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Hausdorff Dimension of Quasi-Circles PUBLICATIONS MATHÉMATIQUES DE L’I.H.É.S. RUFUS BOWEN Hausdorff dimension of quasi-circles Publications mathématiques de l’I.H.É.S., tome 50 (1979), p. 11-25 <http://www.numdam.org/item?id=PMIHES_1979__50__11_0> © Publications mathématiques de l’I.H.É.S., 1979, tous droits réservés. L’accès aux archives de la revue « Publications mathématiques de l’I.H.É.S. » (http:// www.ihes.fr/IHES/Publications/Publications.html) implique l’accord avec les conditions géné- rales d’utilisation (http://www.numdam.org/conditions). Toute utilisation commerciale ou im- pression systématique est constitutive d’une infraction pénale. Toute copie ou impression de ce fichier doit contenir la présente mention de copyright. Article numérisé dans le cadre du programme Numérisation de documents anciens mathématiques http://www.numdam.org/ HAUSDORFF DIMENSION OF QUASI-CIRCLES by RUFUS BOWEN (1) Let G be the group of all linear fractional transformations taking the unit disk U onto itself. One calls a discrete subgroup F C G a surface group if U/F is a compact surface without braitch points. This paper concerns the relation between two such groups I\ and I\ yielding the same topological surface. This is a classical and well- developed problem [6]; what is novel here is the application of the Gibbs measures of statistical mechanics and dynamical systems. The groups r\ and Fg as above are isomorphic since each is isomorphic to the fundamental group of the surface. Furthermore, for any isomorphism a : Fi—^Fg there is an interesting homeomorphism h•.Sl—^Sl of the circle S1^^^:]^^!} so that: A(40=a(^)A(0 for ^eFi, ^eS1.' This homeomorphism A, which is unique, is called the boundary correspondence (Fenchel and Nielsen, see [n], [14], or [22]). Let G* be the group of linear fractional trans- formations taking S1 onto itself. (G has index 2 in G*). We will give a new proof of the following result of Mostow [13]: Theorem 2. — The boundary homeomorphism h is a linear fractional transformation if it is absolutely continuous. Now recall the quasi-Fuchsian group A==A(Fi, Fg, a) [6] associated to the pair of Fuchsian groups F^, Fg, and a given abstract isomorphism a: Fi—^Fg. This is a discrete group A of linear fractional transformations of the extended complex plane (C == S2) which simultaneously uniformizes the surfaces U/F^ in the following sense: (i) There is a Jordan curve y m S2 (called a quasi-circle) with ^M^T ^or a^ elements ^ in A, and each orbit of A is dense in y* (*) Partially supported by NSF MCS74-19388^01. 259 12 RUFUS BOWEN (ii) There are analytic diffeomorphisms ^: U->D^ where D^, Dg are connected components of S^y so that a,(^)=^-lo^Fo^ belongs to F, for all ^eA and a,: A-^F, is an isomorphism, and (iii) agoa^1 is the given isomorphism a : r^-^rg. The curve y is generally not smooth nor even rectifiable [5, p. 263]. Y is a round circle (Theorem i) Y is a genuine quasi-circle (Theorem 2) FIG. i Theorem 2. — Suppose I\ and I\ are not conjugate as Fuchsian groups^ namely via (BeG*. Then the Hausdorff dimension a of y is greater than i. Furthermore O<^(Y)<OO, where v^ is a-dimensional Hausdorff measure^ and ^ | y is ergodic under A. The paper starts with a variant of the Nielsen development. This associates symbol sequences (generalized c< decimal expansion 33) to points in SMn a way determined by r^. The reader familiar with dynamical systems will recognize this as a cc Markov partition for 1^ 55. Via this construction functions and measures are transferred from the circle (where they have geometric meaning) to the Cantor set of symbol sequences (where they can be analyzed). The paper ends with results on Schottky groups, where the symbolic sequences are quite transparent. The author thanks Dennis Sullivan for introducing him to Kleinian groups and Hedlund for his paper [9] which motivated the present one. i. Nielsen Development. Let FOG* be a surface group. A piecewise smooth map f:Sl->Sl is called Markov for F if one can partition S1 into segments Ii, ..., 1^ so that: (i) f\^k=fk\^ some /^er, and (ii) for each k, f(l^) is the union of various Ij's. 260 HAUSDORFF DIMENSION OF QUASI-GIRCLES 13 Condition (ii) can be rephrased as follows. Letting W be the set of endpoints of the I^'s, consider each such point to be really two points, depending on which 1^; it is associated with. Then condition (ii) is equivalent to ./(W) C W. The idea of a Markov map is to replace the action of the group F on S1 by the single map f. For each g^2 let O^ be a surface group of genus g whose fundamental domain R in U is a regular 4^-sided noneuclidean polygon ([9], [12], [18, p. 89]). The Nielsen development is a certain Markov map for O^ ([9], [14, pp. 211-217]). We shall construct variants which are more suitable for our purposes. Each angle ofRis a= — and each vertex of R belongs to 4^ distinct translates <p(R), (pe0^. The net 91 is defined to be the collection of all sides and vertices of all translates <p(R), (pe^. This net has the following crucial property: (*) the entire noneuclidean geodesic passing through any edge in 91 is contained in 91. Let V be the set of vertices in 91 which are adjacent in 91 to vertices of R but are not themselves vertices ofR. The set Vis contained in the set of vertices of the noneucli- dean polygon R which is the union of R plus all the translates <p(R) which touch R. The polygon R is convex since each interior angle equals 2a<7i;. For each vertex^ of 91 let W be the set of 4^ points on S1 which are the points at infinity of the 2g noneuclidean geodesies in 91 passing through p. Letting W== U Wy, property (*) implies that: WgCW for q a vertex of R. Recall now a set of generators for 0^ (fo]? E1^], [18]). Divide the sides of R into g groups of 4 consecutive sides; label thej-th group ^, ^, a^~1, b^1. Gall a^ and a^1 (and b^ and b^1) corresponding sides. For each side s of R there is an element ^se(^g so that: (p^)=:Rn(pg(R)==side corresponding to s. The set {93} generates 0.. Let Jg be the smaller segment of S1 whose endpoints are the points at infinity of the noneuclidean geodesic through s. Lemma L — <p,CLnW)CW. Proof. — The geodesies passing through nonconsecutive sides of R do not intersect. One way of checking this well-known fact is by an area argument. If two such geodesies intersected (perhaps at oo), then, they together with m sides (m^-1) of R would form a polygon containing R and with noneuclidean area less than mn^^g—3)^. This polygon contains at least 2g translates of R, each having area (4^—4.)n. So 2s{4•g—4)^4«?—'33 which is impossible since g^-2. We claim that J^nWC U W where Tg is the set of vertices in 91 adjacent to p(=Ts * an endpoint ofs. Now (p^(T^)CVu{ vertices ofR} because 9^) is a side ofR. Hence the above claim would yield: 9,(J,nW)CTs\Js ; ^^U W.CW.p 261 14 RUFUS BO WEN Suppose ^eJ,nW but u^ U, Wp. Let y be a geodesic in SR from some ^eV\T, to u. Let ^, ^ be the vertices of R on the geodesic a through s; let r^, ^ be the vertices of R adjacent to ^, ^ respectively and exterior to the domain bounded by a and Jg. Let (B, be the geodesic containing a side of R, passing through r, but not q^ Then (B, and a do not intersect as they pass through nonconsecutive sides of an image 9(R). Since the endpoints of J, are in W^uW^, and fteT,, one has ueintj,. Therefore the geodesic y must cut the geodesic a; this intersection ^is a vertex of the net. If q lay between q^ and q^, then ^ would be an endpoint of s and ue U W . Suppose 'q lies between q^ and S1 on a. PE s Consider the region Q in the unit disk exterior to the half disks bounded by the geodesies a, (B^ and Pa. Because (B^, [Bg are sides of the convex polygon K, the point qeR above lies in Q. Because Qis convex the geodesic ^ C y from y to ^lies in Q^. ^ must now intersect r^; this intersection is a vertex of the next, hence r^ or q^ Either case gives a contradiction. • Let v be a vertex of R, belonging to the sides s and ^' of R. We will construct a segment J(y)Cint(J,nJ,,). Let j^'eV be the vertices of the net 91 adjacent to v in yi and lying on the geodesies a and a' through j and s\ Choose vertices q, q' of R so that ^, v,p\ q' are consecutive vertices of a translate <p(R), ^eO.- The geodesies (B, p' containing j&y, j^' do not intersect; they intersect S1 at points w{v}, ^(^eint^nj^). LetJ(y) be the interval [w{v), ^'(^)]CS1. The endpoints w{v), w\v) of]{v) are in,W; arguments analogous to the proof of Lemma i show that WnintJ(y)==0. Theorem 0. — A surface group FCG has transitive Markov maps on S1.
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