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ℓ i (α) i(α, λ) σ converges to , ℓσi (β) i(β, λ) where ℓσi (α) is the length of closed σi-geodesic which is homotopic to α, and i(α, λ) is the intersection number of α and λ which is a generalization of the geometric intersection number of simple closed curves (see [11, 89, 90]). The geometry of T (S) and complex of curves are well described by Minsky in [78]. They are related by the collar lemma (see [22, 46]). The collar lemma implies that there is a universal constant ǫ > 0 such that for any distinct α, β ∈ C0(S) and σ ∈ T (S), if ℓσ(α) < ǫ and ℓσ(β) < ǫ then the two geodesic representatives of α and β are disjoint, i.e. α and β are on a same simplex in the complex of curves. Therefore C1(S) could be considered as the nerve of the family of regions
T (α)= {σ ∈ T (S) | ℓσ(α) <ǫ}, α ∈C0(S).
2 Although T (S) is not Gromov hyperbolic, Masur and Minsky showed that T (S) is Gromov hyperbolic modulo this family of regions (see [67, 78, 79]). A lamination µ ∈ ML(S) is called a filling lamination if i(µ, µ′) = 0 then
support(µ) = support(µ′), for any µ′ ∈ ML(S). Minsky wrote EL(S) to denote the image of filling lamina- tions in UML(S). In the celebrated proof of Thurston’s ending lamination con- jecture, by Brock, Canary and Minsky, the laminations in EL(S) are appeared as ending laminations of Kleinian surface groups without accidental parabolics (see [86, 88]). Ending Lamination Conjecture. A hyperbolic 3-manifold with finitely gen- erated fundamental group is uniquely determined by its topological type and its end invariants. The proof of the ending lamination conjecture and the recent proof of Marden’s tameness conjecture by Agol([2]) give us rough picture of hyperbolic 3-manifolds. See [60] for the original conjecture, and see [24] for another proof by Calegari- Gabai. Tameness Conjecture. A hyperbolic 3-manifold with finitely generated fun- damental group is homeomorphic to the interior of a compact manifold with boundary. The Gromov boundary of C(S) is homeomorphic to the Gromov boundary of its 1-skeleton C1(S) because C(S) is quasi-isometric to C1(S). In [67], Masur and Minsky showed that the relative hyperbolic space, which is roughly T (S) modulo the regions T (α), is quasi-isometric to C1(S). Therefore we can expect some relation between Thurston boundary of T (S) and Gromov boundary of C(S) (see [78] for the first question of Minsky on this relation). In fact, Klarreich showed that ∂∞C(S) is homeomorphic to EL(S) (see [37] for a new proof by Hamenst¨adt). Theorem 1.1 (Klarreich [51]). There is a homeomorphism
k : ∂∞C(S) → EL(S) such that for any sequence αi in C0(S), αi converges to α ∈ ∂∞C(S) if and only if αi, considered as a subset of UML(S), converges to k(α). In the following theorem of Bers, T (Σ) stands for the Teichm¨uller space of Σ with geodesic boundaries (see [8, 9, 22]). Theorem 1.2 (Bers [9]). Let Σ be a compact Riemann surface with genus g ≥ 2 from which n points and m disks have been removed. For any σ ∈ T (Σ), there exist 3g − 3+ n + m disjoint geodesics, which are not boundary parallel, whose lengths are bounded by a constant L which depends only on g,n,m, and the largest length of the geodesics homotopic to the boundaries of Σ.
3 The constant L is called a Bers constant. Notice that for the closed surface S, for any σ ∈ T (S), S has a pants decomposition with total length bounded by a constant L which depends only on the genus g. In fact, Buser and Sepp¨al¨a([23]) showed that we can choose L = 21g(3g − 3). Throughout this thesis, we write L to denote a fixed Bers constant. Motivated by Theorem 1.2, we define a function Φ on T (S) as follows. For σ ∈ T (S), let
Φ(σ) = a pants decomposition whose total length is bounded by L, where all pants curves are geodesics in σ. Let u : PML(S) → UML(S) be the quotient map by forgetting measure. Suppose that
σi ∈ T (S) converges to [λ] ∈ PML(S) in T (S) (1) and αi is a pants curve in Φ(σi). From Theorem 1.1, the following question is immediate. Question 1.3. Suppose that λ is a filling lamination. Is the geodesic represen- tative of αi converging to u([λ]) in UML(S)? This question is the motivation of all the work in this thesis. In Chapter 5, we will solve this question positively. Notice that if we identify ∂∞C(S) with EL(S) via the homeomorphism in Theorem 1.1, then we have
Theorem 1.4. If λ is a filling lamination, then αi converges to u([λ]) in C(S)∪ ∂∞C(S). Suppose that λ is not necessarily a filling lamination in eq. (1). Is there any relation between the limit point of αi and u([λ])? The following simple example shows that a limit of αi and u([λ]) could be disjoint.
Example 1.5. Consider a fixed γ ∈C0(S). Suppose that σi ∈ T (S) converges to [γ] ∈ PML(S) and ℓσi (γ) → 0, where we consider γ as an element of ML(S) via the counting measure. Since ℓσi (γ) → 0, there exists a pants decomposition Φ(σi) whose total length is bounded by L and
γ ∈ Φ(σi) for all large enough i.
Then we can find a pants curve αi in Φ(σi) such that a limit point of αi is disjoint from γ in UML(S). A nonempty geodesic lamination µ ∈ GL(S) is called minimal if no proper subset of µ is a geodesic lamination. For example, any closed geodesic is a minimal lamination. The following theorem on the structure of a geodesic lam- ination on S works for any hyperbolic surface of finite type (see [25] §4.2, [28] §4 or [101] §8). Theorem 1.6 (Structure of Geodesic Lamination). A geodesic lamination on S is the union of finitely many minimal sublaminations and of finitely many infinite isolated leaves whose ends spiral along the minimal sublaminations.
4 Furthermore, if [λ] ∈ PML(S) then we can decompose u([λ]) as a finite disjoint union of minimal laminations,
u([λ]) = λ1 ∪ λ2 ∪···∪ λm. (2)
An essential subsurface F of S is a subsurface of S whose boundaries are all homotopically non-trivial geodesics. Throughout this thesis, we assume that all essential subsurfaces of S are open, i.e. the boundaries are not included (see Figure 1). Note that two distinct boundaries of F could be a same curve in S, where F is the completion of F with the path-metric in F .
F
Figure 1: An essential subsurface
Suppose that µ ∈ UML(S). An essential subsurface F is called filled by µ, if for any simple closed curve α in F which is not parallel to a boundary of F , α intersects µ. Suppose that σi ∈ T (S) converges to [λ] ∈ PML(S), and let u([λ]) = λ1 ∪ λ2 ∪···∪ λm be the decomposition into minimal sublaminations. The main theorem of this thesis is
Theorem 1.7 (Main Theorem). If Φ(σi) converges to a geodesic lamination ν in Hausdorff metric topology, then u([λ]) ⊂ ν. There is no minimal lamination which fills a pair of pants (see [91] §2.6), therefore in eq. (2), if λj is not a simple closed curve then the essential sub- surface filled by λj is at least a 1-holed torus or 4-holed sphere. For a closed annulus Y , Minsky defined the arc complex A(Y ). In this complex vertices are essential homotopy classes, rel endpoints, of properly embedded arcs, and sim- plices are sets of vertices with representatives with disjoint interiors. Note that the endpoints are not allowed to move in the boundary. Let A0(Y ) be the set of vertices and A1(Y ) the 1-skeleton, and give shortest-path metrics to A(Y ) and A1(Y ) as in C(S). For a,b ∈ A0(Y ), we write a · b to denote the algebraic intersection number. Suppose that in eq. (2), λ1 is a simple closed curve. Consider an annular covering Y of S in which a neighborhood of λ1 lifts homeomorphically. The following corollary follows from the main theorem.
5 Corollary 1.8. Suppose that αi meets λ1 for all i. Then |a1 · ai| approaches to ∞, where a1 ∈ A0(Y ) is a lift of α1 in Y and so is ai.
Suppose that in eq. (2), λ2 is not a simple closed curve. Suppose that F is an essential subsurface of S which is filled by λ2. Notice that there exists a pants curve αi in Φ(σi) such that αi ∩ F 6= ∅ for all i. Let βi be a component of αi ∩ F . We can define a simple closed curve β˜i from βi canonically (see Chapter 5). We will prove the following theorem which is a generalization of Theorem 1.4.
Theorem 1.9. The geodesic representative of β˜i converges to λ2 in UML(F ). In Chapter 2 and Chapter 3, we study some preliminaries. We prove the main theorem in Chapter 4, and we prove Corollary 1.8 and Theorem 1.9 in Chapter 5.
Acknowledgements. This paper is a part of Ph.D thesis of the author (May 2005, Stony Brook university). The author would like to express his deepest gratitude to his advisor Yair Minsky for suggesting this thesis topic, and for his patient guidance and continuous encouragement.
2 The Thurston Boundary of Teichm¨uller Space
Let S be a closed oriented surface of genus g ≥ 2. In this chapter we study the Teichm¨uller space T (S) and its Thurston boundary PML(S). See [43] and [105] as references.
2.1 Teichm¨uller space and its boundaries A conformal structure σ on S is determined by an atlas of coordinate neighbor- hoods (Uα,zα), where {Uα} is an open cover of M, and zα : Uα → C has the −1 property that zα ◦ zβ is analytic whenever defined. Teichm¨uller space T (S) is the space of conformal structures on S, where two structures are considered to be equivalent if there is a conformal map between them isotopic to the iden- tity. Recall that two diffeomorphisms f and g on S are called isotopic if there exists a diffeomorphism H(x, t) = (ht(x),t) : S × [0, 1] → S × [0, 1] such that h0(x)= f(x) and h1(x)= g(x) for all x ∈ S. By the uniformization theorem, T (S) can be considered as the space of complete hyperbolic metrics on S with finite area. The area of hyperbolic surface (S, σ) is equal to −2πχ(S), where χ(S) = 2(1 − g) is the Euler characteristic of S (see [4] for details). As usual, two closed curves α, β : [0, 1] → S are called free homotopic if there exists a continuous mapping F : [0, 1] × [0, 1] → S such that F (t, 0) = α(t), F (t, 1) = β(t) and F (0,s)= F (1,s) for all t,s ∈ [0, 1]. The following well-known lemma will be used frequently in this thesis (see [4] §B.4 for a proof).
6 Lemma 2.1. Suppose that (S, σ) is a hyperbolic surface. Then each free homo- topy class of closed curves contains a unique geodesic representative. Two closed curves α, β : [0, 1] → S are called isotopic if there exists a diffeo- morphism G : S × [0, 1] → S × [0, 1] such that for all x ∈ S and t,s ∈ [0, 1],
G(x, t) = (gt(x),t), g0(x)= x and g1(α(s)) = β(s).
Free homotopic simple curves on a connected surface are isotopic, too. There is well-known classification of Isom Hn into elliptic, parabolic and 2 hyperbolic isometries. We write H to denote the compactification of H2 by the circle at infinity. The following two lemmas will be useful in Section 3.2 (see [4] §B.4 for a proof).
Lemma 2.2. Every non-trivial elements of π1(S) are hyperbolic isometry.
Lemma 2.3. Suppose that α is a non-trivial simple closed geodesic in S. Then 2 any two lifts of α into H2 can not meet in the whole H .
The following lemma will be useful in Chapter 4 (see [4] §B.4 for a proof). Lemma 2.4. Suppose that α and β are non-intersecting, non-isotopic and non- trivial simple closed curves in a hyperbolic surface (S, σ). Then the geodesic representatives of α and β are non-intersecting. In particular, if α is in a subsurface F of S, then the geodesic representative of α is in F , too.
n−1 n The standard boundary S∞ of H is defined by the equivalence classes of geodesic rays (see [4] §A.5). The Teichm¨uller boundary of T (S) is defined in the same way. But there are distinct geodesic rays from a point in T (S) that always remain within a bounded distance of each other (see [66]). The mapping class group MCG(S) is the group of orientation preserving diffeomorphisms of S modulo those which are isotopic to identity, i.e.
MCG(S) = Diff+(S)/Diff0(S).
The mapping class group acts by isometries on T (S) and its quotient space is called moduli space of S. See [63] for a picture of moduli space. Kerckhoff proved in his thesis that the action of MCG(S) does not in general extend to Teichm¨uller boundary. Let V(S) be the set of representations of π1(S) into PSL(2, C) up to conju- gacy with compact-open topology. The product T (S) × T (S) can be identified with an open subset of V(S), consisting of faithful representations whose images are quasi-Fuchsian groups, by Bers simultaneous uniformization. If we fix the first factor, then we get a holomorphic embedding of T (S) into V(S) which is called a Bers slices. Although this embedding depends on the fixed first fac- tor, there is a biholomorphic mapping between any two slices. The closure of this slice is compact, and is called a Bers compactification (see [7, 74]). But Kerckhoff and Thurston showed that (see Theorem 1 and Theorem 2 of [49])
7 1. For each genus g ≥ 2, there are Bers slices for which the canonical homeo- morphisms do not extend to homeomorphisms on their compactifications. 2. For g = 2, there is a Bers slice for which the action of the mapping class group does not extend continuously to its compactification. In the famous 1976 preprint, which is published in [105] later, Thurston introduced the space of projective measured laminations on S, which will be denoted by PML(S), and a compactification of T (S) whose boundary is equal to PML(S). Thurston boundary PML(S) is a natural boundary of T (S), in the sense that the action of mapping class group extends continuously to the Thurston compactification T (S)= T (S) ∪ PML(S). Masur([66]) showed that Teichm¨uller boundary and Thurston boundary are same almost everywhere, but not everywhere. With this compactifcation, Thurston classified surface diffeomorphisms as periodic, reducible or pseudo-Anosov, which is a generalization of the well- known classification of elements of SL(2, Z). Thurston boundary was also used by Kerckhoff to solve the Nielsen realization problem, i.e. every finite subgroup of MCG(S) can be realized as a group of isometries of some hyperbolic structure on S (see [48] for the proof).
2.2 Measured laminations In this section we study measured laminations. See [14, 25, 28, 40, 56] and [101] as references. Consider a fixed hyperbolic structure σ on S. A geodesic lamination µ is a closed subset of S, which is a disjoint union of simple geodesics which are called leaves of µ. The leaves of a geodesic lamination are complete, i.e. each leaf is either closed or has infinite length in both of its ends, and a geodesic lamination is determined by its support, i.e. a geodesic lamination is 1 a union of geodesics in just one way. Using S∞, a geodesic lamination on (S, σ) can be naturally related to a geodesic lamination on (S, σ′) for any σ′ ∈ T (S). We write GL(S) to denote the space of geodesic laminations on S, which is equipped with the Hausdorff metric on closed subsets. Note that GL(S) is compact and therefore, in particular, every infinite sequence of nontrivial simple closed geodesics has a convergent subsequence. For an arbitrary topological space X, the Chabauty topology on the set of closed subsets of X has the following sub-bases.
(i) O1(K)= {A | A ∩ K = ∅} where K is compact.
(ii) O2(U)= {A | A ∩ U 6= ∅} where U is open. If X is compact and metrizable, in particular for S, the Chabauty topology agrees with the topology induced by the Hausdorff metric. The following lemma will turn out to be useful (see [25] §3.1). Lemma 2.5 (Geometric Convergence). Suppose that X is a locally compact metric space. A sequence An of closed subsets of X converges to a closed subset A in Chabauty topology if and only if
8 (i) If xnk ∈ Ank converges to x ∈ X then x ∈ A.
(ii) If x ∈ A, then there exists a sequence xn ∈ An which converges to x. In H2, a geodesic is determined by an element of the open M¨obius band
1 1 M = (S∞ × S∞ − ∆)/Z2, where ∆ = {(x, x)} is the diagonal and Z2 acts by interchanging coordinates. A geodesic in H2 projects to a simple geodesic on S if and only if the cov- ering translates of its pairs of end points never strictly separate each other. Notice that a geodesic lamination could be considered as a closed subset of M. The Chabauty topology on GL(S) as closed subsets of M is equivalent to the Chabauty topology on GL(S) as closed subsets of H2. Therefore
Lemma 2.6. If µi converges to µ in GL(S) with Hausdorff metric topology, then for any geodesic ℓ ⊂ µ, there exist geodesics ℓi ⊂ µi which converge to ℓ. A geodesic lamination is called maximal if each complementary region is isometric to an ideal triangle. A nonempty geodesic lamination is called minimal if no proper subset is a geodesic lamination. For example, any simple closed geodesic is a minimal lamination. The following lemma is about the structure of minimal laminations (see [25] §4.2 for a proof). Lemma 2.7 (Structure of minimal lamination). If µ is a minimal lami- nation then either µ is a single geodesic or consists of uncountable leaves. The following theorem is about the structure of geodesic laminations.
Theorem 1.6 (Structure of Geodesic Lamination). A geodesic lamination on S is the union of finitely many minimal sublaminations and of finitely many infinite isolated leaves whose ends spiral along the minimal sublaminations.
α β
Figure 2: α and β have same measure
A transverse measure on a geodesic lamination µ is a rule, which assigns to each transverse arc α a measure that is supported on µ ∩ α, which is invariant
9 under a map from α to another arc β if it takes each point of intersection of α with a leaf of µ to a point of intersection β with the same leaf (see Figure 2). A measured lamination on S is a geodesic lamination µ with a transverse measure of full support, i.e. if α ∩ µ 6= ∅ then α has nonzero measure for any transverse arc α. For example, a simple closed geodesic equipped with counting measure is a measured lamination. We write ML(S) to denote the space of measured laminations on S. There is a natural action of R+ on ML(S). Suppose that r> 0. The measured lamination rµ has the same geodesic lamination as µ with the transverse measure scaled by r. We write PML(S) to denote the set of equivalence classes of projective measured laminations. The support of a measured lamination has no infinite isolated leaves, there- fore by Theorem 1.6, it is a finite disjoint union of minimal sublaminations. For a transverse arc α to a measured lamination µ, let θ be the angle between leaves of µ and α, measured counterclockwise from α to µ. The total angle of α is defined by θ(α, µ)= θ dµ. Zα Thurston gave ML(S) a topology with following basis,
B(µ, α1, · · · , αn,ǫ)= {ν ∈ ML(S) :
| (µ(αk),θ(αk, µ)) − (ν(αk),θ(αk, ν)) | <ǫ, k =1, · · · ,n}, where {αk} is a finite set of transverse arcs to µ, and ǫ > 0. The following theorem was proved by Thurston. Theorem 2.8 (Thurston). (1) ML(S) is homeomorphic to the open ball B6g−6 and PML(S) is homeomorphic to the sphere S6g−7.
(2) R ×C0(S) is dense in ML(S), and C0(S) is dense in PML(S).
Suppose that α, β ∈C0(S). The geometric intersection number i(α, β) is the minimal number of intersections of any two their representatives.
For a transverse arc α to µ ∈ ML(S), we write α dµ to denote integration of the transverse measure over α. For a simple closedR curve γ, let
i(µ, γ) = inf′ dµ, γ Zγ′ where the infimum is taken over all the simple closed curves γ′ which is homo- topic to γ. For a general transverse arc α, we define
i(µ, α) = inf′ dµ, α Zα′ where the infimum is taken over all the arcs α′ which is homotopic to α with endpoints fixed. Note that, in both cases, the infimum is realized by the unique geodesic in the corresponding homotopy class. + Suppose that µ ∈ ML(S), γ ∈ C0(S) and r ∈ R , let i(µ,rγ) = ri(µ, γ). The following theorem of Thurston is useful in Chapter 5 (see [104] for a proof).
10 Theorem 2.9 (Continuity of the intersection number). The intersection number i extends to a continuous symmetric function on ML(S) × ML(S).
2.3 Topology of T (S) In this section we study the topology of the Thurston compactification T (S). References are [31, 90] and [104]. The topology on T (S)= T (S) ∪ PML(S) is determined by the following two properties. (P1) T (S) is open in T (S).
(P2) σi ∈ T (S) converge to [λ] ∈ PML(S) if and only if, for all simple closed curves α, β on S with i(β, λ) 6= 0,
ℓ i (α) i(α, λ) σ converges to , ℓσi (β) i(β, λ)
where ℓσi (α) is the length of closed σi-geodesic which is homotopic to α. The following trivial lemma will turn out to be useful (see [71] for a proof). Lemma 2.10. For any infinite sequence of distinct simple closed curves in S, there is a subsequence αi and ci > 0 such that
ci → 0 and ciαi → µ for some µ ∈ ML(S) −{0}.
Suppose that µi ∈ ML(S). We write µi → ∞ to denote that there exists α ∈C0(S) such that i(α, µi) converges to ∞. The following theorem is the most useful theorem in this thesis.
Theorem 2.11 (Theorem 2.2 of [104]). A sequence σi ∈ T (S) converges to [λ] ∈ PML(S) if and only if there is a sequence µi ∈ ML(S) converging projectively to λ such that µi → ∞ and ℓσ1 (µi) → ∞ but ℓσi (µi) remains bounded, and for all ν ∈ ML(S), there exists a constant C > 0 such that
i(ν, µi) ≤ ℓσi (ν) ≤ i(ν, µi)+ Cℓσ1 (ν).
In particular, for each γ ∈C0(S), there exists a constant Γ > 0, which does not depend on i such that
i(γ, µi) ≤ ℓσi (γ) ≤ i(γ, µi)+Γ. (3)
The measured laminations µi were constructed in [31], [90] and [103] §9. In these constructions, the associated measured foliations are constructed first, and the measured laminations µi are induced later. In the following paragraphs, we study measured foliations and the construction of µi following Papadopoulos (see [90] for details). Another good French reference for this construction is [31]. A measured foliation F on S is a foliation with finite number of singularities equipped with a invariant transverse measure, i.e. F is determined by a finite number of points pk ∈ S and an atlas of coordinate neighborhoods 2 (xi,yi) : Ui → R
11 on the complement of {pk} such that xj = fij (xi,yi) and yj = ±yi + C for any overlapping coordinate neighborhoods (xj ,yj), where C is a constant and the transverse measure is dy. The singularities have p-pronged saddles with p ≥ 3. Suppose that F is a measured foliation on S and α ∈C0(S). Let
i(F, α) = inf′ |dy|, α Zα′ where the infimum is taken over all the representatives α′ in the class α. Two measured foliations F and G are called equivalent if i(F, α) = i(G, α) for all α ∈C0(S). We write MF(S) to denote the set of equivalence classes of measured foliations. Measured foliations and measured laminations are related by the following theorem (see [56] for the proof). Theorem 2.12. There is a homeomorphism h : MF(S) → ML(S) which is identity on R ×C0(S) and preserves the intersection number. Suppose that µ is a maximal geodesic lamination on S and σ ∈ T (S). Thurston constructed a measured foliation Fµ(σ), which is called horocyclic foliation as follows. Since µ is maximal, the complementary components of µ are all isometric to ideal triangles. In each of these components, define a partial foliation, i.e. a foliation whose support is subsurface, whose leaves are inter- sections of the triangle and horocycle centered at vertices of the triangle (see Figure 3).
Figure 3: The partial foliation
Notice that the horocycle meets the triangle with right angles, and the non- foliated region is equal to a little triangle whose edges are subarcs of horocycles which meet tangentially at their endpoints. These partial foliations in the ideal triangles fit together on the surface and define a partial foliation on S. The transverse measure on this partial foliation is uniquely determined by the fact that on the leaves of µ this transverse measure is equal to the hyperbolic dis- tance.
12 Suppose that µ is a maximal geodesic lamination on S. Let MF(µ) be the subset of MF(S) consisting of equivalence classes which has representative transverse to µ. In [103] §9, Thurston showed
Theorem 2.13. The map φµ : T (S) → MF(µ) such that φµ(σ)= Fµ(σ) is a homeomorphism.
Suppose that σi ∈ T (S) converges to [λ] ∈ PML(S) in T (S). Then the mea- sured lamination µi, obtained from Fµ(σi) by Theorem 2.12, is the lamination in Theorem 2.11.
3 Complex of Curves
In this chapter we study the complex of curves and its Gromov boundary. All theories in this chapter work for any orientable surface of finite type, i.e. surface with genus g and n punctures. Throughout this chapter, we write Σ = Σg,n to denote an orientable surface with genus g and n punctures. As before, we write MCG(Σ) to denote the mapping class group of Σ, and T (Σ) the Teichm¨uller space of Σ.
3.1 Complex of curves In [39], Harvey introduced the complex of curve C(Σ) to study the action of MCG(Σ) at the infinity of T (Σ). This complex encodes the asymptotic geom- etry of Teichm¨uller space, similarly as the Tits buildings for symmetric spaces. Let S(Σ) be the set of isotopy classes of essential, unoriented, non-boundary parallel simple closed curves in Σ. The vertices of C(Σ) are elements of S(Σ), i.e. C0(Σ) = S(Σ), and the k-simplices of C(Σ) are subsets {α1, · · · , αk+1} of S(Σ) with mutually disjoint representatives. Notice that C(Σ) is empty if g =0 and n ≤ 3. The maximal dimension of simplices is called the dimension of C(Σ), and it is equal to 3g + n − 4. Masur and Minsky defined a metric on C(Σ) by making each simplex regular Euclidean with side length 1 and taking shortest-path metric. Let dC denote this metric on C(Σ). The mapping class group acts on C(Σ) and Ivanov proved that, if g ≥ 2 then all automorphisms of C(Σ) are given by elements of MCG(Σ) (see [44] for the proof, and [52] for the related work of Korkmaz). Luo([59]) generalized this result by showing that, if 3g + n − 4 ≥ 1 and (g,n) 6= (1, 2), then all automorphisms of C(Σ) are given by elements of MCG(Σ). In [38], Harer showed that C(Σg,n) is homotopic to a wedge of spheres of dimension r, where
2g + n − 3 if g > 0 and n> 0 r = 2g − 2 if n =0 n − 4 if g =0. If Σ is a torus, once-punctured torus or 4-times punctured sphere, then any two essential simple closed curves intersects, i.e. there is no edge in C(Σ). Masur and Minsky introduced a new definition for these cases so that it has edges. In
13 this definition, {α, β} is an edge if α 6= β, and α and β have the lowest possible intersection number. For the tori this is 1, and for 4-holed sphere this is 2 (see [41] for Hatcher-Thurston complex which is related to this definition).
3.2 Relative twist number in annulus complex In this section we study the relative twist number in annulus complex, which was introduced by Minsky in [30], of two simple closed curves around a fixed simple closed curve. See [30] §2.1, [69] §2.4 and [86] §4 as references. An annular domain in Σ is an annulus with incompressible boundary. A complex, which is called an annulus complex, is defined for such annuli to keep track of Dehn twisting around their cores. 1 Consider an oriented annulus Y = S × [0, 1]. We write A0(Y ) to denote the set of arcs joining S1 ×{0} to S1 ×{1}, up to homotopy with endpoints fixed. As in C(Σ), we put an edge between any two elements of A0(Y ) which have representatives with disjoint interiors, and define the annular complex A(Y ) as for the complex of curves. We also make A(Y ) a metric space with edge length 1 as in the curve complex. Let dY denote the path-metric.
b
a
Figure 4: |a · b| =2
Suppose that a,b ∈ A0(Y ) and they do not share any endpoints. Notice that a and b inherit orientations from the orientation of [0, 1]. Therefore we can define the algebraic intersection number a · b (for example, see Figure 4). Let a·a = 0. Consider a lift of a ∈ A0(Y ) to the covering space Y = R×[0, 1] which has endpoints (a0, 0) and (a1, 1). Notice that these endpoints are determined e by a, up to Z, and a · b = ⌊b1 − a1⌋−⌊b0 − a0⌋, where ⌊x⌋ denotes the largest integer less than or equal to x. It follows that
a · c = a · b + b · c + ∆ with ∆ ∈{0, 1, −1} (4) for all a, b, c ∈ A0(Y ) such that the intersection numbers are defined. With an inductive argument, we can also check
dY (a,b)=1+ |a · b| for all distinct a,b ∈ A0(Y ). (5)
14 Fix a ∈ A0(Y ). From eq. (4) and eq. (5), we can show that the map f : A0(Y ) → Z with f(b)= a · b is a quasi-isometry. Thus A(Y ) is quasi-isometric to Z. For a fixed finite generating set of MCG(Σ), let || · || be the minimal word length with respect to these generators. Masur and Minsky introduced the relative twist number, and Farb, Lubotzky and Minsky proved that every Dehn twist has linear growth in MCG(Σ). Theorem 3.1 (Theorem 1.1 of [30]). For all Dehn twist t, there exists a constant c> 0 such that ||tm|| ≥ c|m| for all m.
a 2 H
b
Yα
Figure 5: Yα
The relative twist number is defined as follows. For a fixed essential simple 2 closed curve α in Σ, let gα be an isometry of H representing the conjugacy class of α. Let 2 Y = Yα = H \ Fix(gα) /
τα(β,γ)= {b · c | b ∈ liftα(β) and c ∈ liftα(γ)} .
15 From eq. (4), we have diam(τα(β,γ)) ≤ 2. In [30], Farb, Lubotzky and Minsky used the following equations to prove Theorem 3.1.
n (i) If t = Tα is the leftward Dehn twist on α, then τα(β,t (β)) ⊂{n,n +1}. (ii) If β and γ intersect α, then their geometric intersection number bounds their relative twisting, i.e.
max |τα(β,γ)|≤ i(β,γ)+1.
(iii) If β, γ and δ intersect α, then
max τα(β,δ) ≤ max τα(β,γ)+max τα(γ,δ)+2
min τα(β,δ) ≥ min τα(β,γ) + min τα(γ,δ) − 2.
3.3 The theorem of Masur and Minsky, and of Klarreich In this section we study a theorem of Masur and Minsky, and of Klarreich. References are [16, 37, 51, 67] and [78]. If Σ is a torus, once-punctured torus or 4-times punctured sphere, then the complex of curves is the Farey graph (see 3 Figure 6). Minsky showed that (C(Σ), dC ) is 2 -hyperbolic space for these cases (see [78] §3 for the proof). For ξ(Σ) = 3g + n> 4, the following lemma gives an upper bound of dC.
1/0
-3 3
-2 2
-3/2 3/2
-1 1
-2/3 2/3
-1/2 1/2
1/3 -1/3
0
Figure 6: Complex of curves of torus
Lemma 3.2 (Lemma 1.1 of [16]). If ξ(Σ) > 4 then dC (α, β) ≤ i(α, β)+1. Recall the classifications of elements of the mapping class group into periodic, reducible and pseudo Anosov elements. An element h ∈ MCG(Σ) is called
16 pseudo Anosov if there exist r> 1 and a pair of measured foliations F s and F u such that 1 h(F s)= F s and h(F u)= rF u. r If (C(Σ), dC ) is a bounded metric space with upper bound K, then it is triv- ially a K-hyperbolic space. But from the following proposition, it is clear that diam(C(Σ)) = ∞. Proposition 3.3 (Proposition 4.6 of [68]). If ξ(Σ) > 4 then there exists c> 0 such that, for any pseudo-Anosov h ∈ MCG(Σ), γ ∈ S(Σ) and n ∈ Z, we have n dC(h (γ),γ) ≥ c|n|.
In [68], Masur and Minsky proved that (C(Σ), dC ) is δ-hyperbolic for the case ξ(Σ) = 3g + n> 4, too.
Theorem 3.4 (Theorem 1.1 of [67]). C(Σg,n) is a δ-hyperbolic space, where δ depends only on g and n. Masur and Minsky used Teichm¨uller theory in the proof of Theorem 3.4. In their proof, the constant δ is not constructive because it contains a compactness arguments on the spaces of quadratic differentials. In 2002, Bowditch proved Theorem 3.4 in a more combinatorial way, and showed that the number δ is bounded by a logarithmic function of 3g + n − 4. Since C(Σ) is δ-hyperbolic, we can consider its Gromov boundary. For the case of torus, the boundary can be identified with the set of irrational numbers. For general cases, it is clear that a sequence of curves αn, whose distance from a fixed curve is going to infinity, must converge to a maximal lamination. In fact, Klarreich showed that the Gromov boundary of complex of curves is home- omorphic to the space of topological equivalence classes of filling lamination. Theorem 1.1 (Klarreich [51]). There is a homeomorphism
k : ∂∞C(Σ) → EL(Σ) such that for any sequence αn in S(Σ), αn converges to α ∈ ∂∞C(Σ) if and only if αn, considered as a subset of UML(Σ), converges to k(α). Klarreich used Teichm¨uller theory and the results of Masur and Minsky in [67], to prove Theorem 1.1. It is clear that ∂∞C(Σ) is homeomorphic to the Gromov boundary of its 1-skeleton C1(Σ) because they are quasi-isometric. Suppose that ǫ> 0 satisfies the collar lemma. For each α ∈C0(Σ), let
T (α)= {σ ∈ T (Σ) | ℓσ(α) <ǫ}.
Then a collection of sets T (α1), · · · , T (αn) has nonempty intersection if and only if α1, · · · , αn form a simplex in C(S). The set Tel(Σ) is defined from T (Σ), 1 by adding a new point Pα for each set T (α) and an interval of length 2 from Pα to each point in T (α). Tel(Σ) equipped with the minimal path-metric is called
17 the relative Teichm¨uller space following the terminology of Farb([29]). In [67], Masur and Minsky showed that Tel(Σ) is quasi-isometric to C1(Σ), and Klarreich showed that the Gromov boundary of Tel(Σ) is homeomorphic to the space of topological equivalence classes of minimal singular foliations on Σ, which is homeomorphic to EL(Σ).
4 Proof of Main Theorem (Theorem 1.7)
Suppose that σi ∈ T (S) converges to [λ] ∈ PML(S) in T (S). Recall the map
Φ(σi) = a pants decomposition whose total length is bounded by L, where L is a fixed Bers constant and all pants curves are geodesics in σi. Recall also the quotient map u : PML(S) → UML(S) by forgetting measure. Notice that Φ(σi) can be considered as a sequence in GL(S). Since GL(S) is compact, it has a convergent subsequence. In this chapter we prove our main theorem.
Main Theorem. If Φ(σi) converge to ν ∈ GL(S) in Hausdorff metric topology, then u([λ]) ⊂ ν.
Consider the decomposition u([λ]) = λ1 ∪ λ2 ∪··· λm as a finite disjoint union of minimal laminations. To prove λj ⊂ ν for all 1 ≤ j ≤ m, it is enough to show that λ1 ⊂ ν and λ2 ⊂ ν, assuming that λ1 is a simple closed curve and λ2 is not a simple closed curve.
4.1 Proof of λ1 ⊂ ν
Recall that λ1 is a simple closed curve. If λ1 ⊂ Φ(σi) for infinitely many i, then it is clear that λ1 ⊂ ν. If λ1 ⊂ Φ(σi) for only finitely many i, then there exists N1 > 0 such that λ1 6⊂ Φ(σi) for all i>N1. Since Φ(σi) is a pants decomposition, there exists a pants curve αi in Φ(σi) such that αi ∩ λ1 6= ∅ for all i>N1. Choose xi ∈ αi ∩ λ1 and a limit point x of xi. By Lemma 2.5, there exists a leaf ℓ of ν such that x ∈ ℓ. If ℓ = λ1, we are done. To get a contradiction, suppose that ℓ 6= λ1. Choose an open neighborhood U of x which is isometric to an open subset of H2 (see Figure 7). Since S is compact, by Lemma 2.6, there exists a pants curve αi in Φ(σi) such that an arc βi ⊂ αi approaches to ℓ ∩ U. Therefore there exists N2 > 0 such that
i(αi, λ) ≥ dλ1 = r> 0 for all i>N2, Zβi where r> 0 is the transverse measure on λ1. Choose µj ∈ ML(S) which converges to λ in PML(S) as in Theorem 2.11. Since µj → ∞, there exists a sequence cj > 0 such that cj µj converges to λ in ML(S) with limj→∞ cj = 0. Notice that there exists N3 > 0 which does not depend on i such that r i(α ,c µ ) ≥ for all i,j >N3. i j j 2
18 x λ1
ℓ
Figure 7: The open neighborhood U
Therefore i(αi, µj ) approaches to ∞ as i, j → ∞. But from eq. (3), we have i(αi, µi) ≤ ℓσi (αi) ≤ L for all i. This is a contradiction.
4.2 Proof of λ2 ⊂ ν
Recall that λ2 is not a simple closed curve. Let F be the essential subsurface of S which is filled by λ2. Since Φ(σi) is a pants decomposition for all i, we have ν ∩ F 6= ∅. To get contradictions, suppose that λ2 6⊂ ν in the next two paragraphs. Suppose that ν ∩ λ2 = ∅. Notice that ν ∪ λ2 is a geodesic lamination, too. By Theorem 1.6, we can decompose ν ∪ λ2 as a finite disjoint union of minimal lamination, including λ2, and finite number of infinite isolated leaves. Since λ2 is a filling lamination in F , any isolated infinite leaf can not intersect F . Thus ν ∩ F = ∅. This is a contradiction. Suppose that ν ∩ λ2 6= ∅. There exists a leaf ℓ of ν which intersects λ2 transversely. Choose an open neighborhood V which is isometric to an open 2 subset of H , and in which ℓ intersects λ2 transversely (see Figure 8). As in Section 4.1, there exists a pants curve αi in Φ(σi) and arc βi ⊂ αi such that βi converges to ℓ ∩ V . Therefore there exist r> 0 and N > 0 such that
i(αi, λ) ≥ dλ2 = r> 0 for all i>N. Zβi
As in Section 4.1, using Theorem 2.11, we can show that this is a contradiction.
5 Proof of Corollary 1.8 and Theorem 1.9
In this chapter we prove Corollary 1.8 and Theorem 1.9, and show that Theorem 1.4 comes from Theorem 1.9.
19 λ2
ℓ
Figure 8: The open neighborhood V
5.1 Proof of Corollary 1.8
Suppose that σi ∈ T (S) converges to [λ] ∈ PML(S) in T (S), and let u([λ]) = λ1 ∪ λ2 ∪···∪ λm be the decomposition as a finite disjoint union of minimal laminations.
Y
p
λ1 S
U
Figure 9: The annular cover Y
Suppose that λ1 is a simple closed curve. Suppose also that αi is a pants curve in Φ(σi) and αi ∩ λ1 6= ∅ for all i. Construct an annular covering Y of S in which a neighborhood U of λ1 lifts homeomorphically (see Figure 9). We may assume that U is a closed collar around λ1 and U does not intersect λj for all j 6= 1. Let ai ∈ lift(αi). In this section we prove Corollary 1.8.
Corollary 1.8. |a1 · ai| approaches to infinity as i increases.
Proof. To get a contradiction, suppose that |a1 · ai| does not approach to ∞.
20 Then there exists a subsequence of ai, which we will call ai again for the sake of simplicity, such that |a1 · ai| = k for all i for some k ∈ N ∪{0}. We may assume that a1 · ai = k without loss of generality.
2 H
K
p
βi
U
Figure 10: A lift of βi in K
Suppose that i ≥ 3. From eq. (4), we have a1 · ai = a1 · a2 + a2 · ai + ∆, where ∆ ∈ {−1, 0, 1}. Therefore
|a2 · ai|≤ 1 for all i ≥ 3.
This equation implies that αi = p(ai) does not change so much in U for i ≥ 2. Let βi be a component of αi ∩ U (see Figure 10). We can find a compact set 2 2 K ⊂ H such that there exists a lift of βi in K for all i. A geodesic arc in H is determined by its two endpoints. Therefore there exists a subsequence of βi, which we will call βi again, which converges to a geodesic arc β in Hausdorff metric topology with i(β, λ1) 6= 0. For this subsequence
βi ⊂ αi ⊂ Φ(σi),
Φ(σi) still converges to ν in Hausdorff metric topology. Notice that λ1 6⊂ ν. This is a contradiction to Theorem 1.7.
5.2 Proof of Theorem 1.9
In this section we prove Theorem 1.9. Suppose that λ2 is not a simple closed curve and let F be the subsurface of S which is filled by λ2 (see Figure 11).
21 βi
F
Figure 11: βi
Notice that there exists a pants curve αi in Φ(σi) with αi ∩ F 6= ∅ for all i. Let βi be a component of αi ∩ F . As in Lemma 2.2 of [69], let
β˜i be a non-peripheral essential component of boundary
of regular neighborhood of βi ∪ ∂F , where F is the completion of F with path-metric. Note that two different components of ∂F could be a same curve in S. Notice that if βi is a closed curve, then β˜i is homotopic to βi. Notice also that if βi is an arc, then there are two cases as in Figure 12.
βi
βi
∂F ∂F ∂F
CaseI CaseII
Figure 12: Two Cases
Recall that λ2 is not a closed curve. Since F is filled by λ2, it can not be a disk, an annulus or a pants. Therefore for both cases, the regular neighborhood of
22 βi ∪ ∂F has a boundary component which is non-peripheral and essential in F . Suppose that ∂F = {γ1, · · · ,γk} and let
ℓσi (∂F )= ℓσi (γ1)+ · · · ℓσi (γk). ˜ ˜ Let ℓσi (βi) be the length of geodesic representative of βi in σi. From the defi- nition of β˜i, we have ˜ ℓσi (βi) < 2ℓσi (αi)+ ℓσi (∂F ). (6) We now prove Theorem 1.9.
Theorem 1.9. The geodesic representative of β˜i converges to λ2 in UML(F ). To prove Theorem 1.9, it is enough to show that the geodesic representative of β˜i converges to λ2 in UML(S). For any limit point [β] of [β˜i] in PML(S), we will show that i(β, λ2) = 0. Then Theorem 1.9 follows from the fact that λ2 fills F .
Lemma 5.1. If [β] is a limit point of [β˜i] in PML(S), then i(β, λ2)=0.
Proof. Suppose that [β] is a limit point of [β˜i]. After possibly restricting to a subsequence, we may assume that [β˜i] converges to [β] in PML(S). There exist a constant K > 0 and a sequence bi > 0 such that biβ˜i converges to β in ML(S) with bi ≤ K for all i. Choose µi ∈ ML(S) as in Theorem 2.11. Since µi converges to [λ] ∈ PML(S) with µi → ∞, there exists a sequence ci > 0 such that ciµi con- verges to λ in ML(S) with ci → 0. By eq. (3), there exists Γ > 0 which does not depend on i such that i(∂F , µi) ≤ ℓσi (∂F ) ≤ i(∂F, µi) + Γ. Therefore
i(∂F,ciµi) ≤ ciℓσi (∂F ) ≤ i(∂F,ciµi)+ ciΓ. Since i(∂F, λ) = 0, from the continuity of the intersection number, we have
lim ciℓσi (∂F )=0. (7) i→∞ From eq. (3) and eq. (6), we have ˜ ˜ i(βi, µi) ≤ ℓσi (βi) ≤ 2ℓσi (αi)+ ℓσi (∂F ) Therefore ˜ i(biβi,ciµi) ≤ K 2ciL + ciℓσi (∂F ) . Hence from eq. (7) and the continuity of the intersection number, we have i(β, λ) =0. Thus i(β, λ2)=0.
5.3 Theorem 1.4 follows from Theorem 1.9
Suppose that σi ∈ T (S) converges to [λ] ∈ PML(S) in T (S), and αi is a pants curve in Φ(σi). If λ is a filling lamination, from Theorem 1.9, then αi converges to u([λ]) in UML(S). Therefore if we identify ∂∞C(S) with EL(S) via the homeomorphism k in Theorem 1.1, we have
Theorem 1.4. If λ is a filling lamination, then αi converges to u([λ]) in C(S)∪ ∂∞C(S).
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