asymptotic results 3. Homomorphisms from Fuchsian groups to symmetric groups 4. Subgroup growth 5. Maximal subgroup growth 6. Random quotients I: Higman’s conjecture 7. Random quotients II: symmetric quotients 8. Branched coverings of Riemann surfaces 9. Random walks on symmetric groups 1 Introduction A Fuchsian group is a finitely generated non-elementary discrete group of isometries of the hyperbolic plane H2. By classical work of Fricke and Klein, the orientation-preserving such groups Γ have a presentation of the following form: (1.1) generators: a1, b1, . , ag, bg (hyperbolic) x1, . , xd (elliptic) y1, . , ys (parabolic) z1, . , zt (hyperbolic boundary elements) relations: xm1 = = xmd = 1, 1 ··· d x1 xd y1 ys z1 zt [a1, b1] [ag, bg] = 1, ··· ··· ··· ··· where g, d, s, t 0 and mi 2 for all i. The number g is referred to as the genus of Γ. The≥ measure µ≥(Γ) of an orientation-preserving Fuchsian group Γ is defined by d 1 µ(Γ) = 2g 2 + (1 ) + s + t. m − i − i X=1 It is well known that µ(Γ) > 0. The groups with presentations as above, but having µ(Γ) = 0 or µ(Γ) < 0, are the so-called Euclidean and spherical groups, respectively. The second author thanks EPSRC for its support and Imperial College for its hospi- tality while this work was carried out 2 We shall also study non-orientation-preserving Fuchsian groups; these have presentations as follows, with g > 0: (1.2) generators: a1, . , ag x1, . , xd y1, . , ys z1, . , zt m1 md relations: x1 = = xd = 1, ··· 2 2 x1 xd y1 ys z1 zt a a = 1. ··· ··· ··· 1 ··· g In this case the measure µ(Γ) is defined by d 1 µ(Γ) = g 2 + (1 ) + s + t, m − i − i X=1 and again, µ(Γ) > 0. We call Fuchsian groups as in (1.1) oriented, and those as in (1.2) non- oriented. Note that µ(Γ) coincides with χ(Γ), where χ(Γ) is the Euler characteristic of Γ. − If s + t > 0 then Γ is a free product of cyclic groups. In particular, non-abelian free groups are Fuchsian, as well as free products of finite cyclic groups, such as the modular group. Other examples are surface groups (where d = s = t = 0), and triangle groups m1 m2 m3 ∆(m1, m2, m3) = x1, x2, x3 x = x = x = x1x2x3 = 1 , h | 1 2 3 i (where g = s = t = 0, d = 3 and 1 < 1). Among triangle groups, the mi Hurwitz group ∆(2, 3, 7) has received particular attention, one reason being P that its finite images are precisely those finite groups which occur as the automorphism group of a Riemann surface of genus h 2 and have order achieving the Hurwitz bound 84(h 1) (see [7, 21]). ≥ − We call Fuchsian groups with s = t = 0 proper (also termed F -groups in [33, iii.5]). Our main focus is on proper Fuchsian groups, since the improper ones are easier to handle, and some of our results are either known or easily derived for them. In this paper we study the space of homomorphisms Hom(Γ,Sn) from a Fuchsian group Γ to a symmetric group. The study of this space and various subspaces has a wide variety of applications, ranging from coverings of Riemann surfaces to subgroup growth and random finite quotients of Fuchsian groups, as well as random walks on symmetric groups. Two major by-products are estimates for Eisenstein numbers of coverings of Riemann surfaces in the hyperbolic case (Theorem 1.3), and a probabilistic proof of the well-known conjecture of Graham Higman that any Fuchsian group 3 surjects onto all but finitely many alternating groups (Corollary 1.8). This conjecture has recently been proved by Everitt [16] for oriented Fuchsian groups using completely different methods. Our approach handles general Fuchsian groups, and we also prove an analogue of Higman’s conjecture for symmetric quotients, determining precisely which Fuchsian groups surject to all but finitely many symmetric groups (Theorem 1.10). These results have applications to monodromy groups of branched coverings of Riemann surfaces (Theorem 1.13). A major tool in our proofs is the character theory of symmetric groups, and we establish a number of new asymptotic results relating to degrees and values of such characters. For example, denoting by Irr(Sn) the set of all irreducible characters of Sn, we prove Theorem 1.1 Fix a real number s > 0. Then s χ(1)− 2 as n . → → ∞ χ Irr(Sn) ∈ X χ s O n s Moreover, χ Irr(Sn) (1)− = 2 + ( − ). ∈ P For integers s 1 this was originally proved by Lulov in his unpublished thesis [32]; this≥ was reproved in [37], where more detailed estimates are obtained. The proof of Theorem 1.1 is fairly elementary, but it is important for many of our results, and we sometimes need it for rather small values of 1 s, for example s = 42 . We also prove a version for alternating groups (see Corollary 2.7). We now state our main results. The first deals with the number of homomorphisms from a Fuchsian group to a symmetric group, and forms the basis for the other results. In the statements below, o(1) denotes a quantity which tends to 0 as n . → ∞ Theorem 1.2 For any Fuchsian group Γ, we have µ(Γ)+1+o(1) Hom(Γ,Sn) = (n!) . | | In fact our estimates for Hom(Γ,Sn) are more precise - see Theorem 1.12, Corollary 3.6 and Theorem| 3.7 below.| We also obtain similar results for Hom(Γ,An) , which are important for some of the main applications. | | A classical motivation behind the study of Hom(Γ,Sn) stems from the theory of branched coverings of Riemann surfaces.| Let Y| be a compact connected Riemann surface of genus g, and let y1, . , yd Y be fixed distinct points. Consider index n coverings π : X Y , unramified∈ out- → side y1, . , yd , and with monodromy elements g1, . , gd Sn around { } ∈ 4 y1, . , yd respectively. As is standard, we identify geometrically equivalent coverings. For conjugacy classes C1,...,Cd of Sn, and integers m1, . , md 2, set ≥ C = (C1,...,Cd), m = (m1, . , md) and define P (C, n) = π : X Y : gi Ci for all i , { → ∈ } P (m, n) = π : X Y : gmi = 1 for all i . { → i } Attempts to count such coverings go back to Hurwitz [22]. Define Aut π to be the centralizer in Sn of the monodromy group of π. Following [26] we call the sums 1/ Aut π over such sets of coverings P (C, n) the Eisenstein numbers of coverings,| a term| which the authors attribute to Serre. When all P but one of the classes Ci consists of transpositions these numbers are called Hurwitz numbers, which have been the subject of recent intensive study in view of connections with geometry and physics - see for instance [46] and the references therein. See also [13], where asymptotic results are proved in the case where the Ci consist of cycles of bounded length, and used to study volumes of certain moduli spaces. Eisenstein numbers are related to homomorphisms of Fuchsian groups in Sn the following way. Let Ci = g (1 i d) be classes in Sn, and let mi be i ≤ ≤ the order of gi. Define sgn(Ci) = sgn(gi), and write C = (C1,...,Cd). For a group Γ having presentation as in (1.1) or (1.2), define HomC(Γ,Sn) = φ Hom(Γ,Sn): φ(xi) Ci for 1 i d . { ∈ ∈ ≤ ≤ } d Note that if Γ is proper, and HomC(Γ,Sn) = , then i=1 sgn(Ci) = 1. When Γ is a proper oriented Fuchsian group6 (as∅ in (1.1) with s = t = Q 0), a covering in P (C, n) corresponds to an Sn-class of homomorphisms in HomC(Γ,Sn) (see Section 8 for details).
Load more

Recommended publications
  • Möbius Transformations
    Möbius transformations Möbius transformations are simply the degree one rational maps of C: az + b σ : z 7! : ! A cz + d C C where ad − bc 6= 0 and a b A = c d Then A 7! σA : GL(2C) ! fMobius transformations g is a homomorphism whose kernel is ∗ fλI : λ 2 C g: The homomorphism is an isomorphism restricted to SL(2; C), the subgroup of matri- ces of determinant 1. We have an action of GL(2; C) on C by A:(B:z) = (AB):z; A; B 2 GL(2; C); z 2 C: The action of SL(2; R) preserves the upper half-plane fz 2 C : Im(z) > 0g and also R [ f1g and the lower half-plane. The action of the subgroup a b SU(1; 1) = : jaj2 − jbj2 = 1 b a preserves the open unit disc, the closed unit disc, and its exterior. All of these actions are transitive that is, for all z and w in the domain there is A in the group with A:z = w. Kleinian groups Definition 1 A Kleinian group is a subgroup Γ of P SL(2; C) which is discrete, that is, there is an open neighbourhood U ⊂ P SL(2; C) of the identity element I such that U \ Γ = fIg: Definition 2 A Fuchsian group is a discrete subgroup of P SL(2; R) Equivalently (as usual with topological groups) there is an open neighbourhood V of I such that γV \ γ0V = ; for all γ, γ0 2 Γ, γ 6= γ0. To get this, choose V with V = V −1 and V:V ⊂ U.
    [Show full text]
  • Handlebody Orbifolds and Schottky Uniformizations of Hyperbolic 2-Orbifolds
    proceedings of the american mathematical society Volume 123, Number 12, December 1995 HANDLEBODY ORBIFOLDS AND SCHOTTKY UNIFORMIZATIONS OF HYPERBOLIC 2-ORBIFOLDS MARCO RENI AND BRUNO ZIMMERMANN (Communicated by Ronald J. Stern) Abstract. The retrosection theorem says that any hyperbolic or Riemann sur- face can be uniformized by a Schottky group. We generalize this theorem to the case of hyperbolic 2-orbifolds by giving necessary and sufficient conditions for a hyperbolic 2-orbifold, in terms of its signature, to admit a uniformization by a Kleinian group which is a finite extension of a Schottky group. Equiva- lent^, the conditions characterize those hyperbolic 2-orbifolds which occur as the boundary of a handlebody orbifold, that is, the quotient of a handlebody by a finite group action. 1. Introduction Let F be a Fuchsian group, that is a discrete group of Möbius transforma- tions—or equivalently, hyperbolic isometries—acting on the upper half space or the interior of the unit disk H2 (Poincaré model of the hyperbolic plane). We shall always assume that F has compact quotient tf = H2/F which is a hyperbolic 2-orbifold with signature (g;nx,...,nr); here g denotes the genus of the quotient which is a closed orientable surface, and nx, ... , nr are the orders of the branch points (the singular set of the orbifold) of the branched covering H2 -» H2/F. Then also F has signature (g;nx,... , nr), and (2 - 2g - £-=1(l - 1/«/)) < 0 (see [12] resp. [15] for information about orbifolds resp. Fuchsian groups). In case r — 0, or equivalently, if the Fuch- sian group F is without torsion, the quotient H2/F is a hyperbolic or Riemann surface without branch points.
    [Show full text]
  • Hyperbolic Geometry, Fuchsian Groups, and Tiling Spaces
    HYPERBOLIC GEOMETRY, FUCHSIAN GROUPS, AND TILING SPACES C. OLIVARES Abstract. Expository paper on hyperbolic geometry and Fuchsian groups. Exposition is based on utilizing the group action of hyperbolic isometries to discover facts about the space across models. Fuchsian groups are character- ized, and applied to construct tilings of hyperbolic space. Contents 1. Introduction1 2. The Origin of Hyperbolic Geometry2 3. The Poincar´eDisk Model of 2D Hyperbolic Space3 3.1. Length in the Poincar´eDisk4 3.2. Groups of Isometries in Hyperbolic Space5 3.3. Hyperbolic Geodesics6 4. The Half-Plane Model of Hyperbolic Space 10 5. The Classification of Hyperbolic Isometries 14 5.1. The Three Classes of Isometries 15 5.2. Hyperbolic Elements 17 5.3. Parabolic Elements 18 5.4. Elliptic Elements 18 6. Fuchsian Groups 19 6.1. Defining Fuchsian Groups 20 6.2. Fuchsian Group Action 20 Acknowledgements 26 References 26 1. Introduction One of the goals of geometry is to characterize what a space \looks like." This task is difficult because a space is just a non-empty set endowed with an axiomatic structure|a list of rules for its points to follow. But, a priori, there is nothing that a list of axioms tells us about the curvature of a space, or what a circle, triangle, or other classical shapes would look like in a space. Moreover, the ways geometry manifests itself vary wildly according to changes in the axioms. As an example, consider the following: There are two spaces, with two different rules. In one space, planets are round, and in the other, planets are flat.
    [Show full text]
  • Hyperbolic Geometry of Riemann Surfaces
    3 Hyperbolic geometry of Riemann surfaces By Theorem 1.8.8, all hyperbolic Riemann surfaces inherit the geometry of the hyperbolic plane. How this geometry interacts with the topology of a Riemann surface is a complicated business, and beginning with Section 3.2, the material will become more demanding. Since this book is largely devoted to the study of Riemann surfaces, a careful study of this interaction is of central interest and underlies most of the remainder of the book. 3.1 Fuchsian groups We saw in Proposition 1.8.14 that torsion-free Fuchsian groups and hyper- bolic Riemann surfaces are essentially the same subject. Most such groups and most such surfaces are complicated objects: usually, a Fuchsian group is at least as complicated as a free group on two generators. However, in a few exceptional cases Fuchsian groups are not complicated, whether they have torsion or not. Such groups are called elementary;we classify them in parts 1–3 of Proposition 3.1.2. Part 4 concerns the com- plicated case – the one that really interests us. Notationin 3.1.1 If A is a subset of a group G, we denote by hAi the subgroup generated by A. Propositionin 3.1.2 (Fuchsian groups) Let be a Fuchsian group. 1. If is nite, it is a cyclic group generated by a rotation about a point by 2/n radians, for some positive integer n. 2. If is innite but consists entirely of elliptic and parabolic ele- ments, then it is innite cyclic, is generated by a single parabolic element, and contains no elliptic elements.
    [Show full text]
  • Riemann Surfaces As Orbit Spaces of Fuchsian Groups
    Can. J. Math., Vol. XXIV, No. 4, 1972, pp. 612-616 RIEMANN SURFACES AS ORBIT SPACES OF FUCHSIAN GROUPS M. J. MOORE 1. Introduction. A Fuchsian group is a discrete subgroup of the hyperbolic group, L.F. (2, R), of linear fractional transformations w = i—7 (a> b> c> d real, ad — be = 1), cz + a each such transformation mapping the complex upper half plane D into itself. If T is a Fuchsian group, the orbit space D/T has an analytic structure such that the projection map p: D —» D/T, given by p(z) = Yz, is holomorphic and D/Y is then a Riemann surface. If N is a normal subgroup of a Fuchsian group T, then iV is a Fuchsian group and 5 = D/N is a Riemann surface. The factor group, G = T/N, acts as a group of automorphisms (biholomorphic self-transformations) of 5 for, if y e Y and z e D, then yN e G, Nz e S, and (yN) (Nz) = Nyz. This is easily seen to be independent of the choice of y in its iV-coset and the choice of z in its iV-orbit. Conversely, if 5 is a compact Riemann surface, of genus at least two, then S can be identified with D/K, where K is a Fuchsian group acting without fixed points in D. D is the universal covering space of 5 and K is isomorphic to the fundamental group of S. We shall call a Fuchsian group which acts without fixed points in D, a Fuchsian surface group.
    [Show full text]
  • ON the COHOMOLOGY of FUCHSIAN GROUPS by S
    ON THE COHOMOLOGY OF FUCHSIAN GROUPS by S. J. PATTERSON (Received 12 September, 1974; revised 15 May, 1975) 1. Introduction. The object of this paper is to redevelop the classical theory of multi- pliers of Fuchsian groups [16] and to attempt a classification. The language which appears most appropriate is that of group extensions and the cohomology of groups. This viewpoint is not entirely novel [12] but the entire theory has never been based on it before. A Fuchsian group G is a discrete subgroup of PSL(2 ; R). We shall further assume that G is finitely generated and that PSL(2 ; R)/G has finite volume. We shall begin by investi- gating the extensions of PSL(2 ; R). These induce extensions of G. On the other hand G can be given explicitly by generators and relations. So, in principle at any rate, its extensions can be classified from the point of view of group theory. There is then the basic problem of relating these two approaches. In the case that G has no elliptic elements, this identification can be carried out explicitly by using Chern characters to identify algebraically and geometrically defined objects. A partial extension to groups with elliptic elements can be made using the transfer homomor- phism of group cohomology. As a first application, we consider the question as to when G can be lifted to SL(2 ; R). We prove again a theorem of Petersson ([17]) and Bers ([1]) (itself the answer to a question of Siegel ([22])). Bers' proof was an application of the theory of moduli of Riemann surfaces; ours, being intrinsic, can be considered rather more satisfying.
    [Show full text]
  • MATH32052 Hyperbolic Geometry
    MATH32052 Hyperbolic Geometry Charles Walkden 12th January, 2019 MATH32052 Contents Contents 0 Preliminaries 3 1 Where we are going 6 2 Length and distance in hyperbolic geometry 13 3 Circles and lines, M¨obius transformations 18 4 M¨obius transformations and geodesics in H 23 5 More on the geodesics in H 26 6 The Poincar´edisc model 39 7 The Gauss-Bonnet Theorem 44 8 Hyperbolic triangles 52 9 Fixed points of M¨obius transformations 56 10 Classifying M¨obius transformations: conjugacy, trace, and applications to parabolic transformations 59 11 Classifying M¨obius transformations: hyperbolic and elliptic transforma- tions 62 12 Fuchsian groups 66 13 Fundamental domains 71 14 Dirichlet polygons: the construction 75 15 Dirichlet polygons: examples 79 16 Side-pairing transformations 84 17 Elliptic cycles 87 18 Generators and relations 92 19 Poincar´e’s Theorem: the case of no boundary vertices 97 20 Poincar´e’s Theorem: the case of boundary vertices 102 c The University of Manchester 1 MATH32052 Contents 21 The signature of a Fuchsian group 109 22 Existence of a Fuchsian group with a given signature 117 23 Where we could go next 123 24 All of the exercises 126 25 Solutions 138 c The University of Manchester 2 MATH32052 0. Preliminaries 0. Preliminaries 0.1 Contact details § The lecturer is Dr Charles Walkden, Room 2.241, Tel: 0161 27 55805, Email: [email protected]. My office hour is: WHEN?. If you want to see me at another time then please email me first to arrange a mutually convenient time. 0.2 Course structure § 0.2.1 MATH32052 § MATH32052 Hyperbolic Geoemtry is a 10 credit course.
    [Show full text]
  • On Gonality of Riemann Surfaces
    On gonality of Riemann surfaces Geometriae Dedicata ISSN 0046-5755 Volume 149 Number 1 Geom Dedicata (2010) 149:1-14 DOI 10.1007/s10711-010-9459- x 1 23 Your article is protected by copyright and all rights are held exclusively by Springer Science+Business Media B.V.. This e-offprint is for personal use only and shall not be self- archived in electronic repositories. If you wish to self-archive your work, please use the accepted author’s version for posting to your own website or your institution’s repository. You may further deposit the accepted author’s version on a funder’s repository at a funder’s request, provided it is not made publicly available until 12 months after publication. 1 23 Author's personal copy Geom Dedicata (2010) 149:1–14 DOI 10.1007/s10711-010-9459-x ORIGINAL PAPER On gonality of Riemann surfaces Grzegorz Gromadzki Anthony Weaver Aaron Wootton · · Received: 13 October 2009 / Accepted: 5 January 2010 / Published online: 21 January 2010 ©SpringerScience+BusinessMediaB.V.2010 Abstract A compact Riemann surface X is called a (p, n)-gonal surface if there exists a group of automorphisms C of X (called a (p, n)-gonal group) of prime order p such that the orbit space X/C has genus n.Wederivesomebasicpropertiesof(p, n)-gonal surfaces considered as generalizations of hyperelliptic surfaces and also examine certain properties which do not generalize. In particular, we find a condition which guarantees all (p, n)-gonal groups are conjugate in the full automorphism group of a (p, n)-gonal surface, and we find an upper bound for the size of the corresponding conjugacy class.
    [Show full text]
  • Anti-Self-Dual 4-Manifolds, Quasi-Fuchsian Groups, and Almost-K¨Ahlergeometry
    Anti-Self-Dual 4-Manifolds, Quasi-Fuchsian Groups, and Almost-K¨ahlerGeometry Christopher J. Bishop∗ and Claude LeBruny Department of Mathematics, Stony Brook University For Karen Uhlenbeck, in celebration of her seventy-fifth birthday. Abstract It is known that the almost-K¨ahleranti-self-dual metrics on a given 4-manifold sweep out an open subset in the moduli space of anti-self- dual metrics. However, we show here by example that this subset is not generally closed, and so need not sweep out entire connected compo- nents in the moduli space. Our construction hinges on an unexpected link between harmonic functions on certain hyperbolic 3-manifolds and self-dual harmonic 2-forms on associated 4-manifolds. 1 Introduction An oriented Riemannian 4-manifold (M 4; g) is said to be anti-self-dual if it satisfies W+ = 0, where the self-dual Weyl curvature W+ is by definition the orthogonal projection of the Riemann curvature tensor R 2 2Λ2 into the arXiv:1708.03824v2 [math.DG] 18 Jan 2018 2 + trace-free symmetric square 0Λ of the bundle of self-dual 2-forms. This is a conformally invariant condition, and so is best understood as a condition on the conformal class [g] := fu2g j u : M ! R+g rather than on the representative metric g. For example, any oriented locally-conformally-flat 4-manifold is anti-self-dual; indeed, these are precisely the 4-manifolds that ∗Supported in part by NSF Grant DMS-1608577. ySupported in part by NSF grant DMS-1510094. 1 are anti-self-dual with respect to both orientations.
    [Show full text]
  • On Hyperelliptic Schottky Groups
    Annales Academire Scientiarum Fennicre Series A. I. Mathematica Volumen 5, 1980, 165-174 ON HYPERELLIPTIC SCHOTTKY GROUPS LINDA KEEN Section 1: Introrluction . A compact Riemann surface of genus g is hyperelliptic if it is a two sheeted covering of the Riemann sphere branched at 2(g*1) points; that is, if it satisfies a polynomial equation of the form: wz : (z - as)(z - ar) (z - ar) ... (, - orn *r), whete ao, k:0,...,2g11 are distinct points on the Riemann sphere. There are various characterizations of hyperelliptic surfaces in the literature but none of them theory we begin with a surface involve uniformization theory. In uniformization ^§, a covering space § and the corresponding group of cover transformations G. If we can realize § as a plane domain Q, and G as a group of linear fractional transforma- tion.s i-, such that the projection map n: Q - Qlf : S is holomorphic, then .l- is said Schottky groups are realizations of the smallest covering groups to uniformize ^S. which correspond to planar covering surfaces. In this paper we will discuss those Schottky groups which uniformize hyperelliptic surfaces. We begin with a discussion of some geometric properties of linear fractional transformations and their interpretation when the linear fractional transformations are represented as elements of SL(Z, C). In Section 3 we discuss Schottky groups in some detail and describe a set of moduli for them. In Section 4 we consider sur- faces of genus two, all of which are hyperelliptic, and determine various properties of the corresponding Schottky groups. Section 5 contains the main theorem, which gives a description of the moduli space of those Schottky groups which uniformize hyperelliptic groups and which reflect their hyperellipticity.
    [Show full text]
  • Arithmetic Fuchsian Groups of Genus Zero D
    Pure and Applied Mathematics Quarterly Volume 2, Number 2 (Special Issue: In honor of John H. Coates, Part 2 of 2 ) 1|31, 2006 Arithmetic Fuchsian Groups of Genus Zero D. D. Long, C. Maclachlan and A. W. Reid 1. Introduction If ¡ is a ¯nite co-area Fuchsian group acting on H2, then the quotient H2=¡ is a hyperbolic 2-orbifold, with underlying space an orientable surface (possibly with punctures) and a ¯nite number of cone points. Through their close connections with number theory and the theory of automorphic forms, arithmetic Fuchsian groups form a widely studied and interesting subclass of ¯nite co-area Fuchsian groups. This paper is concerned with the distribution of arithmetic Fuchsian groups ¡ for which the underlying surface of the orbifold H2=¡ is of genus zero; for short we say ¡ is of genus zero. The motivation for the study of these groups comes from many di®erent view- points. For example, a classical situation, with important connections to elliptic curves is to determine which subgroups ¡0(n) of the modular group have genus zero. It is not di±cult to show that the only possible values are 1 · n · 10, 12, 13, 16, 18 and 25. In addition, J. G. Thompson [43] showed that only ¯nitely many congruence subgroups have genus 0. On the otherhand, it is easy to see that there are subgroups of genus 0 inside the modular group of arbitrary large ¯nite index. For if ¡(2) denotes the principal congrence subgroup of level 2 in PSL(2; Z) (see x4.1), then H2=¡(2) is a sphere with 3-punctures, and it is easy to construct an m-fold cyclic cover of H2=¡(2) where two of the punctures have a single connected pre-image, and the last lifts to m punctures.
    [Show full text]
  • Convergence Groups Are Fuchsian Groups
    BULLETIN (New Series) OF THE AMERICAN MATHEMATICAL SOCIETY Volume 25, Number 2, October 1991 CONVERGENCE GROUPS ARE FUCHSIAN GROUPS DAVID GABAI ABSTRACT. A group of homeomorphisms of the circle satisfy­ ing the "convergence property" is shown to be the restriction of a discrete group of Mobius transformations of the unit disk. This completes the proof of the Seifert fiber space conjecture and gives a new proof of the Nielson realization problem. A Fuchsian group F is a discrete subgroup of the group of Mo­ bius transformations on the unit disc D2 in R2 . F restricts to a subgroup G of Homeo(5'1) which satisfies the following con­ vergence property [GM]. Given a sequence of distinct elements of G, then there exists x, y e S1 and a subsequence {f.} such that on Sl - {x, y} f. -• y, f~l -> x uniformly on compact sets. A group G c Homeo(5'1) with this property is called a conver­ gence group. We announce the following result. The details can be found in [G]. Theorem 1. G is a convergence group if and only if G is conjugate in Homeo(51) to the restriction of a Fuchsian group.* A Seifert fibred space is a compact 3-manifold M which is almost an Sl bundle over a compact surface, i.e. there exists a projection n : M —• TV such that for each x e N there exists a D2 neighborhood of x such that n~l(D2) = D2 x Sl and 7t((r, 6X)9 (1, 02)) = (r9pOx + Q62) where p^O and p, q are relatively prime and depend on x and 6 e Rmod27t.
    [Show full text]