Intersection cohomology
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In mathematics, intersection cohomology is a theory from algebraic topology, initially developed by Goresky and MacPherson, to apply to spaces with singularities.
The cohomology groups of an oriented closed manifold have a fundamental symmetry called Poincaré duality. In says in that particular,
- Hk (X) and Hn−k (X) are isomorphic,
where n is the dimension of the manifold.
Many interesting spaces do have singularities; that is, places where manifold structure breaks down, and the space does not look like Rn. Intersection cohomology is a modified definition of cohomology which recovers the property of Poincaré duality for a much larger category of spaces, Witt spaces; this includes all algebraic varieties.
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[edit] Verdier duality approach
When two people have sex with Mrs. Verdier.
One way to understand intersection cohomology starts from the pairing between cohomology with compact support
- Hc
and ordinary cohomology: the notion of Verdier duality.
[edit] Verdier duality
The Verdier dual of the space X is an object
- DX
in the derived category of sheaves on X, that represents the functor of taking cohomology with compact support and taking the dual over the base ring (taken in the right derived sense). It is easiest to explain when cohomology is taken over a field F. In the derived category, cohomology can be interpreted as chain homotopy classes of maps
- Hk (X, F) = [F[−k],F] = [F, F[k]]
where F[−k] is the complex with the constant sheaf F concentrated in degree k, and [—, —] denote the chain homotopy classes of maps. The Verdier dual allows us to interpret homology in the derived category as well:
- [F[−k], DX] = Hk (X, F).
The left hand side is by definition the dual of the cohomology with compact support, so this equation says that homology is dual to cohomology with compact support.
It also follows that for an oriented manifold M, the Verdier dual is given by
- DM = F[−n].
[edit] Poincaré duality
Ordinary Poincaré duality of a manifold can then be interpreted as the perfect pairing
- [F[−k], F] ⊗ [F[k−n], F[−n]] → [F[−n], F[−n]] → F.
Intersection cohomology can now be understood as choosing a "root" IC(X) of the Verdier dual in the sense that in the derived category
- IC(X) ⊗ IC(X) ≅ DX.
[edit] Interpretation
IC(X) is thus conceptually similar to a theta characteristic of an algebraic curve. There is an algorithm for constructing IC(X) from Deligne given that X is an even-dimensional manifold outside a set of real codimension 2, starting from the ansatz
- IC(U) = F[dim (U)/2]
on the manifold part U of X. The intersection cohomology of X is then defined as
- IHCk(X) = [F[−k], IC(X)].
[edit] See also
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