Quasiconformal groups and a theorem of Biship and Jones

We provide new bounds on the exponent of convergence of a planar discrete quasiconformal group in terms of the associated dilatation and the Hausdorff dimension of its conical limit set. In doing so, we use these bounds to realize a theorem of C. Bishop and P. Jones as an asymptotic limit in the dilatation.     

Convergence Groups, Hausdorff Dimension, and a Theorem of Sullivan and Tukia

We show that a discrete, quasiconformal group preserving ⁿ has the property that its exponent of convergence and the Hausdorff dimension of its limit set detect the existence of a non-empty regular set on the sphere at infinity to ⁿ . This generalizes a result due separately to Sullivan and Tukia, in which it is further assumed that the group act isometrically on ⁿ , i.e. is a Kleinian group. From this generalization we are able to extract geometric information about infinite-index subgroups within certain of these groups.     

Isomorphisms of Sobolev Spaces on Riemannian Manifolds and Quasiconformal Mappings

We prove that a measurable mapping of domains in a complete Riemannian manifold induces an isomorphism of Sobolev spaces with the first generalized derivatives whose summability exponent equals the (Hausdorff) dimension of the manifold if and only if the mapping coincides with some quasiconformal mapping almost everywhere.

     

Hausdorff dimension distribution of quasiconformal mappings on the Heisenberg group

We construct quasiconformal mappings on the Heisenberg group which change the Hausdorff dimension of Cantor-type sets in an arbitrary fashion. On the other hand, we give examples of subsets of the Heisenberg group whose Hausdorff dimension cannot be lowered by any quasiconformal mapping. For a general set of a certain Hausdorff dimension we obtain estimates of the Hausdorff dimension of the image set in terms of the magnitude of the quasiconformal distortion.     

The limit sets of Schottky quasiconformal groups are uniformly perfect

In this paper we study Schottky quasiconformal groups. We show that the limit sets of Schottky quasiconformal groups are uniformly perfect, and that the limit set of a given discrete non-elementary quasiconformal group has positive Hausdorff dimension.

     

Fuchsian Groups, Quasiconformal Groups, and Conical Limit Sets

We construct examples showing that the normalized Lebesgue measure of the conical limit set of a uniformly quasiconformal group acting discontinuously on the disc may take any value between zero and one. This is in contrast to the cases of Fuchsian groups acting on the disc, conformal groups acting discontinuously on the ball in dimension three or higher, uniformly quasiconformal groups acting discontinuously on the ball in dimension three or higher, and discrete groups of biholomorphic mappings acting on the ball in several complex dimensions. In these cases the normalized Lebesgue measure is either zero or one.

     

Dimension of the boundary of quasiconformal Siegel disks

We study quasiconformal Siegel disks with critical points in their boundaries. The main result asserts that every subarc of the boundary of the Siegel disk has the Hausdorff dimension strictly larger than 1 and that the boundary does not have a tangent at any point. Oblatum 19-V-2000 & 4-X-2001?Published online: 18 January 2002     

Dimension of quasicircles

We introduce canonical antisymmetric quasiconformal maps, which minimize the quasiconformality constant among maps sending the unit circle to a given quasicircle. As an application we prove Astala’s conjecture that the Hausdorff dimension of a k-quasicircle is at most 1+k ².     

Enroulements des géodésiques sous la mesure de Patterson-Sullivan

Let M be a geometrically finite hyperbolic surface having at least one cusp, and infinite volume. We obtain the limit law under the Patterson-Sullivan measure on T¹ M of the normalized integral along the geodesics of M of any 1-form closed near the cusps. This limit law is stable with parameter 2δ − 1, where δ is the Hausdorff dimension of the limit set of the subgroup Γ of Möbius isometries associated with M.     

Hausdorff dimension of quasi-cirles of polygonal mappings and its applications

We show that the Hausdorff dimension of quasi-circles of polygonal mappings is one.Furthermore,we apply this result to the theory of extremal quasiconformal mappings.Let [μ] be a point in the universal Teichmller space such that the Hausdorff dimension of fμ(Δ) is bigger than one.We show that for every kn∈(0,1) and polygonal differentials ψn,n=1,2,...,the sequence {[kn ψn/|ψn|]} cannot converge to [μ] under the Teichmer metric.     

关于小伸缩商拟共形群的几个定理

本文研究了单位球Bn上小伸缩商拟共形群的离散性质，给出了一个收敛定理，并且证明了在一定限制条件下任意一个非初等非离散小伸缩商拟共形群含有一个二元生成的非初等非离散子群。     

Quasiconformal motions and isomorphisms of continuous families of Möbius groups

In their paper [17], Sullivan and Thurston introduced the notion of quasiconformal motions, and proved an extension theorem for quasiconformal motions over an interval. We prove some new properties of (normalized) quasiconformal motions of a closed set E in the Riemann sphere, over connected Hausdorff spaces. As a spin-off, we strengthen the result of Sullivan and Thurston, and show that if a quasiconformal motion of E over an interval has a certain group-equivariance property, then the extended quasiconformal motion can be chosen to have the same group-equivariance property. Our main theorem proves a result on isomorphisms of continuous families of Möbius groups arising from a group-equivariant quasiconformal motion of E over a path-connected Hausdorff space. Our techniques connect the Teichmüller space of the closed set E with quasiconformal motions of E.     

拟Fuchs群的收敛指数

根据H2上的双曲距离在拟共形变换下的拟不变性,给出了K-拟共形抛物循环Fuchs群的收敛指数的估计．     

Invariant measures for parabolic IFS with overlaps and random continued fractions

We study parabolic iterated function systems (IFS) with overlaps on the real line. An ergodic shift-invariant measure with positive entropy on the symbolic space induces an invariant measure on the limit set of the IFS. The Hausdorff dimension of this measure equals the ratio of entropy over Lyapunov exponent if the IFS has no ``overlaps.' We focus on the overlapping case and consider parameterized families of IFS, satisfying a transversality condition. Our main result is that the invariant measure is absolutely continuous for a.e. parameter such that the entropy is greater than the Lyapunov exponent. If the entropy does not exceed the Lyapunov exponent, then their ratio gives the Hausdorff dimension of the invariant measure for a.e. parameter value, and moreover, the local dimension of the exceptional set of parameters can be estimated. These results are applied to a family of random continued fractions studied by R. Lyons. He proved singularity above a certain threshold; we show that this threshold is sharp and establish absolute continuity for a.e. parameter in some interval below the threshold.

     

Homological Dimension and Critical Exponent of Kleinian Groups

We prove an inequality between the relative homological dimension of a Kleinian group and its critical exponent. As an application of this result we show that for a geometrically finite Kleinian group Γ, if the topological dimension of the limit set of Γ equals its Hausdorff dimension, then the limit set is a round sphere. Received: March 2007, Revision: October 2007, Accepted: October 2007     

Mesures de Patterson-Sullivan dans les espaces symétriques

Let X = G/K be a symmetric space of noncompact type, Γ a Zariski-dense subgroup of G with critical exponent δ(Γ). We show that all Γ-invariant conformal densities of dimension δ(Γ) (e.g. Patterson-Sullivan densities) have their support contained in a same and single G-orbit on the geometric boundary of X. In the lattice case, we explicitly determine δ(Γ) and this G-orbit, and we establish the uniqueness of such densities.     

单位球B~n上小伸缩商拟共形群

本文研究了单位球Bn上小伸缩商拟共形群的离散性质,给出了几个离散判别准则和收敛定理.     

Stable windings on hyperbolic surfaces

Let ℳ be a geometrically finite hyperbolic surface with infinite volume, having at least one cusp. We obtain the limit law under the Patterson-Sullivan measure on T ¹ℳ of the windings of the geodesics of ℳ around the cusps. This limit law is stable with parameter 2δ− 1, where δ is the Hausdorff dimension of the limit set of the subgroup Γ of M?bius isometries associated with ℳ. The normalization is t ^{−1/(2δ−1)}, for geodesics of length t. Our method relies on a precise comparison between geodesics and diffusion paths, for which we need to approach the Patterson-Sullivan measure mentioned above by measures that are regular along the stable leaves. Received: 8 October 1999 / Revised version: 2 June 2000 / Published online: 21 December 2000