Picard group


In mathematics, the Picard group of a ringed space X, denoted by Pic, is the group of isomorphism classes of invertible sheaves on X, with the group operation being tensor product. This construction is a global version of the construction of the divisor class group, or ideal class group, and is much used in algebraic geometry and the theory of complex manifolds.
Alternatively, the Picard group can be defined as the sheaf cohomology group
For integral schemes the Picard group is isomorphic to the class group of Cartier divisors. For complex manifolds the exponential sheaf sequence gives basic information on the Picard group.
The name is in honour of Émile Picard's theories, in particular of divisors on algebraic surfaces.

Examples

and since we have because is contractible, then and we can apply the Dolbeault isomorphism to calculate by the Dolbeault-Grothendieck lemma.

Picard scheme

The construction of a scheme structure on the Picard group, the Picard scheme, is an important step in algebraic geometry, in particular in the duality theory of abelian varieties. It was constructed by, and also described by and. The Picard variety is dual to the Albanese variety of classical algebraic geometry.
In the cases of most importance to classical algebraic geometry, for a non-singular complete variety V over a field of characteristic zero, the connected component of the identity in the Picard scheme is an abelian variety written Pic0. In the particular case where V is a curve, this neutral component is the Jacobian variety of V. For fields of positive characteristic however, Igusa constructed an example of a smooth projective surface S with Pic0 non-reduced, and hence not an abelian variety.
The quotient Pic/Pic0 is a finitely-generated abelian group denoted NS, the Néron-Severi group of V. In other words the Picard group fits into an exact sequence
The fact that the rank of NS is finite is Francesco Severi's theorem of the base; the rank is the Picard number of V, often denoted ρ. Geometrically NS describes the algebraic equivalence classes of divisors on V; that is, using a stronger, non-linear equivalence relation in place of linear equivalence of divisors, the classification becomes amenable to discrete invariants. Algebraic equivalence is closely related to numerical equivalence, an essentially topological classification by intersection numbers.

Relative Picard scheme

Let f: X →S be a morphism of schemes. The relative Picard functor is given by: for any S-scheme T,
where is the base change of f and fT * is the pullback.
We say an L in has degree r if for any geometric point s → T the pullback of L along s has degree r as an invertible sheaf over the fiber Xs