# Picard group

Template:Distinguish
In mathematics, the **Picard group** of a ringed space *X*, denoted by Pic(*X*), is the group of isomorphism classes of invertible sheaves (or line bundles) 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

- The Picard group of the spectrum of a Dedekind domain is its
*ideal class group*.

- The invertible sheaves on projective space
**P**^{n}(*k*) for*k*a field, are the twisting sheaves so the Picard group of**P**^{n}(*k*) is isomorphic to**Z**.

- The Picard group of the affine line with two origins over
*k*is isomorphic to**Z**.

## Picard scheme

The construction of a scheme structure on (representable functor version of) 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 Template:Harvtxt, and also described by Template:Harvtxt and Template:Harvtxt. 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 Pic^{0}(*V*). 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 Pic^{0}(*S*) non-reduced, and hence not an abelian variety.

The quotient Pic(*V*)/Pic^{0}(*V*) is a finitely-generated abelian group denoted NS(*V*), the **Néron–Severi group** of *V*. In other words the Picard group fits into an exact sequence

The fact that the rank is finite is Francesco Severi's **theorem of the base**; the rank is the **Picard number** of *V*, often denoted ρ(*V*). Geometrically NS(*V*) 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.

## See also

## References

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