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In mathematics, the Fulton–Hansen connectedness theorem is a result from intersection theory in algebraic geometry, for the case of subvarieties of projective space with codimension large enough to make the intersection have components of dimension at least 1. It is named after William Fulton and Johan Hansen, who proved it in 1979.
One says that “the affine plane does not have a good intersection theory”, and intersection theory on non-projective varieties is much more difficult. A line on a P 1 × P 1 (which can also be interpreted as the non-singular quadric Q in P 3) has self-intersection 0, since a line can be moved off itself. (It is a ruled surface.)
In 1996 he received the Steele Prize for mathematical exposition for his text Intersection Theory. [1] Fulton is a member of the United States National Academy of Sciences since 1997; a fellow of the American Academy of Arts and Sciences from 1998, and was elected a foreign member of the Royal Swedish Academy of Sciences in 2000. [3]
That is, a scheme-theoretic multiplicity of an intersection may differ from an intersection-theoretic multiplicity, the latter given by Serre's Tor formula. Solving this disparity is one of the starting points for derived algebraic geometry, which aims to introduce the notion of derived intersection.
Fulton and MacPherson extended the Chow ring to singular varieties by defining the "operational Chow ring" and more generally a bivariant theory associated to any morphism of schemes. [13] A bivariant theory is a pair of covariant and contravariant functors that assign to a map a group and a ring respectively.
Let X be a Riemann surface.Then the intersection number of two closed curves on X has a simple definition in terms of an integral. For every closed curve c on X (i.e., smooth function :), we can associate a differential form of compact support, the Poincaré dual of c, with the property that integrals along c can be calculated by integrals over X:
For example, the expected dimension of intersection of and is , the intersection of and has expected dimension , and so on. The definition of a Schubert variety states that the first value of j {\displaystyle j} with dim ( V j ∩ w ) ≥ i {\displaystyle \dim(V_{j}\cap w)\geq i} is generically smaller than the expected value n − k + i ...
In algebraic geometry, the flag bundle of a flag [1]: = of vector bundles on an algebraic scheme X is the algebraic scheme over X: : such that () is a flag of vector spaces such that is a vector subspace of () of dimension i.