Prym Varieties and Teichmüller Curves

Transcription

1 Prym Varieties and Teichmüller Curves The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation McMullen, Curtis T Prym varieties and Teichmüller curves. Duke Mathematical Journal 133(3): Published Version Citable link Terms of Use This article was downloaded from Harvard University s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at nrs.harvard.edu/urn-3:hul.instrepos:dash.current.terms-ofuse#laa

2 Foliations of Hilbert modular surfaces Curtis T. McMullen 21 February, 2005 Abstract The Hilbert modular surface X D is the moduli space of Abelian varieties A with real multiplication by a quadratic order of discriminant D > 1. The locus where A is a product of elliptic curves determines a finite union of algebraic curves X D (1) X D. In this paper we show the lamination X D (1) extends to an essentially unique foliation F D of X D by complex geodesics. The geometry of F D is related to Teichmüller theory, holomorphic motions, polygonal billiards and Lattès rational maps. We show every leaf of F D is either closed or dense, and compute its holonomy. We also introduce refinements T N (ν) of the classical modular curves on X D, leading to an explicit description of X D (1). Contents 1 Introduction Quaternion algebras Modular curves and surfaces Laminations Foliations of Teichmüller space Genus two Holomorphic motions Quasiconformal dynamics Further results Research supported in part by the NSF and the Guggenheim Foundation.

3 1 Introduction Let D > 1 be an integer congruent to 0 or 1mod 4, and let O D be the real quadratic order of discriminant D. The Hilbert modular surface X D = (H H)/SL(O D O D) is the moduli space for principally polarized Abelian varieties A τ = C 2 /(O D O D τ) with real multiplication by O D. Let X D (1) X D denote the locus where A τ is isomorphic to a polarized product of elliptic curves E 1 E 2. The set X D (1) is a finite union of disjoint, irreducible algebraic curves ( 4), forming a lamination of X D. Note that X D (1) is preserved by the twofold symmetry ι(τ 1,τ 2 ) = (τ 2,τ 1 ) of X D. In this paper we will show: Theorem 1.1 Up to the action of ι, the lamination X D (1) extends to a unique foliation F D of X D by complex geodesics. (Here a Riemann surface in X D is a complex geodesic if it is isometrically immersed for the Kobayashi metric.) Holomorphic graphs. The preimage X D (1) of X D (1) in the universal cover of X D gives a lamination of H H by the graphs of countably many Möbius transformations. To foliate X D itself, in 6 we will show: Theorem 1.2 For any (τ 1,τ 2 ) X D (1), there is a unique holomorphic function f : H H such that f(τ 1 ) = τ 2 and the graph of f is disjoint from X D (1). The graphs of such functions descend to X D, and form the leaves of the foliation F D ( 7). The case D = 4 is illustrated in Figure 1. Modular curves. To describe the lamination X D (1) explicitly, recall that the Hilbert modular surface X D is populated by infinitely many modular curves F N [Hir], [vg]. The endomorphism ring of a generic Abelian variety in F N is a quaternionic order R of discriminant N 2. In general F N can be reducible, and R is not determined up to isomorphism by N. In 3 we introduce a refinement F N (ν) of the traditional modular curves, such that the isomorphism class of R is constant along 1

4 Figure 1. Foliation of the Hilbert modular surface X D, D = 4. F N (ν) and F N = F N (ν). The additional finite invariant ν ranges in the ring O D /( D) and its norm satisfies N(ν) = N mod D. The curves T N = F N/l 2 can be refined similarly, and we obtain: Theorem 1.3 The locus X D (1) X D is given by X D (1) = T N ((e + D)/2), where the union is over all integral solutions to e 2 + 4N = D, N > 0. Remark. Although X D (1) = T (D e 2 )/4 when D is prime, in general (e.g. for D = 12,16,20,21,...) the locus X D (1) cannot be expressed as a union of the traditional modular curves T N ( 3). Here is a corresponding description of the lamination X D (1). Given N > 0 such that D = e 2 + 4N, let Λ N D = U = µ bd : a µ a,b Z, µ O D, det(u) = N and µ ±(e + D)/2 in O D /( D) Let Λ D be the union of all such Λ N D. Choosing a real place ι 1 : O D R, we can regard Λ D as a set of matrices in GL + 2 (R), acting by Möbius transformations on H. Theorem 1.4 The lamination X D (1) of H H is the union of the loci τ 2 = U(τ 1 ) over all U Λ D.. 2

5 We also obtain a description of the locus X D (E) X D where A τ admits an action of both O D and O E ( 3). Quasiconformal dynamics. Although its leaves are Riemann surfaces, F D is not a holomorphic foliation. Its transverse dynamics is given instead by quasiconformal maps, which can be described as follows. Let q = q(z)dz 2 be a meromorphic quadratic differential on H. We say a homeomorphism f : H H is a Teichmüller mapping relative to q if it satisfies f/ f = αq/ q for some complex number α < 1; equivalently, if f has the form of an orientation-preserving real-linear mapping f(x + iy) = a b x = D q (f) x c d y y in local charts where q = dz 2 = (dx + idy) 2. Fix a transversal H s = {s} H to F D. Any g SL(O D O D) acts on H H, permuting the leaves of F D. The permutation of leaves is recorded by the holonomy map φ g : H s H s, characterized by the property that g(s,z) and (s,φ g (z)) lie on the same leaf of F D. In 8 we will show: Theorem 1.5 The holonomy acts by Teichmüller mappings relative to a fixed meromorphic quadratic differential q on H s. For s = i and g = ( ) a b c d, we have D q (φ g ) = ( ) a b c d PSL2 (R). On the other hand, for z H s we have φ g (z) = (a z b )/( c z + d ); in particular, the holonomy acts by Möbius transformations on H s. Here (x + y D) = (x y D). Note that both Galois conjugate actions of g on R 2 appear, as different aspects of the holonomy map φ g. Quantum Teichmüller curves. For comparison, consider an isometrically immersed Teichmüller curve f : V M g, 3

6 generated by a holomorphic quadratic differential (Y, q) of genus g. For simplicity assume Aut(Y ) is trivial. Then the pullback of the universal curve X = f (M g,1 ) gives an algebraic surface p : X V with p 1 (v) = Y for a suitable basepoint v V. The surface X carries a canonical foliation F, transverse to the fibers of p, whose leaves map to Teichmüller geodesics in M g,1. The holonomy of F determines a map π 1 (V,v) Aff + (Y,q) giving an action of the fundamental group by Teichmüller mappings; and its linear part yields the isomorphism π 1 (V,v) = PSL(Y,q) PSL 2 (R), where PSL(Y, q) is the stabilizer of (Y, q) in the bundle of quadratic differentials QM g M g. (See e.g. [V1], [Mc4, 2].) The foliated Hilbert modular surface (X D, F D ) presents a similar structure, with the fibration p : X V replaced by the holomorphic foliation A D coming from the level sets of τ 1 on X D = H H. This suggests that one should regard (X D, A D, F D ) as a quantum Teichmüller curve, in the same sense that a 3-manifold with a measured foliation can be regarded as a quantum Teichmüller geodesic [Mc3]. Question. Does every fibered surface p : X C admit a foliation F by Riemann surfaces transverse to the fibers of p? Complements. We conclude in 9 by presenting the following related results. 1. Every leaf of F D is either closed or dense. 2. When D d 2, there are infinitely many eigenforms for real multiplication by O D that are isoperiodic but not isomorphic. 3. The Möbius transformations Λ D give a maximal top-speed holomorphic motion of a discrete subset of H. 4. The foliation F 4 also arises as the motion of the Julia set in a Lattès family of iterated rational maps. 4

7 The link with complex dynamics was used to produce Figure 1. Notes and references. The foliation F D is constructed using the connection between polygonal billiards and Hilbert modular surfaces presented in [Mc4]. For more on the interplay of dynamics, holomorphic motions and quasiconformal mappings, see e.g. [MSS], [BR], [Sl], [Mc2], [Sul], [McS], [EKK] and [Dou]. A survey of the theory of holomorphic foliations of surfaces appears in [Br1]; see also [Br2] for the Hilbert modular case. I would like to thank G. van der Geer, B. Gross and the referees for useful comments and suggestions. 2 Quaternion algebras In this section we consider a real quadratic order O D acting on a symplectic lattice L, and classify the quaternionic orders R End(L) extending O D. Quadratic orders. Given an integer D > 0, D 0 or 1mod 4, the real quadratic order of discriminant D is given by O D = Z[T]/(T 2 + bt + c), where D = b 2 4c. Let K D = O D Q. Provided D is not a square, K D is a real quadratic field. Fixing an embedding ι 1 : K D R, we obtain a unique basis K D = Q 1 Q D such that ι 1 ( D) > 0. The conjugate real embedding ι 2 : K D R is given by ι 2 (x) = ι 1 (x ), where (a + b D) = (a b D). Square discriminants. The case D = d 2 can be treated similarly, so long as we regard x = d 2 as an element of K D satisfying x 2 = d 2 but x Q. In this case the algebra K D = Q Q is not a field, so we must take care to distinguish between elements of the algebra such as and the corresponding real numbers x = d d 2 K D, ι 1 (x) = d d = 0, and ι 2 (x) = d + d = 2d. Trace, norm and different. For simplicity of notation, we fix D and denote O D and K D by K and O. 5

8 The trace and norm on K are the rational numbers Tr(x) = x + x and N(x) = xx. The inverse different is the fractional ideal O = {x K : Tr(xy) Z y O}. It is easy to see that O = D 1/2 O, and thus the different D = (O ) 1 O is the principal ideal ( D). The trace and norm descend to give maps satisfying Tr,N : O /D Z/D, Tr(x) 2 = 4N(x)mod D. (2.1) When D is odd, Tr : O /D Z/D is an isomorphism, and thus (2.1) determines the norm on O /D. On the other hand, when D = 4E is even, we have an isomorphism O /D = Z/2E Z/2 given by a + b E (a,b), and the trace and norm on O /D are given by Tr(a,b) = 2amod D, N(a,b) = a 2 Eb 2 modd. Symplectic lattices. Now let L ( ) = (Z 2g, 0 I I 0 ) be a unimodular symplectic lattice of genus g. (This lattice is isomorphic to the first homology group H 1 (Σ g, Z) of an oriented surface of genus g with the symplectic form given by the intersection pairing.) Let End(L) = M 2g (Z) denote the endomorphism ring of L as a Z-module. The Rosati involution T T on End(L) is defined by the condition Tx,y = x,t y ; it satisfies (ST) = T S, and we say T is self-adjoint if T = T. Specializing to the case g = 2, let L denote the lattice with the unimodular symplectic form L = O O x,y = Tr(x y) = Tr K Q (x 1y 2 x 2 y 1 ). A standard symplectic basis for L (satisfying a i b j = δ ij ) is given by (a 1,a 2,b 1,b 2 ) = ((1,0),(γ,0),(0, γ / D),(0,1/ D)), (2.2) 6

9 where γ = (D + D)/2. The lattice L comes equipped with a proper, self-adjoint action of O, given by k (x 1,x 2 ) = (kx 1,kx 2 ). (2.3) Conversely, any proper, self-adjoint action of O on a symplectic lattice of genus two is isomorphic to this model (see e.g. [Ru], [Mc7, Thm 4.1]). (Here an action of R on L is proper if it is indivisible: if whenever T End(L) and mt R for some integer m 0, then T R.) Matrices. The natural embedding of L = O O into K K determines an embedding of matrices and hence a diagonal inclusion M 2 (K) End(L Q), K End(L Q) extending the natural action (2.3) of O on L. Every T End(L Q) can be uniquely expressed in the form T(x) = Ax + Bx, A,B M 2 (K), where (x 1,x 2 ) = (x 1,x 2 ); and we have T (x) = A x + (B ) x, (2.4) where ( ) ( ) a b c d = d b c a. The automorphisms of L as a symplectic O-module are given, as a subgroup of M 2 (K), by SL(O O ) = a b O D c d O : ad bc = 1 O. Compare [vg, p.12]. Integrality. An endomorphism T End(L Q) is integral if it satisfies T(L) L. Lemma 2.1 The endomorphism φ(x) = ax + bx of K satisfies φ(o) O iff a,b O and a + b O. Proof. Since x x DZ for all x O, the conditions on a,b imply φ(x) = a(x x ) + (a + b)x O for all x O. Conversely, if φ is integral, then φ(1) = a + b O, and thus a(x x ) O for all x O, which implies a D 1/2 O = O. 7

10 Corollary 2.2 The endomorphism T(x) = kx + ( ) a bd c d x is integral iff we have a,b,c,d,k O and k + a,k d O. Proof. This follows from the preceding Lemma, using the fact that kx+dx maps O to O iff kx dx maps O to O. Quaternion algebras. A rational quaternion algebra is a central simple algebra of dimension 4 over Q. Every such algebra has the form Q = Q[i,j]/(i 2 = a,j 2 = b,ij = ji) = ( ) a,b Q for suitable a,b Q. Any q Q satisfies a quadratic equation q 2 Tr(q)q + N(q) = 0, where Tr, N : Q Q are the reduced trace and norm. An order R Q is a subring such that, as an additive group, we have R = Z 4 and Q R = Q. Its discriminant is the square integer N 2 = det(tr(q i q j )) > 0, where (q i ) 4 1 is an integral basis for R. The discriminants of a pair of orders R 1 R 2 are related by N 1 /N 2 = R 2 /R 1 2. Generators. We say V End(L) is a quaternionic generator if: 1. V = V, 2. V 2 = N Z, N 0, 3. V k = k V for all k K, and 4. k + D 1/2 V End(L) for some k K. These conditions ) imply that Q = K KV is a quaternion algebra isomorphic to. Conversely, we have: ( D, N Q Theorem 2.3 Any Rosati-invariant quaternion algebra Q with K Q End(L Q) contains a unique pair of primitive quaternionic generators ±V. 8

11 (A generator is primitive unless (1/m)V,m > 1 is also a generator.) Proof. By a standard application of the Skolem-Noether theorem, we can write Q = K KW with 0 W 2 Q and Wk = k W for all k K. Then KW coincides with the subalgebra of Q anticommuting with the selfadjoint element D, so it is Rosati-invariant. The eigenspaces of KW are exchanged by multiplication by D, so up to a rational multiple there is a unique nonzero V KW with V = V. A suitable integral multiple of V is then a generator, and a rational multiple is primitive. Corollary 2.4 Quaternionic extensions K Q End(L) correspond bijectively to pairs of primitive generators ±V End(L). Generator matrices. We say U M 2 (K) is a quaternionic generator matrix if it has the form U = µ bd (2.5) a µ with a,b Z, µ O and N = det(u) 0. Theorem 2.5 The endomorphism V (x) = Ux is a quaternionic generator iff U is a quaternionic generator matrix. Proof. By (2.4) the condition V = V is equivalent to U = U, and thus U can be written in the form (2.5) with a,b Q and µ K. Assuming U = U, we have N = det(u) = UU = UU = V 2, so V 2 0 det(u) 0. The condition that D 1/2 (k+v ) is integral for some k implies, by Corollary 2.2, that the coefficients of U satisfy a,b Z and µ O; and given such coefficients for U, the endomorphism D 1/2 (k + V ) is integral when k = µ. ( The invariant ν(u). Given generator matrix U = µ bd a µ ), let ν(u) denote the image of µ in the finite ring O /D. It is easy to check that ν(u) = ±ν(g Ug 1 ) for all g SL(O O ), and that its norm satisfies N(ν(U)) N modd. (2.6) 9

12 Quaternionic orders. Let V (x) = Ux, and let R U = (K KV ) End(L). Then R U is a Rosati-invariant order in the quaternion algebra generated by V. Clearly O R U, so we can also regard (R U, ) as an involutive algebra over O. We will show that N = det(u) and ν(u) determine (R U, ) up to isomorphism. Models. We begin by constructing a model algebra (R N (ν), ) over O D for every ν O /D with N(ν) = N 0mod D. Let Q N = K KV be the abstract quaternion algebra with the relations V 2 = N and V k = k V. Define an involution on Q N by (k 1 + k 2 V ) = (k 1 k 2 V ), and let R N(ν) be the order in Q N defined by R N (ν) = {α + βv : α,β O,α + βν O.} (2.7) Note that O D O, so the definition of R N (ν) depends only on the class of ν in O /D. To check that R N (ν) is an order, note that (α + βv )(γ + δv ) = (κ + λv ) = (αγ Nβδ ) + (αδ + βγ )V ; since N N(ν) = νν modd, we have κ + νλ (αγ + νν βδ ) + ν(αδ + βγ ) = (α + βν)(γ + δ ν ) + α(γ γ + νδ ν δ ) mod O, and thus R U is closed under multiplication. Theorem 2.6 The quaternionic order R N (ν) has discriminant N 2. Proof. Note that the inclusions O O V R N (ν) O O V each have index D. The quaternionic order O O V has discriminant D 2 N 2, since V 2 = N and Tr O V = 0, and thus R N (ν) has discriminant N 2. 10

13 Theorem 2.7 We have (R N (ν), ) = (R M (µ), ) iff N = M and ν = ±µ. Proof. The element V R N (ν) is, up to sign, the order s unique primitive generator, in the sense that V = V, V k = k V for all k O D, V 2 0, k + D 1/2 V R N (ν) for some k K, and V is not a proper multiple of another element in R N (ν) with the same properties. Thus the structure of (R N (ν), ) as an O D -algebra determines V R N (ν) up to sign, and V determines N = V 2 and the constant ν O /D in the relation α+βν O defining R N (ν) K KV. Theorem 2.8 If U is a primitive generator matrix, then we have where N = det(u) and ν = ν(u). (R U, ) = (R N (ν), ) Proof. Setting V (x) = Ux, we need only verify that (K KV ) End(L) coincides with the order R N (ν) defined by (2.7). To see this, let T(x) = αx + βv (x) = αx + β µ bd x a µ in K KV. By Corollary 2.2, T is integral iff (i) aβ,bβ,µβ,µ β O, (ii) α O, (iii) α + βµ O and (iv) α + βµ O. Using (iii), condition (iv) can be replaced by (iv ) β(µ µ )/ D O. Since U is primitive, the ideal (a,b,µ,(µ µ )/ D) is equal to O. Thus (i) and (iv ) together are equivalent to the condition β O, and we are left with the definition of R N (ν). 11

14 Remark. In general, the invariants det(u) and ν(u) do not determine the embedding R U End(L) up to ( conjugacy. ) For example, ( ) when D is odd, the generator matrices U 1 = 0 D 2 D 0 and U 2 = 0 D have the same invariants, but the corresponding endomorphisms are not conjugate in End(L) because L/V 1 (L) = (Z/D Z/D 2 ) 2 while L/V 2 (L) = Z/D Z/D 2 Z/D 3. Extra quadratic orders. Finally we determine when the algebra R N (ν) contains a second, independent quadratic order O E. Theorem 2.9 The algebra (R N (ν), ) contains a self-adjoint element T O D generating a copy of O E iff there exist e,l Z such that and (e + E D)/2 + lν = 0mod D. Proof. Given e,l as above, let ED = e 2 + 4Nl 2, l 0 T = α + βv = D 1/2 ( e + E D 2 + lv Then we have T = T, T R N (ν) and T 2 et + (E E 2 )/4 = 0; therefore Z[T] = O E. A straightforward computation shows that, conversely, any independent copy of O E in R N (ν) arises as above. For additional background on quaternion algebras, see e.g. [Vi], [MR] and [Mn]. 3 Modular curves and surfaces In this section we describe modular curves on Hilbert modular surfaces from the perspective of the Abelian varieties they determine. Abelian varieties. A principally polarized Abelian variety is a complex torus A = C g /L equipped with a unimodular symplectic form x,y on L = Z 2g, whose extension to L R = C g satisfies x,y = ix,iy and x,ix 0. ). 12

15 The ring End(A) = End(L) End(C g ) is Rosati invariant, and coincides with the endomorphism ring of A as a complex Lie group. We have Tr(TT ) 0 for all T End(A). Every Abelian variety can be presented in the form A = C g /(Z g ΠZ g ), where Π is an element of the Siegel upper halfplane H g = {Π M g (C) : Π t = Π and Im(Π) is positive-definite}. ( ) The symplectic form on L = Z g ΠZ g is given by 0 I I 0. Any two such presentations of A differ by an automorphism of L, so the moduli space of abelian varieties of genus g is given by the quotient space A g = H g /Sp 2g (Z). Real multiplication. As in 2, let D > 0 be the discriminant of a real quadratic order O D, and let K = O Q. Fix a real place ι 1 : K R, and set ι 2 (k) = ι 1 (k ). We will regard K as a subfield of the reals, using the fixed embedding ι 1 : K R. The case D = d 2 is treated with the understanding that the real numbers (k,k ) implicitly denote (ι 1 (k),ι 2 (k)), k K. An Abelian variety A A 2 admits real multiplication by O D if there is a self-adjoint endomorphism T End(A) generating a proper action of Z[T] = O D on A. Any such variety can be presented in the form A τ = C 2 /(O D O D τ) = C2 /φ τ (L), (3.1) where τ = (τ 1,τ 2 ) H H and where L = O O is embedded in C 2 by the map φ τ (x 1,x 2 ) = (x 1 + x 2 τ 1,x 1 + x 2 τ 2). As in 2, the symplectic form on L is given by x,y = Tr K Q (x y), and the action of O D on C 2 L is given simply by k (z 1,z 2 ) = (kz 1,k z 2 ). Eigenforms. The Abelian variety A τ comes equipped with a distinguished pair of normalized eigenforms η 1,η 2 Ω(A τ ). Using the isomorphism H 1 (A τ, Z) = L, these forms are characterized by the property that ( φ τ (C) = C ) η 1, η 2. (3.2) C 13

16 Modular surfaces. If we change the identification L = H 1 (A τ, Z) by an automorphism g of L, we obtain an isomorphic Abelian variety A g τ. Thus the moduli space of Abelian varieties with real multiplication by O D is given by the Hilbert modular surface X D = (H H)/SL(O D O D ). The point g(τ) is characterized by the property that φ g τ = χ(g,τ) φ τ g 1 for some matrix χ(g,τ) GL 2 (C); explicitly, we have a b ( aτ1 b (τ 1,τ 2 ) = c d cτ 1 + d, a τ 2 b ) c τ 2 + d (3.3) and χ(g,τ) = (d cτ 1) 1 0. (3.4) 0 (d c τ 2 ) 1 A point [τ] X D gives an Abelian variety [A τ ] A 2 with a chosen embedding O D End(A τ ). Similarly, a point τ X D = H H gives an Abelian variety with a distinguished isomorphism or marking, L = H 1 (A τ, Z), sending O D into End(A τ ). Modular embedding. The modular embedding p D : X D A 2 is given by [τ] [A τ ]. To write p D explicitly, note that the embedding φ τ : L C 2 can be expressed with respect to the basis (a 1,a 2,b 1,b 2 ) for L given in (2.2) by the matrix φ τ = 1 γ τ 1γ / D τ 1 / D 1 γ τ 2 γ/ D τ 2 / = (A,B). D Consequently we have A τ = C 2 /(Z 2 ΠZ 2 ), where Π = p D (τ) = A 1 B = 1 τ 1(γ ) 2 + τ 2 γ 2 τ 1 γ τ 2 γ. D τ 1 γ τ 2 γ τ 1 + τ 2 The map X D p D (X D ) has degree two. 14

17 Modular curves. Given a matrix U(x) = ( a b c d ) M2 (K) End(L) such that U = U, let V (x) = Ux and define It is straightforward to check that { H U = H U = {τ H H : V End(A τ )}. (τ 1,τ 2 ) : τ 2 = dτ 1 + b cτ 1 + a } ; (3.5) indeed, when τ 1 and τ 2 are related as above, the map φ τ : L C 2 satisfies φ τ (V (x)) = 0 a + cτ 1 a + c τ 2 0 φ τ (x), exhibiting the complex-linearity of V. Note that H U = if det(u) < 0. We now restrict attention to the case where U is a generator matrix. Then by the results of 2, we have: Theorem 3.1 The ring End(A τ ) contains a quaternionic order extending O D if and only if τ H U for some generator matrix U. Let F U X D denote the projection of H U to the quotient (H H)/SL(O D O D ). Following [Hir, 5.3], we define the modular curve F N by F N = {F U : U is a primitive generator matrix with det(u) = N.}. It can be shown that F N is an algebraic curve on X D. To describe this curve more precisely, let F N (ν) = {F U : U is primitive, det(u) = N and ν(u) = ±ν}, where ν O D /D D. Note that we have F N (ν) N(ν) = N modd by equation (2.6), F N (ν) = F N ( ν), and F N = F N (ν). The results of 2 give the structure of the quaternion ring generated by V (x) = Ux. Theorem 3.2 The curve F N (ν) X D coincides with the locus of Abelian varieties such that O D R End(A τ ), for some properly embedded quaternionic order (R, ) isomorphic to (R N (ν), ). 15

18 Corollary 3.3 The curve F N is the locus where O D End(A τ ) extends to a properly embedded, Rosati-invariant quaternionic order of discriminant N 2. Two quadratic orders. We can now describe the locus X D (E) of Abelian varieties with an independent, self-adjoint action of O E. (We do not require the action of O E to be proper.) To state this description, it is useful to define: and Then Theorem 2.9 implies: T N = {F U : det(u) = N} = F N/l 2, T N (ν) = {F U : det(u) = N,ν(U) = ±ν}. Theorem 3.4 The locus X D (E) is given by X D (E) = T N ((e + E D)/2), where the union is over all N > 0 and e Z such that ED = e 2 + 4N. Corollary 3.5 We have X D (1) = {T N ((e + D)/2) : e 2 + 4N = D}. Refined modular curves. To conclude we show that in general the expression F N = F N (ν) gives a proper refinement of F N. First note: Theorem 3.6 We have F N (ν) = F N iff ±ν are the only solutions to N(ξ) = N modd, ξ O D /D D. Corollary 3.7 If D = p is prime, then F N = F N (ν) whenever F N (ν). Proof. In this case, according to (2.1), the norm map N : O D /D D Tr = Z/p Z/p is given by N(ξ) = ξ 2 /4. Since F N (ν), we have N(ν) = N; and since Z/p is a field, ±ν are the only solutions to this equation. 16

19 Corollary 3.8 When D is prime, we have X D (E) = T (ED e 2 )/4. Now consider the case D = 21, the first odd discriminant which is not a prime. Then the norm map is still given by N(ξ) = ξ 2 /4 on O D /D D = Z/D, but now Z/D is not a field. For example, the equation ξ 2 = 1mod D has four solutions, namely ξ = 1,8,13 or 20. These give four solutions to the equation N(ξ) = 5, and hence contribute two distinct terms to the expression F 5 = F 5 (ν) = F 5 ((1 + 21)/2) F 5 ((8 + 21)/2). Only one of these terms appears in the expression for X D (1). In fact, since 21 = = , by Corollary 3.5 we have X 21 (1) = F 3 F 5 ((1 + 21)/2) F 3 F 5. (The full curve F 3 appears because the only solutions to N(ξ) = ξ 2 /4 = 3mod 21 are ξ = ±3.) Using Theorem 3.6, it is similarly straightforward to check other small discriminants; for example: Theorem 3.9 For D 30 we have X D (1) = e 2 +4N=D T N when D = 4,5,8,9,13,17,25 and 29, but not when D = 12,16,20,21,24 or 28. Notes. For more background on modular curves( and surfaces, ) see [Hir], [HZ2], [HZ1], [BL], [Mc7, 4] and [vg]. Our U = µ bd a µ corresponds to the skew-hermitian matrix B = ( a µ ) D µ bd in [vg, Ch. V]. Note that (3.3) agrees with the standard action (aτ + b)/(cτ + d) up to the automorphism ) ( ) a b c d of SL 2 (K). We remark that X D can also be presented ( a b c d as the quotient (H H)/SL 2 (O D ), using the fact that D = D; on the other hand, the surfaces (H H)/SL 2 (O D ) and X D are generally not isomorphic (see e.g. [HH].) It is known that the intersection numbers T N,T M form the coefficients of a modular form [HZ1], [vg, Ch. VI]. The results of [GKZ] suggest that the intersection numbers of the refined modular curves T N (ν) may similarly yield a Jacobi form. 4 Laminations In this section we show algebraically that X D (1) gives a lamination of H H by countably many disjoint hyperbolic planes. We also describe these 17

20 laminations explicitly for small values of D. Another proof of laminarity appears in 7. Jacobian varieties. Let Ω(X) denote the space of holomorphic 1-forms on a compact Riemann surface X. The Jacobian of X is the Abelian variety Jac(X) = Ω(X) /H 1 (X, Z), polarized by the intersection pairing on 1-cycles. In the case of genus two, any principally polarized Abelian variety A is either a Jacobian or a product of polarized elliptic curves. The latter case occurs iff A admits real multiplication by O 1, generated by projection to one of the factors of A = B 1 B 2. In particular, we have: Theorem 4.1 For any D 4, the locus of Jacobian varieties in X D is given by X D X D (1). Laminations. To describe X D (1) in more detail, given N > 0 such that D = e 2 + 4N let Λ N D = {U M 2 (K) : U is a generator matrix, det(u) = N and ν(u) ±(e + D)/2 mod D D }, and let Λ D be the union of all such Λ N D. Note that if U is in Λ D, then U,U and U are also in Λ D. By Corollary 3.5, the preimage of X D (1) in X D = H H is given by: X D (1) = {H U : U Λ D }. Note that each H U is the graph of a Möbius transformation. Theorem 4.2 The locus X D (1) gives a lamination of H H by countably many hyperbolic planes. (This means any two planes in X D (1) are either identical or disjoint.) For the proof, it suffices to show that the difference g h 1 of two Möbius transformations in Λ D is never elliptic. Since Λ D is invariant under U U = (det U)U 1, this in turn follows from: Theorem 4.3 For any U 1,U 2 Λ D, we have Tr(U 1 U 2 ) 2 4det(U 1 U 2 ). Proof. By the definition of Λ D, we can write D = e 2 i +4det(U i) = e 2 i +4N i, where e i 0. We can also assume that U i = µ i b i D a i 18 µ i

21 satisfies µ i (x i + y i D)/2 (ei + D)/2 mod D D (replacing U i with U i if necessary). It follows that y i is odd and x i = e i modd, which implies Tr(U 1 U 2 ) Tr(µ 1 µ 2 ) = (x 1 x 2 + Dy 1 y 2 )/2 (e 1 e 2 D)/2 modd. (4.1) (The factor of 1/2 presents no difficulties, because x i is even when D is even.) Now suppose Tr(U 1 U 2 ) 2 < 4det(U 1 U 2 ) = 4N 1 N 2. (4.2) Then we have Tr(U 1 U 2 ) < 2 N 1 N 2 D/2, and thus (4.1) implies But this implies 4Tr(U 1 U 2 ) 2 = (D e 1 e 2 ) 2 contradicting (4.2). Tr(U 1 U 2 ) = (e 1 e 2 D)/2. (D e 2 1)(D e 2 2) = (4N 1 )(4N 2 ) = 16det(U 1 U 2 ), Small discriminants. To conclude we record a few cases where Λ D admits a particularly economical description. For concreteness, we will present Λ D as a set matrices in GL + 2 (R) using the chosen real place ι 1 : K R. This works even when D = d 2, since both µ and µ appear on the diagonal of U Λ D (no information is lost). Under the standard action ( ) a b c d z = (az + b)/(cz + d) of GL + 2 (R) on H, we can then write X D (1) = {(τ 1,τ 2 ) : τ 2 = U(τ 1 )}. ΛD This holds despite the twist in the definition (3.5) of H U, because Λ D is invariant under ( ) a b c d ( d b c a ). Theorem 4.4 For D = 4, 5, 8, 9 and 13 respectively, we have: Λ 4 = {U M 2 (Z) : det(u) = 1 and U ( 0 )mod 4}, Λ 5 = {U = Λ 8 = Λ 1 8 Λ 2 8 = ( ) µ bd a µ { U = : det(u) = 1}, ( ) µ bd a µ } : det(u) = 1 or 2, Λ 9 = {U M 2 (Z) : det(u) = 2 and U ( 0 )mod 9}, { ( ) } and Λ 13 = Λ 1 13 Λ 3 13 = U = µ bd a µ : det(u) = 1 or 3, 19

22 where it is understood that a,b Z and µ O D. Proof. Recall from Theorem 3.9 that X D (1) = e 2 +4N=D T N when D = 4,5,8,9 and 13. When this equality holds, we can ignore the condition on ν(u) in the definition of Λ D. The cases D = 5,8 and 13 then follow directly from the definition ( of ) Λ N D. For D = 9, we note that any integral matrix satisfying det x 9b a y = 2 also satisfies x + y = 0mod 3, and thus it can be ( ) written in the form µ bd a µ with µ = (x y) + (x + y) 9/3 2 Similar considerations apply when D = 4. 5 Foliations of Teichmüller space In this section we introduce a family of foliations F i of Teichmüller space, related to normalized Abelian differentials and their periods τ ij = b i ω j. We then show: Theorem 5.1 There is a unique holomorphic section of the period map τ ii : T g H through any Y T g. Its image is the leaf of F i containing Y. The case g = 2 will furnish the desired foliations of Hilbert modular surfaces. Abelian differentials. Let Z g be a smooth oriented surface of genus g. Let T g be the Teichmüller space of Riemann surfaces Y, each equipped with an isotopy class of homeomorphism or marking Z g Y. The marking determines a natural identification between H 1 (Z g ) and H 1 (Y ) used frequently below. Let ΩT g T g denote the bundle of nonzero Abelian differentials (Y,ω), ω Ω(Y ). For each such form we have a period map I(ω) : H 1 (Z g, Z) C given by I(ω) : C C ω. There is a natural action of GL+ 2 (R) on ΩT g, satisfying I(A ω) = A I(ω) (5.1) 20

23 under the identification C = R 2 given by x + iy = (x,y). Each orbit GL + 2 (R) (Y,ω) projects to a complex geodesic f : H T g, which can be normalized so that f(i) = Y and ν = df dt = i ω t=i 2 ω The subspace of H 1 (Z g, R) spanned by (Re ω,im ω) is constant along each orbit (cf. [Mc7, 3]). Symplectic framings. Now let (a 1,...,a g,b 1,...,b g ) be a real symplectic basis for H 1 (Z g, R) (with a i,b i = b i,a i = 1 and all other products zero). Then for each Y T g, there exists a unique basis (ω 1,...,ω g ) of Ω(Y ) such that a i ω j = δ ij. The period matrix then determines an embedding τ ij (Y ) = b i ω j τ : T g H g. This agrees with the usual Torelli embedding, up to composition with an element of Sp 2g (R). Note that Im(τ ii (Y )) > 0 since Im τ is positive definite. The normalized 1-forms (ω i ) give a splitting Ω(Y ) = g 1 Cω i = g 1 F i(y ), and corresponding subbundles F i T g ΩT g. Complex subspaces. Let (a i,b i ) denote the dual basis for H1 (Z g, R), and let S i be the span of (a i,b i ). It easy to check that the following conditions are equivalent: 1. S i is a complex subspace of H 1 (Y, R) = Ω(Y ). 2. S i is spanned by (Reω i,im ω i ). 3. The period matrix τ(y ) satisfies τ ij = 0 for all j i. 21

24 Let T g (S i ) T g denote the locus where these condition hold. Note that condition (3) defines a totally geodesic subset H i = H Hg 1 H g such that T g (S i ) = τ 1 (H i ). Foliations. Next we show that the complex geodesics generated by the forms (Y,ω i ) give a foliation of Teichmüller space. Theorem 5.2 The sub-bundle F i T g ΩT g is invariant under the action of GL + 2 (R), as is its restriction to T g(s i ). Proof. The invariance of F i T g is immediate from (5.1). To handle the restriction to T g (S i ), recall that the span W of (Re ω i,im ω i ) is constant along orbits; thus the condition W = S i characterizing T g (S i ) is preserved by the action of GL + 2 (R). Corollary 5.3 The foliation of F i T g by GL + 2 (R) orbits projects to a foliation F i of T g by complex geodesics. Corollary 5.4 The locus T g (S i ) is also foliated by F i : any leaf meeting T g (S i ) is entirely contained therein. Proof of Theorem 5.1. The proof uses Ahlfors variational formula [Ah] and follows the same lines as the proof of [Mc4, Thm. 4.2]; it is based on the fact that the leaves of F i are the geodesics along which the periods of ω i change most rapidly. Let s : H T g be a holomorphic section of τ ii. Let v TH be a unit tangent vector with respect to the hyperbolic metric ρ = dz /(2Im z) of constant curvature 4, mapping to Ds(v) T Y T g. By the equality of the Teichmüller and Kobayashi metrics [Gd, Ch. 7], Ds(v) is represented by a Beltrami differential ν = ν(z)dz/dz on Y with ν 1. But s is a section, so the composition τ ii s : H H is the identity; thus the norm of its derivative, given by Ahlfors formula as / D(τ ii s)(ν) = ωi 2 ν ω i 2, is one. It follows that ν = ω i /ω i up to a complex scalar of modulus one, and thus Ds(v) is tangent to the complex geodesic generated by (Y,ω i ). Equivalently, s(h) is everywhere tangent to the foliation F i ; therefore its image is the unique leaf through Y. Y Y 22

25 6 Genus two We can now obtain results on Hilbert modular surfaces by specializing to the case of genus two. In this section we will show: Theorem 6.1 There is a unique holomorphic section of τ 1 passing through any given point of H H X D (1). Here τ 1 : H H H is simply projection onto the first factor. This result is a restatement of Theorem 1.2; as in 1, we assume D 4. Framings for real multiplication. Let g = 2, and choose a symplectic isomorphism L = H 1 (Z g, Z) = O D O D. We then have an action of O D on H 1 (Z g, Z), and the elements {a,b} = {(1,0),(0,1)} in L give a distinguished basis for H 1 (Z g, Q) = L Q = K 2 as a vector space over K = O D Q. Using the two Galois conjugate embeddings K R, we obtain an orthogonal splitting H 1 (Z g, R) = L R = V 1 V 2 such that k (C 1,C 2 ) = (kc 1,k C 2 ). The projections (a i,b i ) of a,b L to each summand yield bases for V i, which taken together give a standard symplectic basis for H 1 (Z g, R). (Note that (a i,b i ) is generally not an integral sympletic basis; indeed, when K is a field, the elements (a i,b i ) do not even lie in H 1 (Z g, Q).) Let S D i H 1 (Z g, R) be the span of the dual basis a i,b i. Theorem 6.2 The ring O D End(L) acts by real multiplication on Jac(Y ) if and only if Y T g (S D 1 ). Proof. Since g = 2 we have S2 D = (S1 D), and thus T g (S1 D) = T g(s2 D). But Jac(Y ) has real multiplication iff S1 D and SD 2 are complex subspaces of H 1 (Y, R) = Ω(Y ) so the result follows. (Cf. [Mc4, Lemma 7.4].) 23

26 Sections. Let E D = X D X D (1) denote the space of Jacobians in X D, and ẼD = H H X D (1) its preimage in the universal cover. (The notation comes from [Mc7, 4], where we consider the space of eigenforms ΩE D as a closed, GL + 2 (R)-invariant subset of ΩM g.) By the preceding result, the Jacobian of any Y T g (S1 D ) is an Abelian variety with real multiplication. Moreover, the marking of Y determines a marking L = H 1 (Y, Z) = H 1 (Jac(Y ), Z) of its Jacobian, and thus a map Jac : T g (S D 1 ) ẼD = X D X D (1). The basis (a i,b i ) yields a pair of normalized forms ω 1,ω 2 Ω(Y ). Similarly, we have a pair of normalized eigenforms η 1,η 2 Ω(A τ ) for each τ X D, characterized by (3.2). Under the identification Ω(Y ) = Ω(Jac(Y )), we find: Theorem 6.3 The forms ω i and η i are equal for any Y T g (S1 D ). Thus Jac(Y ) = A (τ1,τ 2 ), where τ ( ) 1 0 = τ ij (Y ) = ω j. (6.1) 0 τ 2 b i Proof. The period map φ τ : L C 2 for A τ = Jac(Y ) is given by ( ) φ τ (C) = η 1, η 2 = (x 1 + x 2 τ 1,x 1 + x 2τ 2 ), C where C = (x 1,x 2 ) O D O D ; in particular, we have C φ τ (a) = φ τ (1,0) = (1,1). Since φ τ diagonalizes the action of K, we also have ( ) φ τ (C) = η 1, C 1 C 2 η 2 for any C = C 1 + C 2 L R = V 1 V 2. Setting C = a, this implies φ τ (a 1 ) = (1,0) and φ τ (a 2 ) = (0,1); thus a i η j = δ ij, and therefore η i = ω i for i = 1, 2. Similarly, we have φ τ (b) = (τ 1,τ 2 ) = (τ 11,τ 22 ), which implies Y and A τ are related by (6.1). 24

27 Corollary 6.4 We have a commutative diagram T g (S D 1 ) Jac ẼD τ 1 τ 11 H. Proof of Theorem 6.1. Using the Torelli theorem, it follows easily that Jac : T g (S D 1 ) ẼD is a holomorphic covering map. Since H is simplyconnected, any section s of τ 1 lifts to a section Jac 1 s of τ 11. Thus Theorem 5.1 immediately implies Theorem Holomorphic motions In this section we use the theory of holomorphic motions to define and characterize the foliation F D. Holomorphic motions. Given a set E Ĉ and a basepoint s H, a holomorphic motion of E over (H,s) is a family of injective maps F t : E Ĉ, t H, such that F s (z) = z and F t (z) is a holomorphic function of t. A holomorphic motion of E has a unique extension to a holomorphic motion of its closure E; and each map F t : E Ĉ extends to a quasiconformal homeomorphism of the sphere. In particular, F t int(e) is quasiconformal (see e.g. [Dou]). These properties imply: Theorem 7.1 Let P be a partition of H H into disjoint graphs of holomorphic functions. Then: 1. P is the set of leaves of a transversally quasiconformal foliation F of H H; and 2. If we adjoin the graphs of the constant functions f : H H to P, we obtain a continuous foliation of H H. The foliation F D. Recall that every component of X D (1) H H is the graph of a Möbius transformation. By Theorem 6.1, there is a unique partition of H H X D (1) into the graphs of holomorphic maps as well. 25

28 Taken together, these graphs form the leaves of a foliation F D of H H by the preceding result. Since X D (1) is invariant under SL(O D O D ), the foliation F D descends to a foliation F D of X D. To characterize F D, recall that the surface X D admits a holomorphic involution ι(τ 1,τ 2 ) = (τ 2,τ 1 ) which preserves X D (1). Theorem 7.2 The only leaves shared by F D and ι(f D ) are the curves in X D (1). Proof. Let f : H H be a holomorphic function whose graph F is both a leaf of F D and ι( F D ). Then ι(f) is also a graph, so f is an isometry. But if F X D (1) =, then F lifts to a leaf of the foliation F 1 of Teichmüller space, and hence f is a contraction by [Mc4, Thm. 4.2]. Corollary 7.3 The only leaves of F D that are graphs of Möbius transformations are those belonging to X D (1). Complex geodesics. Let us say F is a foliation by complex geodesics if each leaf is a hyperbolic Riemann surface, isometrically immersed for the Kobayashi metric. We can then characterize F D as follows. Theorem 7.4 Up to the action of ι, F D is the unique extension of the lamination X D (1) to a foliation of X D by complex geodesics. Proof. Let F be a foliation by complex geodesics extending X D (1). Then every leaf of its lift F to X D is a Kobayashi geodesic for H H. But a complex geodesic in H H is either the graph of a holomorphic function or its inverse, so every leaf belongs to either F D or ι( F D ). Consequently every leaf of F is a leaf of F D or ι(f D ). Since these foliations have no leaves in common on the open set U = X D X D (1), F coincides with one or the other. Stable curves. The Abelian varieties E F in X D (1) are the Jacobians of certain stable curves with real multiplication, namely the nodal curves Y = E F obtained by gluing E to F at a single point. If we adjoin these stable curves to M 2, we obtain a partial compactification M 2 which maps isomorphically to A 2. The locus X D (1) can then be regarded as the projection to X D of a finite set of GL + 2 (R) orbits in ΩM 2, giving another proof that it is a lamination. 26

29 8 Quasiconformal dynamics In this section we use the relative period map ρ = y 2 y 1 η 1 to define a meromorphic quadratic differential q = (dρ) 2 transverse to F D. We then show the transverse dynamics of F D is given by Teichmüller mappings relative to q. Absolute periods. The level sets of τ 1 form the leaves of a holomorphic foliation ÃD on H H which covers foliation A D of X D. By (3.2), every τ = (τ 1,τ 2 ) determines a pair of eigenforms η 1,η 2 Ω(A τ ) such that the absolute periods η 1, C H 1 (A τ, Z) C are constant along the leaves of ÃD. Since every leaf of F D is the graph of a function f : H H, we have: Theorem 8.1 The foliation A D is transverse to F D. The Weierstrass curve. Recall that E D X D denotes the locus of Jacobians with real multiplication by O D. For [A τ ] = Jac(Y ) E D we can regard the eigenforms η 1,η 2 as holomorphic 1-forms in Ω(Y ) = Ω(A τ ). Let W D E D denote the locus where η 1 has a double zero on Y. By [Mc5] we have: Theorem 8.2 The locus W D is an algebraic curve with one or two irreducible components, each of which is a leaf of F D. We refer to W D as the Weierstrass curve, since η 1 vanishes at a Weierstrass point of Y. Relative periods. Let E D (1,1) = X D (W D X D (1)) denote the Zariski open set where η 1 has a pair of simple zeros, and let ẼD(1,1) be its preimage in the universal cover X D. Let H s = {s} H H H, and let H s = H s ẼD(1,1). For each τ H s, let y 1,y 2 denote the zeros of the associated form η 1 Ω(Y ). We can then define the (multivalued) relative period map ρ s : H s C by ρ s (τ) = y2 y 1 η 1. 27

30 To make ρ s (τ) single-valued, we must (locally) choose (i) an ordering of the zeros y 1 and y 2, and (ii) a path on Y connecting them. Quadratic differentials. Let z be a local coordinate on H s, and recall that the absolute periods of η 1 are constant along H s. Thus if we change the choice of path from y 1 to y 2, the derivative dρ/dz remains the same; and if we interchange y 1 and y 2, it changes only by sign. Thus the quadratic differential q = (dρ/dz) 2 dz 2 is globally well-defined on H s. Theorem 8.3 The form q extends to a meromorphic quadratic differential on H s, with simple zeros where H s meets W D, and simple poles where it meets X D (1). Proof. It is a general result that the period map provides holomorphic local coordinates on any stratum of ΩM g (see [V2], [MS, Lemma 1.1], [KZ]). Thus ρ s H s is holomorphic with dρ s 0, and hence q H s is a nowhere vanishing holomorphic quadratic differential. To see q acquires a simple zero when η 1 acquires a double zero, note that the relative period map ρ(t) = t t (z 2 t)dz = ( 4/3)t 3/2 of the local model η t = (z 2 t)dz satisfies (dρ/dt) 2 = 4t. Similarly, a point of H s X D (1) is locally modeled by the family of connected sums (Y t,η t ) = (E 1,ω 1 )#(E 2,ω 2 ), I with I = [0,ρ(t)] = [0, ± t]. Since (dρ/dt) 2 = 1/(4t), at these points q has simple poles. See [Mc7, 6] for more on connected sums. Teichmüller maps. Now let f : H s H t be a quasiconformal map. We say f is a Teichmüller map, relative to a holomorphic quadratic differential q, if its complex dilatation satisfies µ(f) = ( ) f/ z dz f/ z dz = α q q 28

31 for some α C. This is equivalent to the condition that w = f(z) is real-linear in local coordinates where q = dz 2 and dw 2 respectively. In such charts we can write w = w 0 + D q (f) z, with D q (f) SL 2 (R). We refer to D q (f) as the linear part of f; it is only well-defined up to sign, since z z preserves dz 2. Theorem 8.4 Given g = ( a b c d ) SL(OD O D ) and s H, let H t = g(h s ). Then the linear part of g : H s H t is given by D q (g) z = (d cs) 1 z. Proof. Since the Riemann surfaces Y at corresponding points of H s and H t differ only by marking, the relative period maps ρ s and ρ t differ only by the normalization of η 1. This discrepancy is accounted for by equation (3.4), which gives ρ t /ρ s = χ(g,s) = (d cs) 1. Since the coordinates ρ s and ρ t linearize q, the map D q (g) is given by multiplication by (d cs) 1. Now let C st : H s H t be the unique map such that z and C st (z) lie on the same leaf of F D. Theorem 8.5 ) The linear part of C st is given by D q (C st ) = A t A 1 s, where A u = PSL 2 (R). ( 1 Re(u) 0 Im(u) Proof. By the definition of F D, the forms η 1 at corresponding points of H s and H t are related by some element B GL+ 2 (R) acting on ΩT g. Thus ρ t = B ρ s and therefore D q (C st ) = B. Since the action of B on the absolute periods of η 1 satisfies B(O D O D s) = O D O D t (in the sense of equation (3.1)), we have B(1) = 1 and B(s) = t, and thus B = A t A 1 s as above. Dynamics. Every leaf of F D meets the transversal H s in a single point. Thus the action of g SL(O D O D ) on the space of leaves determines a holonomy map φ g : H s H s, characterized by the property that (s,φ g (z)) lies on the same leaf as g(s,z). Theorem 8.6 The group SL(O D O D ) acts on H s by Teichmüller mappings, satisfying D q (φ g ) = g in the case s = i. 29

32 (As usual we regard g as a real matrix using ι 1 : K R.) Proof. Let g = ( a b c d ), and t = (as b)/( cs + d); then Ht = g(h s ). Since φ g (z) is obtained from g(s,z) by combing it along the leaves of F D back into H s, we have φ g (s,z) = C ts (g(s,z)). Thus the chain rule implies D q (φ g ) z = B z = A s A 1 t (z/( cs + d)). Now assume s = i. Then we have ( B(ai ) b) = A 1 t (t) = i and B( ci + d) = A 1 t (1) = 1; therefore B 1 = d b c a and thus B = ( ) a b c d = g. Corollary 8.7 The foliation F D carries a natural transverse invariant measure. Proof. Since detd q (φ g ) = 1 for all g, the form q gives a holonomyinvariant measure on the transversal H s. Finally we show that, although φ g H s is quasiconformal, its continuous extension to H s is a Möbius transformation. Theorem 8.8 For any g = ( a b c d ) SL(OD O D ) and z H s, we have φ g (z) = (a z b )/( c z + d ). Proof. By Theorem 7.1, the combing maps C st extend to the identity on H s. Thus (t,φ g (z)) = g(s,z), and the result follows from equation (3.3). Note: if we use the transversal H t instead of H s, the holonomy simply changes by conjugation by C st. 9 Further results In this section we summarize related results on the density of leaves, isoperiodic forms, holomorphic motions and iterated rational maps. I. Density of leaves. By [Mc7], the closure of the complex geodesic f : H M 2 generated by a holomorphic 1-form is either an algebraic curve, a Hilbert modular surface or the whole moduli space. Since the leaves of F D are examples of such complex geodesics, we obtain: 30

33 Theorem 9.1 Every leaf of F D is either a closed algebraic curve, or a dense subset of X D. It is easy to see that the union of the closed leaves is dense when D = d 2. On the other hand, the classification of Teichmüller curves in [Mc5] and [Mc6] implies: Theorem 9.2 If D is not a square, then F D has only finitely many closed leaves. These consist of the components of W D X D (1) and, when D = 5, the Teichmüller curve generated by the regular decagon. II. Isoperiodic forms. Next we discuss interactions between the foliations F D and A D. When D = d 2 is a square, the surface X D is finitely covered by a product, and hence every leaf of A D is closed. Theorem 9.3 If D is not a square, then every leaf L of A D is dense in X D, and L F is dense in F for every leaf F of F D. Proof. The first result follows from the fact that SL(O D O D) is a dense subgroup of SL 2 (R), and the second follows from the first by transversality of A D and F D. Let us say a pair of 1-forms (Y i,ω i ) ΩM g are isoperiodic if there is a symplectic isomorphism such that the period maps φ : H 1 (Y 1, Z) H 1 (Y 2, Z) I(ω i ) : H 1 (Y i, Z) C satisfy I(ω 1 ) = I(ω 2 ) φ. Since the absolute periods of η 1 are constant along the leaves of A D, from the preceding result we obtain: Corollary 9.4 The SL 2 (R)-orbit of any eigenform for real multiplication by O D, D d 2, contains infinitely many isoperiodic forms. For a concrete example, let Q C be a regular octagon containing [0, 1] as an edge. Identifying opposite sides of Q, we obtain the octagonal form (Y,ω) = (Q,dz)/ 31

34 of genus two. Let Z[ζ] C denote the ring generated by ζ = (1+i)/ 2 = exp(2πi/8), equipped with the symplectic form Then it is easy to check that: z 1,z 2 = Tr Q(ζ) Q ((ζ + ζ2 + ζ 3 )z 1 z 2 /4). 1. The octagonal form ω has a single zero of order 2, and 2. Its period map I(ω) sends H 1 (Y, Z) to Z[ζ] by a symplectic isomorphism. However, these two properties do not determine (Y,ω) uniquely. Indeed, ω is an eigenform for real multiplication by O 8, so the preceding Corollary ensures there are infinitely many isoperiodic forms (Y i,ω i ) in its SL 2 (R) orbit. In other words we have: Corollary 9.5 There are infinite many fake octagonal forms in ΩM 2. Note that the forms (Y i,ω i ) cannot be distinguished by their relative periods either, since they all have double zeros. A similar statement can be formulated for the pentagonal form on the curve y 2 = x 5 1. III. Top-speed motions. Let F t : E H be a holomorphic motion of E H over (H,s). By the Schwarz lemma, we have df t (z)/dt 1 with respect to the hyperbolic metric on H. Let us say F t is a top-speed holomorphic motion if equality holds everywhere; equivalently, if t F t (z) is an isometry of H for every z E. A top-speed holomorphic motion is maximal if it cannot be extended to a top-speed motion of a larger set E E. Theorem 9.6 For any discriminant D 4, the map F t (U(s)) = U(t), U Λ D gives a maximal top-speed holomorphic motion of E = Λ D s over (H,s). Proof. Let t f(t) = F t (z) be an extension of the motion to a point z E. Then the graph of f is a leaf of F D, since it is disjoint from X D (1). But the only leaves that are graphs of Möbius transformations are those in X D (1), by Corollary