Hodge theory tells us that the cohomology of a compact Kähler manifold is a pure Hodge structure. What if we consider an algebraic variety (over ) in general? The question whether its cohomology should have a pure Hodge structure has negative answer. Consider a nodal elliptic curve .
It turns out that , hence it does not admit a complex structure and, in particular, is not a pure Hodge structure of weight 1. If we resolve the singularity, we get with exceptional divisor such that and we have a long exact sequence of relative homology groups
and the relative exact sequence in cohomology
, we can see
as the extension of
, which are, respectively, pure Hodge structures of weight 0 and 1. More precisely, we have an increasing filtration
such that the graded piece
is a pure Hodge structure of weight 1.
A mixed Hodge structure consists of a free abelian group together with an increasing filtration of
and a dicreasing filtration of
defines a pure Hodge structure of weight
on the graded piece
A morphism of mixed Hodge structures is a -linear map which is compatible with the two filtrations of filtered vector spaces.
Any morphism of mixed Hodge structures is strict, i.e. every element of which is in the image of comes from and similarly the same holds for the weight filtration.
In view of the (singular) case above, we would like to have a (canonical and functorial) result independent of resolution and compactification, which allows us to associate a mixed Hodge structure to any algebraic variety over . Indeed, we have the following.
Quasi-projective varieties have canonical mixed Hodge structure on their cohomology.
Moreover, we get that the filtrations are independent of the choice of resolution, and so, by Hironaka's theorem, the result is extended to any algebraic variety over .