# Chern numbers and Hilbert Modular Varieties

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1 Chern numbers and Hilbert Modular Varieties Dylan Attwell-Duval Department of Mathematics and Statistics McGill University Montreal, Quebec April 9, 2011

2 A Topological Point of View Given a topological group G, F G : Top Set via F G (X ) = {Isomorphism classes of principal G-bundles on X } F G (f ) = f (The map sending a bundle to its pullback induced by f ). We define a characteristic class of principal G-bundles as any natural transformation of F G with a cohomology functor. Examples (Fibers, G, Cohomology functor) Whitney Classes w m (R n, GL n (R), H m (, Z/2Z), m n) Euler Class e (R n, GL n (R) +, H n (, Z)) Chern Classes c m (C n, GL n (C), H 2m (, Z), m n) Pontrjagin Classes p m (R n, GL n (R), H 4m (, Z), m n/2) D. Duval (McGill) Chern Numbers April / 15

3 Chern Classes Definition (ζ := E π B an arbitrary C n -bundle) Chern classes c m can be defined inductively by c n (ζ) = e(ζ R ) and c m (ζ) = π0 1 c i (ζ 0 ), where ζ 0 is a canonically defined bundle whose base space is E 0 (E with the trivial section removed) and whose fiber over (x, v) looks like C n /(C v). Alternative Definition (ζ smooth, B paracompact) Any such bundle ζ has a connection and hence a curvature tensor K C ( 2 τ C Hom(ζ, ζ)). K can be described locally as an n n matrix of 2-forms. It is a theorem that the polynomial det(i + tk ) is independent of and abusing notation we have det(i + tk ) = n (2πi) r c r (ζ)t r. r=0 D. Duval (McGill) Chern Numbers April / 15

4 An Algebraic Approach Chern classes have an alternative characterization in algebraic geometry. Recall the Chow ring A(X ) of a non-singular quasi-projective variety X of dimension n over an algebraically closed field K. Given a locally free sheaf E of rank r on X, let P(E) be the associated projective bundle over X with projection π. Then the line bundle O P(E) (1) produces ζ A 1 (P(E)) and A(P(E)) becomes a free graded A(X )-module generated by {ζ i } r 1 i=0. For i = 1, 2,..., r, define the i th Chern class c i (E) via the relation r ( 1) i π c i (E).ζ r i = ζ r. i=1 When X is a nonsingular projective algebraic variety over C, these two definitions agree via Poincaré duality and Chow s theorem identifying the Chow ring of X with its nonsingular cohomology ring. D. Duval (McGill) Chern Numbers April / 15

5 Properties of Chern Classes Chern polynomial c t (E) := c t (E F) = c t (E) c t (F) r c i (E)t i i=0 c t (ɛ) = 1, where ɛ is the trivial bundle of any dimension c t (Ě) = c t (E), where Ě is the dual bundle of E. If D is a divisor, then c t (L(D)) = 1 + td The splitting principle Examples (c t (X ) is the Chern polynomial for the tangent bundle over X ) c t (A n K ) = 1 c t (P n K ) = (1 + Ht)n+1, where H is the divisor of a hyperplane If X P n is a complete intersection of hypersurfaces of degrees n 1,..., n k then c t (X ) = (1 + Ht) n+1 (1 + n i Ht) 1 A(X ) D. Duval (McGill) Chern Numbers April / 15

6 Why do we care? Why Chern Classes? For paracompact base spaces, every characteristic class defined for C n -bundles with coefficient group Z is a unique polynomial in the Chern classes Chern classes provide obstructions to embeddings of manifolds / smooth varieties (Ex. No abelian surface P 3 C ) Self intersection formula Chern numbers of compact complex manifolds: For such an M of dimension n, fix a bundle E and consider n-cycles (n-forms) arising from intersections (cup products) of the c i (E). Taking degree maps (integrating over M) gives rise to these important invariants. D. Duval (McGill) Chern Numbers April / 15

7 Chern numbers Information contained in Chern numbers Arithmetic genus p a (X ) Geometric genus p g (X ) Hirzebruch-Riemann-Roch formula for holomorphic Euler characteristic χ(e) = deg(ch(e).td(t )) n Intersection numbers of divisors (Ex. c1 n[x ] = ( 1)n deg( n 1 K), for K the canonical divisor of complex n-manifold X ) Various cohomology group dimensions (Ex. c n [X ] = e = ( 1) i rank H i (X, Z), X as above) The Chern numbers of a complex projective manifold (ie. of its tangent sheaf) also uniquely determine its complex cobordism class. This is the focus of the works of Thomas and Vasquez. D. Duval (McGill) Chern Numbers April / 15

8 Hilbert Modular Varieties Fix totally real field F of degree n with Hilbert modular group G. Let Γ = G or some torsion-free arithmetic subgroup of finite index. Let ŶΓ be the minimal compactification of H n /Γ P 1 (C) n. By Hironaka there exists minimal resolutions Z Γ ŶΓ When n = 2, well known canonical construction of Z Γ due to Hirzebruch For a cusp of type (M, V ), decompose R 2 + into simplicial cones spanned by boundary points of M +. Glue together via V. Distinct copies correspond to distinct nonsingular rational curves lying over cusp in resolution. Similar for quotient singularities. H = isotropy subgroup of x, H acts on C 2 via complex rotations (type of H) which identifies neighbourhood of 0 in H\C 2 with neighbourhood of x. Again reduce to finding bases of a lattice M as vertices of simplicial cones. Each cone gives a copy of P 1 lying over 0. D. Duval (McGill) Chern Numbers April / 15

9 Classification of Hilbert Modular Surfaces An important problem is to classify the Z Γ for all Γ. For n = 2, a lot of progress has been made on this problem. Explicit descriptions for certain fields / subgroups exist [6], [7]: F = Q( 13), Γ = principal congruence subgroup of 2, Z Γ is isomorphic to a blow up of the minimal resolution of 4 4 (x 0 :... : x 4 ) P 4 (C) x i = 0, xi 5 = xi 2 12 i=0 F = Q( 6), Γ = principal congruence subgroup of unique prime lying over 2, Z Γ is isomorphic to the minimal resolution of singularities of a certain double covering of P 2 (C). i=0 i=0 j=0 x 3 j D. Duval (McGill) Chern Numbers April / 15

10 One major result for F a real quad. field of discriminant D, Γ = SL 2 (O F ) is the complete understanding of how the corresponding Hilbert modular surface (Z Γ (D)) fits into the Enriques-Kodaira rough classification of surfaces, which classifies a surface X in terms of its Hodge numbers h i,j = dimh j (X, Ω i ). Such a surface is regular and simply connected, hence is either rational, or admits a minimal model that is a K-3, honestly elliptic, or of general type One can use the Chern numbers of Z Γ (D) to determine the classification. For example, if D is a prime congruent with 1 mod 4, the arithmetic genus completely determines the surface type: P a = c2 1 + c 2 12 [Z Γ (D)] 1 = 1 Z Γ (D) is rational = 2 Z Γ (D) is a blown up K-3 = 3 Z Γ (D) is a blown up elliptic 4 Z Γ (D) is of general type. D. Duval (McGill) Chern Numbers April / 15

11 More careful analysis shows that the surface type of general Z Γ (D) can be determined by the Chern numbers c1 2[Z Γ(D)] = K.K and c 2 [Z Γ (D)] = e along with numerical estimates and the intersection numbers of various curves (which are determined by the Chern classes of said curves). This is convenient, as the Chern numbers of these surfaces are readily calculable: c 2 [Z Γ (D)] = 2ζ F ( 1) a 2(Z Γ (D)) a 3(Z Γ (D)) for D 13 c1 2[Z Γ(D)] = 4ζ F ( 1) l 0 a+ 3 (Z Γ(D)) 3 for D 13, where a i is the number of quotient singularities of type (i, 1, ), a 3 + the number of quotient singularities of type (3, 1, 1), l0 the number of curves arising from minimal resolution of cusps of H H Final Result of the Classification of Z Γ (D) Rational for D {5, 8, 12, 13, 17} Blown-up K-3 for D {21, 24, 28, 29, 33, 37, 40, 41} Blown-up honestly elliptic for D {44, 53, 57, 61, 65, 73, 85} General type otherwise. D. Duval (McGill) Chern Numbers April / 15

12 3 Dimensional Hilbert Modular Varieties When n > 2, much less is know about the structure of the Z Γ. Ehlers [1] generalizes Hirzebruch method for resolving singularities however for a cusp singularity (M, V ), there is no canonical decomposition of (M R) + into appropriate simplicial cones. Thomas and Vasquez [2] have explicitly constructed resolutions for a certain family of cubic fields using this method. The surfaces lying over the cusps are usually fairly simple (ex. projective planes, ruled surfaces, Hirzebruch surface, etc). In [3], [4], Thomas and Vasquez classify the varieties Z Γ up to complex cobordism, which by Milnor amounts to calculating the Chern numbers c 3 1 [Z Γ ], c 1 c 2 [Z Γ ], c 3 [Z Γ ]. (Note that these numbers are for a fixed resolution Z Γ.) D. Duval (McGill) Chern Numbers April / 15

13 Calculating the Chern numbers Each of the Chern numbers for such a Z Γ reflects an interesting geometric invariant: c 3 1 [Z Γ] = K 3, c 1 c 2 [Z Γ ] = 24p a, c 3 [Z Γ ] = e. The arithmetic genus p a is actually a birational invariant and hence independent of resolution. It s calculation follows from the following theorem of Hirzebruch and Vigneras: Theorem Let F be a totally real number field of degree n which contains a unit of norm 1, and let Γ be a subgroup of G of modular type. Then p a (Y Γ ) = 1 [G 2 n : Γ] ζ F ( 1) + a r (Γ) r 1 r r 2 D. Duval (McGill) Chern Numbers April / 15

14 The values of the other two invariants are more delicate, and depend explicitly on the resolution chosen. Let ξ = {E 1,..., E h } denote a set of non-singular varieties resolving each cusp x i of Y Γ. Set K 3 (ξ) = h 1 K 3 [E i ] and e(ξ) = h 1 e[e i]. The following theorem is the main result of [4]. Theorem (Thomas and Vasquez) Let Γ be a subgroup of modular type for a totally real cubic number field F. Let Z Γ be a non-singular Hilbert modular variety for Γ, with cusp resolutions ξ as above. Then K 3 [Z Γ ] = a 2 /2 + a 3 /2 + 6a 7 /7 + 7a 9 /6 + K 3 (ξ) 12dζ F ( 1), e[z Γ ] = 5a 2 /2 + 25a 3 / a 7 / a 9 /18 + e(ξ) + 2dζ F ( 1). Here a r = a r (Γ), and d = [G : Γ]. D. Duval (McGill) Chern Numbers April / 15

15 References 1 F. Ehlers, Eine Klasse komplexer Mannigfaltigkeiten... Math. Ann. 218 (1975), E. Thomas, A. Vasquez, On the resolution of cusp singularities... Math. Ann. 247 (1980), E. Thomas, A. Vasquez, Chern numbers of cusp resolutions... J. Reine Angew. Math. 324 (1981), E.Thomas, A. Vasquez, Chern numbers of Hilbert modular varieties. J. Reine Angew. Math. 324 (1981), G. van der Geer, Hilbert modular forms for the field Q( 6), Math. Ann. 233 (1978), G. van der Geer, D. Zagier, The Hilbert modular group for the field Q( 13), Invent. Math. 42 (1977) G. van der Geer. Hilbert Modular Surfaces. Springer-Verlag, D. Duval (McGill) Chern Numbers April / 15

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