Hodge conjecture

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The Hodge conjecture is a major unsolved problem in algebraic geometry which relates the algebraic topology of a non-singular complex algebraic variety and the subvarieties of that variety. More specifically, the conjecture says that certain de Rham cohomology classes are algebraic, that is, they are sums of Poincaré duals of the cohomology classes of subvarieties. The Hodge conjecture is one of the Clay Mathematics Institute's Millenium Prize Problems, so there is a USD $1,000,000 prize for proving the Hodge conjecture.

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[edit] Motivation

Let X be a connected complex manifold of complex dimension n. X is then an orientable smooth manifold of dimension 2n, so its cohomology groups lie in degrees zero through 2n. Assume that X is a Kähler manifold, so that there is a decomposition on its cohomology with complex coefficients:

H^k(X, \mathbf{C}) = \oplus_{p+q=k} H^{p,q}(X),

where Hp,q(X) is the subgroup of cohomology classes which are represented by harmonic forms of type (p, q). That is, these are the cohomology classes represented by differential forms which, in some choice of local coordinates z_1, \ldots, z_n, can be written as a harmonic function times dz_{i_1} \wedge \cdots \wedge dz_{i_p} \wedge d\bar z_{j_1} \wedge \cdots \wedge d\bar z_{j_q}. (See Hodge theory for more details.) Taking wedge products of these harmonic representatives corresponds to the cup product in cohomology, so the cup product is compatible with the Hodge decomposition:

\cup : H^{p,q}(X) \times H^{p',q'}(X) \rightarrow H^{p+p',q+q'}(X).

Since X is a complex manifold, X has a fundamental class.

Let Z be a complex submanifold of X of dimension k, and let i : Z → X be the inclusion map. Choose a differential form α of type (p, q). We can integrate α over Z:

∫ i * α.
Z

To evaluate this integral, choose a point Z and call it 0. Around 0, we can choose local coordinates z_1,\ldots,z_n on X such that Z is just z_{k+1} = \cdots = z_n = 0. If p > k, then α must contain some dzi where zi pulls back to zero on Z. The same is true if q > k. Consequently, this integral is zero if (p, q) ≠ (k, k).

More abstractly, the integral can be written as the cap product of the homology class of Z and the cohomology class represented by α. By Poincaré duality, the homology class of Z is dual to a cohomology class which we will call [Z], and the cap product can be computed by taking the cup product of [Z] and α and capping with the fundamental class of X. Because [Z] is a cohomology class, it has a Hodge decomposition. By the computation we did above, if we cup this class with any class of type (p, q) ≠ (k, k), then we get zero. Because H^{2n}(X, \mathbf{C}) = H^{n,n}(X), we conclude that [Z] must lie in H^{k,k}(X, \mathbf{C}). Loosely speaking, the Hodge conjecture asks:

Which cohomology classes in Hk,k(X) come from complex subvarieties Z?

[edit] Statement of the Hodge conjecture

Let:

\operatorname{Hdg}^k(X) = H^{2k}(X, \mathbf{Q}) \cap H^{k,k}(X).

We call this the group of Hodge classes of degree 2k on X.

The modern statement of the Hodge conjecture is:

Hodge conjecture. Let X be a projective complex manifold. Then every Hodge class on X is a linear combination with rational coefficients of the cohomology classes of complex subvarieties of X.

A projective complex manifold is a complex manifold which can be embedded in complex projective space. Because projective space carries a Kähler metric, the Fubini-Study metric, such a manifold is always a Kähler manifold. By Chow's theorem, a projective complex manifold is also a smooth projective algebraic variety, that is, it is the zero set of a collection of homogenous polynomials.

[edit] Reformulation in terms of algebraic cycles

Another way of phrasing the Hodge conjecture involves the idea of an algebraic cycle. An algebraic cycle on X is a formal combination of subvarieties of X, that is, it is something of the form:

∑ ciZi.
i

The coefficients are usually taken to be integral or rational. We define the cohomology class of an algebraic cycle to be the sum of the cohomology classes of its components. For example, the cohomology class of the above cycle would be:

∑ ci[Zi].
i

Such a cohomology class is called algebraic. With this notation, the Hodge conjecture becomes:

Let X be a projective complex manifold. Then every Hodge class on X is algebraic.

[edit] Known cases of the Hodge conjecture

[edit] Low dimension and codimension

The first result on the Hodge conjecture is due to Solomon Lefschetz. In fact, it predates the conjecture and provided some of Hodge's motivation.

Theorem (Lefschetz theorem on (1,1)-classes) Any element of H^{2}(X, \mathbf{Z}) \cap H^{1,1}(X) is the cohomology class of a divisor on X. In particular, the Hodge conjecture is true for H2.

A very quick proof can be given using sheaf cohomology and the exponential exact sequence. (The cohomology class of a divisor turns out to equal to its first Chern class.) Lefschetz's original proof proceeded by normal functions, which were introduced by Henri Poincaré. However, Griffiths's transversality theorem shows that this approach cannot prove the Hodge conjecture for higher codimensional subvarieties.

By the Hard Lefschetz theorem, one can prove:

Theorem. If the Hodge conjecture holds for Hodge classes of degree p, p < n, then the Hodge conjecture holds for Hodge classes of degree 2n-p.

Combining the above two theorems implies that Hodge conjecture is true for Hodge classes of degree 2n-2. This proves the Hodge conjecture when X has dimension at most three.

The Lefschetz theorem on (1,1)-classes also implies that if all Hodge classes are generated by the Hodge classes of divisors, then the Hodge conjecture is true:

Corollary. If the algebra \operatorname{Hdg}^*(X) = \sum_k \operatorname{Hdg}^k(X) is generated by \operatorname{Hdg}^1(X), then the Hodge conjecture holds for X.

[edit] Abelian varieties

For most abelian varieties, the algebra \operatorname{Hdg}^*(X) is generated in degree one, so the Hodge conjecture holds. In particular, the Hodge conjecture holds for general abelian varieties, for products of elliptic curves, and for simple abelian varieties. However, Mumford constructed an example of an abelian variety where \operatorname{Hdg}^2(X) is not generated by products of divisor classes. André Weil generalized this example by showing that whenever the variety has complex multiplication by an imaginary quadratic field, then \operatorname{Hdg}^2(X) is not generated by products of divisor classes. Moonen and Zahren proved that in dimension less than 5, either \operatorname{Hdg}^*(X) is generated in degree one, or the variety has complex multiplication by an imaginary quadratic field. In the latter case, the Hodge conjecture is only known in special cases.

[edit] Generalizations

[edit] The integral Hodge conjecture

Hodge's original conjecture was:

Integral Hodge conjecture. Let X be a projective complex manifold. Then every cohomology class in H^{2k}(X, \mathbf{Z}) \cap H^{k,k}(X) is the cohomology class of an algebraic cycle with integral coefficients on X.

This is now known to be false. The first counterexample was constructed by Atiyah and Hirzebruch. Using K-theory, they constructed an example of a torsion Hodge class, that is, a Hodge class α such that for some positive integer n, nα = 0. Such a cohomology class cannot be the class of a cycle. Totaro reinterpreted their result in the framework of cobordism and found many examples of torsion classes.

The simplest adjustment of the integral Hodge conjecture is:

Integral Hodge conjecture modulo torsion. Let X be a projective complex manifold. Then every non-torsion cohomology class in H^{2k}(X, \mathbf{Z}) \cap H^{k,k}(X) is the cohomology class of an algebraic cycle with integral coefficients on X.

This is also false. Kollár found an example of a Hodge class α which is not algebraic, but which has an integral multiple which is algebraic.

[edit] The Hodge conjecture for Kähler varieties

A natural generalization of the Hodge conjecture would ask:

Hodge conjecture for Kähler varieties, naive version. Let X be a complex Kähler manifold. Then every Hodge class on X is a linear combination with rational coefficients of the cohomology classes of complex subvarieties of X.

This is too optimistic, because there are not enough subvarieties to make this work. A possible substitute is to ask instead one of the two following questions:

Hodge conjecture for Kähler varieties, vector bundle version. Let X be a complex Kähler manifold. Then every Hodge class on X is a linear combination with rational coefficients of Chern classes of vector bundles on X.
Hodge conjecture for Kähler varieties, coherent sheaf version. Let X be a complex Kähler manifold. Then every Hodge class on X is a linear combination with rational coefficients of Chern classes of coherent sheaves on X.

Claire Voisin proved that the Chern classes of coherent sheaves give strictly more Hodge classes than the Chern classes of vector bundles and that the Chern classes of coherent sheaves are insufficient to generate all the Hodge classes. Consequently, the only known formulations of the Hodge conjecture for Kähler varieties are false.

[edit] The generalized Hodge conjecture

Hodge made an additional, stronger conjecture than the integral Hodge conjecture. Say that a cohomology class on X is of level c if it is the pushforward of a cohomology class on a c-codimensional subvariety of X. The cohomology classes of level at least c filter the cohomology of X, and it is easy to see that the cth step of the filtration N^cH^k(X, \mathbf{Z} satisfies

N^cH^k(X, \mathbf{Z}) \subseteq H^k(X, \mathbf{Z}) \cap (H^{k-c,c}(X) \oplus\cdots\oplus H^{c,k-c}(X)).

Hodge's original statement was:

Generalized Hodge conjecture, Hodge's version. N^cH^k(X, \mathbf{Z}) = H^k(X, \mathbf{Z}) \cap (H^{k-c,c}(X) \oplus\cdots\oplus H^{c,k-c}(X)).

Grothendieck observed that this cannot be true, even with rational coefficients, because the right-hand side is not always a Hodge structure. His corrected form of the Hodge conjecture is:

Generalized Hodge conjecture. N^cH^k(X, \mathbf{Q}) is the largest sub-Hodge structure of H^k(X, \mathbf{Z}) contained in H^{k-c,c}(X) \oplus\cdots\oplus H^{c,k-c}(X).

This version is open.

[edit] Algebraicity of Hodge loci

The strongest evidence in favor of the Hodge conjecture is the algebraicity result of Cattani, Deligne and Kaplan. Suppose that we vary the complex structure of X over a simply connected base. Then the topological cohomology of X does not change, but the Hodge decomposition does change. It is known that if the Hodge conjecture is true, then the locus of all points on the base where the cohomology of a fiber is a Hodge class is in fact an algebraic subset, that is, it is cut out by polynomial equations. Cattani, Deligne, and Kaplan proved that this is always true, without assuming the Hodge conjecture.

[edit] External links

[edit] References

  • Hodge, W. V. D. The topological invariants of algebraic varieties. Proceedings of the International Congress of Mathematicians, Cambridge, Mass., 1950, vol. 1, pp. 182--192.
  • Grothendieck, A. Hodge's general conjecture is false for trivial reasons. Topology 8 1969 299--303.
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