# Cohomology jump loci of quasi-projective varieties

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1 Cohomology jump loci of quasi-projective varieties Botong Wang joint work with Nero Budur University of Notre Dame June

2 Motivation What topological spaces are homeomorphic (or homotopy equivalent) to a smooth complex projective variety (or a compact Kähler manifold)?

3 Motivation What topological spaces are homeomorphic (or homotopy equivalent) to a smooth complex projective variety (or a compact Kähler manifold)? According to classical Hodge theory, if X is a compact Kähler manifold of dimension n, then b 2k (X ) 0 for 1 k n 1 and b 2k 1 (X ) are even for 1 k n.

4 Motivation What topological spaces are homeomorphic (or homotopy equivalent) to a smooth complex projective variety (or a compact Kähler manifold)? According to classical Hodge theory, if X is a compact Kähler manifold of dimension n, then b 2k (X ) 0 for 1 k n 1 and b 2k 1 (X ) are even for 1 k n. In this talk, we will give some constraints of the homotopy type of smooth complex quasi-projective varieties.

5 Non-abelian Hodge theory and cohomology jump loci

6 Non-abelian Hodge theory and cohomology jump loci Our talk only concerns rank one non-abelian Hodge theory.

7 Non-abelian Hodge theory and cohomology jump loci Our talk only concerns rank one non-abelian Hodge theory. If X is connected, a rank one local system on X is equivalent to a C representation of π 1 (X ), i.e., an element in Hom(π 1 (X ), C ) = Hom(H 1 (X, Z), C ).

8 Non-abelian Hodge theory and cohomology jump loci Our talk only concerns rank one non-abelian Hodge theory. If X is connected, a rank one local system on X is equivalent to a C representation of π 1 (X ), i.e., an element in Hom(π 1 (X ), C ) = Hom(H 1 (X, Z), C ). We denote the space of rank one local system on X by M B (X ). M B (X ) is isomorphic to the direct product of (C ) b 1(X ) and a finite abelian group, and hence it is an abelian algebraic group.

9 Non-abelian Hodge theory and cohomology jump loci The non-abelian Hodge theory due to Simpson says the space of local systems is equal to the space of Higgs bundles. In the rank one case, it says when X is a compact Kähler manifold, M B (X ) by definition == Hom(π 1 (X ), C ) = Pic τ (X ) H 0 (X, Ω 1 X ). Here the isomorphism is an isomorphism of real Lie groups, not complex Lie groups.

10 Non-abelian Hodge theory and cohomology jump loci The non-abelian Hodge theory due to Simpson says the space of local systems is equal to the space of Higgs bundles. In the rank one case, it says when X is a compact Kähler manifold, M B (X ) by definition == Hom(π 1 (X ), C ) = Pic τ (X ) H 0 (X, Ω 1 X ). Here the isomorphism is an isomorphism of real Lie groups, not complex Lie groups. However, we will not use this at all in this talk. Instead, we need to introduce our main player, the cohomology jump loci in M B (X ), Σ i def k (X ) = {L M B (X ) dim H i (X, L) k}. They are closed subvarieties defined over Q.

11 The result of Green-Lazarsfeld, Arapura and Simpson The cohomology jump locus is a general notion defined for any connected topological space of the homotopy type of a finite CW-complex. It is a homotopy invariant. In general, it can be any subvariety of a torus.

12 The result of Green-Lazarsfeld, Arapura and Simpson The cohomology jump locus is a general notion defined for any connected topological space of the homotopy type of a finite CW-complex. It is a homotopy invariant. In general, it can be any subvariety of a torus. Theorem (Green-Lazarsfeld, Arapura) If X is a compact Kähler manifold, then Σ i k (X ) M B(X ) is finite union of translates of subtori, for any i, k N.

13 The result of Green-Lazarsfeld, Arapura and Simpson The cohomology jump locus is a general notion defined for any connected topological space of the homotopy type of a finite CW-complex. It is a homotopy invariant. In general, it can be any subvariety of a torus. Theorem (Green-Lazarsfeld, Arapura) If X is a compact Kähler manifold, then Σ i k (X ) M B(X ) is finite union of translates of subtori, for any i, k N. Theorem (Simpson) If X is a smooth complex projective variety, then Σ i k (X ) M B(X ) is finite union of torsion translates of subtori, for any i, k N.

14 The result of Green-Lazarsfeld, Arapura and Simpson The cohomology jump locus is a general notion defined for any connected topological space of the homotopy type of a finite CW-complex. It is a homotopy invariant. In general, it can be any subvariety of a torus. Theorem (Green-Lazarsfeld, Arapura) If X is a compact Kähler manifold, then Σ i k (X ) M B(X ) is finite union of translates of subtori, for any i, k N. Theorem (Simpson) If X is a smooth complex projective variety, then Σ i k (X ) M B(X ) is finite union of torsion translates of subtori, for any i, k N. These two theorems give nontrivial constraints on the possible homotopy type of compact Kähler manifolds and smooth complex projective varieties.

15 The result of Dimca-Papadima, Budur-W. Arapura hinted that at least the first theorem of Green-Lazarsfeld and Arapura should be true for smooth complex quasi-projective varieties.

16 The result of Dimca-Papadima, Budur-W. Arapura hinted that at least the first theorem of Green-Lazarsfeld and Arapura should be true for smooth complex quasi-projective varieties. Theorem (Dimca-Papadima) Suppose X is a smooth complex quasi-projective variety. Any irreducible component of Σ i k (X ) containing the trivial local system is a subtorus.

17 The result of Dimca-Papadima, Budur-W. Arapura hinted that at least the first theorem of Green-Lazarsfeld and Arapura should be true for smooth complex quasi-projective varieties. Theorem (Dimca-Papadima) Suppose X is a smooth complex quasi-projective variety. Any irreducible component of Σ i k (X ) containing the trivial local system is a subtorus. Theorem (Budur-W.) Suppose X is a smooth complex quasi-projective variety. Then Σ i k (X ) is a finite union of torsion translates of subtori.

18 Examples Example (A trivial example) Let X be a compact Riemann surface of genus g 1. Since H 1 (X, Z) = Z 2g, M B (X ) = (C ) 2g.

19 Examples Example (A trivial example) Let X be a compact Riemann surface of genus g 1. Since H 1 (X, Z) = Z 2g, M B (X ) = (C ) 2g. Denote by C X the trivial rank one local system on X. Then H 0 (X, C X ) = C, H 1 (X, C X ) = C 2g, H 2 (X, C X ) = C. Suppose L is a rank one local system on X which is not isomorphic to the trivial local system C X. Then H 0 (X, L) = 0, H 1 (X, L) = C 2g 2, H 2 (X, L) = 0.

20 Examples Example (A trivial example) Let X be a compact Riemann surface of genus g 1. Since H 1 (X, Z) = Z 2g, M B (X ) = (C ) 2g. Denote by C X the trivial rank one local system on X. Then H 0 (X, C X ) = C, H 1 (X, C X ) = C 2g, H 2 (X, C X ) = C. Suppose L is a rank one local system on X which is not isomorphic to the trivial local system C X. Then H 0 (X, L) = 0, H 1 (X, L) = C 2g 2, H 2 (X, L) = 0. Therefore, all the cohomology jump loci are either the whole space M B (X ), or the single point at the trivial local system {C X }, or empty.

21 Examples Example (nontrivial) Let σ = {σ ij } be an invertible n n matrix with entries in Z. Suppose none of the eigenvalues of σ is a root of unity. Define an action of Z on (C ) n C by l (η, λ) = (σ l η, λ + l), where l Z, λ C, η = {η k } (C ) n and σ l η = { j ησ ij j } 1 i n. Now, denote by X the quotient of (C ) n C by Z under the above action. Since C/Z = C, one can easily see that X is a (C ) n fiber bundle over C.

22 Examples Example (nontrivial) Let σ = {σ ij } be an invertible n n matrix with entries in Z. Suppose none of the eigenvalues of σ is a root of unity. Define an action of Z on (C ) n C by l (η, λ) = (σ l η, λ + l), where l Z, λ C, η = {η k } (C ) n and σ l η = { j ησ ij j } 1 i n. Now, denote by X the quotient of (C ) n C by Z under the above action. Since C/Z = C, one can easily see that X is a (C ) n fiber bundle over C. First, we compute π 1 (X ). Since X is a quotient of (C ) n C by Z, π 1 (X ) = Z n σ Z. The condition that none of the eigenvalues of σ is a root of unity implies that the normal subgroup Z n is (almost) equal to the commutator subgroup of π 1 (X ). Hence H 1 (X, Z) = Z, and M B (X ) = Hom(H 1 (X ), C ) = C.

23 Examples Example (continued) Next, we compute the cohomology jump loci Σ 1 1 (X ). For this example, the best way to compute the cohomology of a local system is to use group cohomology. Recall that a rank one local system L on X is equivalent to a C representation of π 1 (X ), which we denote by ρ L. ρ L : π 1 (X ) C puts a π 1 (X )-module structure on C by α t = ρ L (α)t, where α π 1 (X ) and t C. We write C ρl to emphasize the π 1 (X )-module structure on C.

24 Examples Example (continued) Next, we compute the cohomology jump loci Σ 1 1 (X ). For this example, the best way to compute the cohomology of a local system is to use group cohomology. Recall that a rank one local system L on X is equivalent to a C representation of π 1 (X ), which we denote by ρ L. ρ L : π 1 (X ) C puts a π 1 (X )-module structure on C by α t = ρ L (α)t, where α π 1 (X ) and t C. We write C ρl to emphasize the π 1 (X )-module structure on C. Since the universal cover of X is contractible, H i (X, L) = H i (π 1 (X ), C ρl ). Now, using group cohomology, it is not hard to see that Σ 1 1(X ) = {eigenvalues of σ} {1} C = M B (X ).

25 Examples Conclusion Since {eigenvalues of σ} consists of points that are not torsion, by the theorem of Budur-W., X is not homotopy equivalent to any quasi-projective variety.

26 Examples Conclusion Since {eigenvalues of σ} consists of points that are not torsion, by the theorem of Budur-W., X is not homotopy equivalent to any quasi-projective variety. Remark Even when the universal cover of a topological space is not contractible, the first cohomology of a local system can always be computed via group cohomology, and hence only depends on the fundamental group. Thus, both the theorems of Dimca-Papadima and Budur-W. put nontrivial constraints on the possible fundamental groups of a quasi-projective varieties. For example, Z n σ Z is not isomorphic to the fundamental group of any quasi-projective variety.

27 Proof of Theorem of Budur-W. The idea behind the results of Green-Lazarsfeld, Arapura and Dimca-Papadima can be summarized into one sentence: the obstruction of deforming elements in the cohomology group is linear.

28 Proof of Theorem of Budur-W. The idea behind the results of Green-Lazarsfeld, Arapura and Dimca-Papadima can be summarized into one sentence: the obstruction of deforming elements in the cohomology group is linear. However, the proof of the theorem of Budur-W. is arithmetic. We will first work with quasi-projective varieties that are defined over Q.

29 Proof of Theorem of Budur-W. The idea behind the results of Green-Lazarsfeld, Arapura and Dimca-Papadima can be summarized into one sentence: the obstruction of deforming elements in the cohomology group is linear. However, the proof of the theorem of Budur-W. is arithmetic. We will first work with quasi-projective varieties that are defined over Q. Let X be a smooth complex quasi-projective variety, and let X be a good compactification. In other words, X is a smooth projective variety and D = X X is a simple normal crossing divisor. Denote the number of irreducible components of D by n.

30 Proof of Theorem of Budur-W. The following diagram plays an essential role in our proof Z n Z n 0 M DR ( X ) M DR ( X /D) res C n RH RH 0 M B ( X ) M B (X ) ev exp (C ) n 0 0 0

31 Proof of Theorem of Budur-W. This is a diagram of complex algebraic group (possibly not of finite type). All complexes in the diagram are exact. All objects are defined over Q. Horizontal maps are maps defined over Q.

32 Proof of Theorem of Budur-W. This is a diagram of complex algebraic group (possibly not of finite type). All complexes in the diagram are exact. All objects are defined over Q. Horizontal maps are maps defined over Q. M DR ( X ) is the moduli space of rank one flat bundles on X. M DR ( X /D) is the moduli space of rank one flat bundles on X with possible log poles along D. res is the taking residue map. ev is taking monodromy around each D i. RH are the maps of Riemann-Hilbert correspondence. ex is the composition of multiplication by 2πi and then taking exponential.

33 Proof of Theorem of Budur-W. Σ i k (X ) M B(X ) is defined over Q. Using the hypercohomology of logarithmic de Rham complexes, one can define similarly cohomology jump loci Σ i k ( X /D) in M DR ( X /D). Away from some small bad locus, RH(Σ i k ( X /D)) = Σ i k (X ).

34 Proof of Theorem of Budur-W. Σ i k (X ) M B(X ) is defined over Q. Using the hypercohomology of logarithmic de Rham complexes, one can define similarly cohomology jump loci Σ i k ( X /D) in M DR ( X /D). Away from some small bad locus, RH(Σ i k ( X /D)) = Σ i k (X ). Proposition Suppose V M DR ( X /D) and W M B (X ) are closed irreducible subvarieties defined over Q. Moreover, suppose RH(V ) = W. Then W consists of a torsion point.

35 Proof of Theorem of Budur-W. Σ i k (X ) M B(X ) is defined over Q. Using the hypercohomology of logarithmic de Rham complexes, one can define similarly cohomology jump loci Σ i k ( X /D) in M DR ( X /D). Away from some small bad locus, RH(Σ i k ( X /D)) = Σ i k (X ). Proposition Suppose V M DR ( X /D) and W M B (X ) are closed irreducible subvarieties defined over Q. Moreover, suppose RH(V ) = W. Then W consists of a torsion point. The proposition essentially follows from the next two theorems.

36 Proof of Theorem of Budur-W. Theorem (Simpson) Suppose A Σ i k ( X ) and B M B ( X ) are closed irreducible subvarieties defined over Q. Moreover, suppose RH(A) = B. Then B consists of a torsion point.

37 Proof of Theorem of Budur-W. Theorem (Simpson) Suppose A Σ i k ( X ) and B M B ( X ) are closed irreducible subvarieties defined over Q. Moreover, suppose RH(A) = B. Then B consists of a torsion point. Theorem (Gelfond-Schneider) Suppose C C n and D (C ) n are both defined over Q, and suppose exp(c) = D. Then D consists of a torsion point.

38 Proof of Theorem of Budur-W. Now, it follows from the proposition that any irreducible component of Σ i k (X ) consists of a torsion point. By taking a finite cover of X, one can move the point to origin. Then the theorem of Budur-W. follows from the theorem of Dimca-Papadima. Thus we have proved that when X is defined over Q, Σ i k (X ) is finite union of torsion translates of subtori.

39 Proof of Theorem of Budur-W. Now, it follows from the proposition that any irreducible component of Σ i k (X ) consists of a torsion point. By taking a finite cover of X, one can move the point to origin. Then the theorem of Budur-W. follows from the theorem of Dimca-Papadima. Thus we have proved that when X is defined over Q, Σ i k (X ) is finite union of torsion translates of subtori. In general, one can prove that for any smooth complex quasi-projective variety X, there exists another smooth complex quasi-projective variety X defined over Q such that X is homeomorphic to X. Since our statement is topological, it is true for any smooth complex quasi-projective variety.

40 Some further questions Question If X is a compact Kähler manifold or a quasi-kähler manifold, does every irreducible component of Σ i k (X ) contain a torsion point?

41 Some further questions Question If X is a compact Kähler manifold or a quasi-kähler manifold, does every irreducible component of Σ i k (X ) contain a torsion point? Question Can one say anything about the dimensions of Σ i k (X ) when X is a quasi-projective variety?

42 Some further questions Question If X is a compact Kähler manifold or a quasi-kähler manifold, does every irreducible component of Σ i k (X ) contain a torsion point? Question Can one say anything about the dimensions of Σ i k (X ) when X is a quasi-projective variety? Question How about higher rank analog?

43 Some further questions When X is defined over Q, it seems plausible to replace torsion point by a Q-variation of Hodge structure whose Gauss-Manin connection is defined over Q.

44 Some further questions When X is defined over Q, it seems plausible to replace torsion point by a Q-variation of Hodge structure whose Gauss-Manin connection is defined over Q. Being more optimistic, Arapura made the following conjecture, which refines a conjecture of Simpson. Conjecture When X is a smooth projective variety, every nonempty Σ i k (X ) contains a motivic point (a point of geometric origin).

45 Mulţumesc!

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