Dual abelian variety
In mathematics, a dual abelian variety can be defined from an abelian variety A, defined over a field k. A 1-dimensional abelian variety is an elliptic curve, and every elliptic curve is isomorphic to its dual, but this fails for higher-dimensional abelian varieties, so the concept of dual becomes more interesting in higher dimensions.
Definition
Let A be an abelian variety over a field k. We define to be the subgroup consisting of line bundles L such that , where are the multiplication and projection maps respectively. An element of is called a degree 0 line bundle on A.[1]
To A one then associates a dual abelian variety Av (over the same field), which is the solution to the following
- for all , the restriction of L to A×{t} is a degree 0 line bundle,
- the restriction of L to {0}×T is a trivial line bundle (here 0 is the identity of A).
Then there is a variety Av and a line bundle , called the Poincaré bundle, which is a family of degree 0 line bundles parametrized by Av in the sense of the above definition.[2] Moreover, this family is universal, that is, to any family L parametrized by T is associated a unique morphism f: T → Av so that L is isomorphic to the pullback of P along the morphism 1A×f: A×T → A×Av. Applying this to the case when T is a point, we see that the points of Av correspond to line bundles of degree 0 on A, so there is a natural group operation on Av given by tensor product of line bundles, which makes it into an abelian variety.
In the language of representable functors one can state the above result as follows. The contravariant functor, which associates to each k-variety T the set of families of degree 0 line bundles parametrised by T and to each k-morphism f: T → T' the mapping induced by the pullback with f, is representable. The universal element representing this functor is the pair (Av, P).
This association is a duality in the sense that there is a
History
The theory was first put into a good form when K was the field of
For the case of the
- K(L)
of translations on L that take L into an isomorphic copy is itself finite. In that case, the quotient
- A/K(L)
is isomorphic to the dual abelian variety Â.
This construction of  extends to any field K of
- A × Â.
The construction when K has characteristic p uses
The Dual Isogeny
Let be an isogeny of abelian varieties. (That is, is finite-to-one and surjective.) We will construct an isogeny using the functorial description of , which says that the data of a map is the same as giving a family of degree zero line bundles on , parametrized by .
To this end, consider the isogeny and where is the Poincare line bundle for . This is then the required family of degree zero line bundles on .
By the aforementioned functorial description, there is then a morphism so that . One can show using this description that this map is an isogeny of the same degree as , and that .[5]
Hence, we obtain a contravariant endofunctor on the category of abelian varieties which squares to the identity. This kind of functor is often called a dualizing functor.[6]
Mukai's Theorem
A celebrated theorem of Mukai
Recall that if X and Y are varieties, and is a complex of coherent sheaves, we define the Fourier-Mukai transform to be the composition , where p and q are the projections onto X and Y respectively.
Note that is flat and hence is exact on the level of coherent sheaves, and in applications is often a line bundle so one may usually leave the left derived functors underived in the above expression. Note also that one can analogously define a Fourier-Mukai transform using the same kernel, by just interchanging the projection maps in the formula.
The statement of Mukai's theorem is then as follows.
Theorem: Let A be an abelian variety of dimension g and the Poincare line bundle on . Then, , where is the inversion map, and is the shift functor. In particular, is an isomorphism.[8]
Notes
- ^ Milne, James S. Abelian Varieties (PDF). pp. 35–36.
- ^ Milne, James S. Abelian Varieties (PDF). p. 36.
- ^ Mumford, Abelian Varieties, pp.74-80
- ^ Mumford, Abelian Varieties, p.123 onwards
- ^ Bhatt, Bhargav (2017). Abelian Varieties (PDF). p. 38.
- ISBN 978-3-540-78122-6.
- ^ Mukai, Shigeru (1981). "Duality between D(X) and D(\hat{X}) with its application to Picard sheaves". Nagoya Math. 81: 153–175.
- ^ Bhatt, Bhargav (2017). Abelian Varieties (PDF). p. 43.
References
- Milne, James S. (2008). Abelian Varieties (v2.00) (PDF).
- ISBN 978-0-19-560528-0.
- Bhatt, Bhargav (2017). Abelian Varieties (PDF).
This article incorporates material from Dual isogeny on