Local Picard groups and theta series
Jan Hendrik Bruinier December 14, 2001
Mathematisches Institut, Universit¨at Heidelberg, Im Neuenheimer Feld 288, D-69120 Heidelberg, Germany
E-mail: [email protected]
1 Introduction
The purpose of the present note is to present some of the results on local Picard groups and Borcherds products obtained in joint work with E. Freitag [BF].
Let (V, q) be a real quadratic space of signature (2, l) with l > 2 and L ⊂ V an even lattice. To simplify the exposition we assume throughout that L is unimodular. For the general case we refer the reader to [BF].
Let O′(V) be the spinor kernel of the orthogonal group ofV andK a maximal compact subgroup. Then the quotientHl= O′(V)/K is a Hermitean symmetric domain of complex dimension l. The orthogonal group O(L) of L is an arithmetic subgroup of O(V). The group Γ = O(L)∩O′(V) acts properly discontinuously on Hl, and the quotient Hl/Γ is a non-compact normal complex space. Let XL be the Baily-Borel compactification ofHl/Γ.
It is obtained by adjoining finitely many boundary curves toHl/Γ. According to the theory of Baily-Borel, XL has a structure as a projective algebraic variety over C.
The Picard group Pic(XL) is an important invariant of XL. It can be studied by looking at divisors on XL. A particular class of divisors, called Heegner divisors, arises from embedded quotients of smaller dimension l−1 analogous to XL.
Using Borcherds’ lifting from nearly-holomorphic modular forms for SL2(Z) to mero- morphic modular forms for Γ, one obtains explicit relations among these Heegner divisors, and thereby some information about their position in Pic(XL) [Bo1, Bo2].
The local Picard group Pic(XL, s) ofXLat a points∈XL is defined as the direct limit Pic(XΓ, s) = lim−→Pic(Ureg), (1.1) where U runs through all open neighborhoods of s, and Ureg denotes the regular locus of U. The group Pic(XL, s) is zero, if s is a nonsingular point, and a torsion group, if s is an elliptic fixed point. It is much more complicated, if s is a boundary point of XL. We
s does not belong to the zero dimensional boundary components of XL.
By means of certain local Borcherds products the positions of the images of Heegner divisors in Pic(XL, s) can be precisely determined up to torsion. They can be described in terms of the Fourier coefficients of certain theta series attached to the 1 dimensional boundary component containing s.
In contrast to the general study of Pic(XL, s), in our main result we exploit the uni- modularity ofLin a vital way. By means of a result of Waldspurger, it can be proved that a linear combination of Heegner divisors is torsion in the Picard group ofXL, if and only if it is torsion in Pic(XL, s) for every one-dimensional boundary component B and a generic point s∈B. As a consequence we find a converse theorem for the Borcherds lifting [Bo1]:
Any meromorphic modular form for the group Γ, whose divisor is a linear combination of Heegner divisors, must be a Borcherds product. This was also proved in greater generality in [Br1, Br2]. However, in these papers a completely different argument is used, which does not say anything about the local Picard groups of XL.
2 Heegner divisors and modular forms
We stick to the notation introduced in the introduction.
Letmbe a negative integer. Ifλ ∈Lwithq(λ) = m, then (V ∩λ⊥, q) is a real quadratic space of signature (2, l−1). Let O′(V)λ and Kλ be the stabilizers of λ in O′(V) and K, respectively. Then Hλ = O′(V)λ/Kλ is a Hermitean symmetric space isomorphic to Hl−1
and the natural embedding
O′(V)λ/Kλ ,→O′(V)/K
identifies Hλ with an analytic divisor of Hl. The Heegner divisor of discriminant m is defined by
H(m) = ∑
λ∈L q(λ)=m
Hλ.
It is a Γ-invariant divisor on Hl, which is the inverse image of an algebraic divisor on XL (also denoted byH(m)). These Heegner divisors can be viewed as higher dimensional generalizations of Heegner points on modular curves and Hirzebruch-Zagier divisors on Hilbert modular surfaces [Bo2].
With the theory of Borcherds products it is possible to construct explicit relations among Heegner divisors (cf. [Bo1] Theorem 13.3). Since the existence of Borcherds products is controlled by the space Sκ of elliptic cusp forms of weight κ = 1 +l/2 for the group SL2(Z), the position of Heegner divisors in Pic(XL) can be described in terms of such cusp forms (cf. [Bo2] Theorem 3.1). Let Pic(Xf L) be the quotient of Pic(XL) modulo the subgroup generated by the canonical bundle.
Theorem 2.1 (Borcherds). An integral linear combination∑
m<0c(m)H(m)of Heegner divisors is a torsion element of Pic(Xf L), if ∑
m<0c(m)a(−m) = 0 for every cusp form f ∈Sκ with Fourier coefficients a(n). In other words, the generating series
G(τ) =∑
n>0
H(−n)e(nτ)
is a cusp form of weight κ for SL2(Z) with values in the (finite dimensional) subspace P of Pic(Xf L)⊗Z C generated by the Heegner divisors. Here τ denotes the variable in the complex upper half plane H={z ∈C; ℑ(z)>0} and e(τ) = e2πiτ as usual.
This is a sufficient condition for a linear combination of Heegner divisors to being torsion inPic(Xf L). Similar results, but without precise information on the level ofG, were obtained earlier in a completely different way by Kudla-Millson.
3 Local Heegner divisors and theta series
LetB be a 1-dimensional boundary component of XL ands∈B a generic point. We want to determine the position of the image of H(m) in the local Picard group Pic(XL, s) at s.
The 1-dimensional boundary components of XL are parametrized by Γ-classes of 2- dimensional isotropic subspaces of L⊗ZQ (see [BF]). Let F be a 2-dimensional isotropic subspace of L⊗ZQ corresponding to B. It can be shown that the image of H(m) in the local Picard group Pic(XL, s) is given by the local Heegner divisor
HF(m) = ∑
λ∈L∩F⊥ q(λ)=m
Hλ ⊂ Hl.
This divisor is invariant under the stabilizer Γs of s and thereby defines an element of Pic(XL, s).
Let ˜F ⊂L⊗ZQbe a complementary isotropic subspace forF. ThenD=L∩F⊥∩F˜⊥is a negative definite even unimodular lattice of rankl−2. We consider theta series associated with D.
We write (·,·) for the bilinear form corresponding toq(·) such that q(x) = 12(x, x). The homogeneous polynomial
Q(u, v) = (u, v)2− (u, u)(v, v)
l−2 , u, v ∈D⊗ZR, is harmonic in u and v. Hence standard results on theta series imply that
ΘD(τ, v) = ∑
λ∈D
Q(λ, v)e(−q(λ)τ), τ ∈H,
belongs to Sκ for every v ∈ D⊗ZR. The minus sign in the exponential term comes from the fact thatD is negative definite. We denote bySκD the subspace ofSκ generated by the theta series ΘD(τ, v) withv ∈D. Proposition 5.1 of [BF], specialized to our situation that L is unimodular, says:
Theorem 3.1. An integral linear combination m<0c(m)HF(m)of local Heegner divisors is a torsion element of Pic(XL, s), if and only if ∑
m<0c(m)a(−m) = 0 for every f ∈SκD with Fourier coefficients a(n).
In the proof one uses local analogues of Borcherds products or more precisely of the generalized Borcherds products introduced in [Br1]. We briefly indicate how these local Borcherds products are defined, referring to [BF] for more details. We introduce adapted coordinates Z = (z1, z2,z) with respect to F for Z ∈ Hl as in [BF], where z ∈ D⊗Z C, z1, z2 ∈ C, and q(ℑ(Z)) > 0, ℑ(z2) > 0. If e1, e3 ∈ L denote primitive isotropic vectors such thatL∩F =Ze1+Ze3, then the local Heegner divisor HF(m) on Hl can be written as
HF(m) = ∑
λ∈D q(λ)=m
∑
ν1,ν3∈Z
Hλ+ν1e1+ν3e3.
Notice that the first sum over λ is finite. The local Borcherds product associated with HF(m) is defined by
ΨF,m(Z) = ∏
λ∈D q(λ)=m
∏
n∈Z
[1−e(
σn(nz2+ (λ,z)))]
for Z = (z1, z2,z)∈ Hl. Here
σn = {
+1, if n ≥0,
−1, if n <0,
is a sign, which is needed in order that the product converges. It is easily checked that the divisor of ΨF,m(Z) is preciselyHF(m). Consequently the image ofHF(m) in Pic(XL, s) is determined by the automorphy factor
JF,m(g, Z) = ΨF,m(gZ)/
ΨF,m(Z) (g ∈Γs).
It can be computed explicitly in terms of the above polynomials Q(u, v). Thereby the Chern class of HF(m) in H2(Γs,Z) can be described. Combining this with results on the local cohomology due to Ballweg [Ba], the theorem is derived.
The fact that the condition in Theorem 3.1 is both, necessary and sufficient, may be used to infer:
Corollary 3.2. The condition in Theorem 2.1 is also necessary. In other words, the nat- ural map P∗ →Sκ given by ℓ 7→∑
n>0ℓ(H(−n))e(nτ) is surjective. Here P∗ denotes the dual vector space of P.
Proof. Let H = ∑
m<0c(m)H(m) be an integral linear combination of global Heegner divisors which is a torsion element of Pic(Xf L). This means that H is the divisor of a meromorphic modular form for Γ with some multiplier system of finite order. Then the
local Heegner divisors ∑
m<0c(m)HF(m) are clearly torsion elements of Pic(XL, s) for all 1-dimensional boundary componentsB and a generic points ∈B. In view of Theorem 3.1 this implies that
∑
m<0
c(m)a(−m) = 0 (3.1)
for everyf =∑
n>0a(n)e(nτ)∈SκD and every negative definite even unimodular sublattice D⊂L of rankl−2.
According to a theorem of Waldspurger, the sum ∑
DSκD, where D runs through all negative definite even unimodular lattices of rank l−2, is equal to Sκ (see [Wal] and also [EZ] Theorem 7.4). But since every negative definite even unimodular lattice of rank l−2 can be realized as a sublattice of L, we find that (3.1) actually holds for all cusp forms f ∈Sκ with Fourier coefficients a(n).
Corollary 3.3. Any meromorphic modular form for the group Γ, whose divisor is a linear combination of Heegner divisors, must be a Borcherds product in the sense of [Bo1] Theorem 13.3.
Proof. This is an immediate consequence of Corollary 3.2 and [Bo2] Therem 3.1.
References
[Ba] L. Ballweg, Die lokalen Cohomologiegruppen der Baily-Borel-Kompaktifizierung in generischen Randpunkten, Dissertation, University of Heidelberg (1992).
[Bo1] R. E. Borcherds, Automorphic forms with singularities on Grassmannians, Invent.
Math. 132(1998), 491–562.
[Bo2] R. E. Borcherds, The Gross-Kohnen-Zagier theorem in higher dimensions, Duke Math.
J.97 (1999), 219–233.
[Br1] J. H. Bruinier, Borcherds products on O(2, l) and Chern classes of Heegner divisors, preprint (May 2000), to appear in the Springer Lecture Notes in Mathematics series.
[Br2] J. H. Bruinier, Borcherds products and Chern classes of Hirzebruch-Zagier divisors, Invent. math.138 (1999), 51–83.
[BF] J. H. Bruinier and E. Freitag, Local Borcherds Products, Ann. Inst. Fourier, Grenoble 51(2001), 1–27.
[EZ] M. Eichler and D. Zagier, The Theory of Jacobi Forms, Progress in Math.55 (1985), Birkh¨auser.
[Wal] J.-L. Waldspurger, Engendrement par des s´eries thˆeta de certains espaces de formes modulaires, Invent. math.50(1979), 135–168.
