In algebraic geometry, a quotient stack is a stack that parametrizes equivariant objects. Geometrically, it generalizes a quotient of a scheme or a variety by a group: a quotient variety, say, would be a coarse approximation of a quotient stack.

The notion is of fundamental importance in the study of stacks: a stack that arises in nature is often either a quotient stack itself or admits a stratification by quotient stacks (e.g., a Deligne–Mumford stack.) A quotient stack is also used to construct other stacks like classifying stacks.

An orbifold is an example of a quotient stack.[citation needed]

Contents

DefinitionEdit

A quotient stack is defined as follows. Let G be an affine smooth group scheme over a scheme S and X a S-scheme on which G acts. Let   be the category over the category of S-schemes:

  • an object over T is a principal G-bundle   together with equivariant map  ;
  • an arrow from   to   is a bundle map (i.e., forms a cartesian diagram) that is compatible with the equivariant maps   and  .

Suppose the quotient   exists as an algebraic space (for example, by the Keel–Mori theorem). The canonical map

 ,

that sends a bundle P over T to a corresponding T-point,[1] need not be an isomorphism of stacks; that is, the space "X/G" is usually coarser. The canonical map is an isomorphism if and only if the stabilizers are trivial (in which case   exists.)[citation needed]

In general,   is an Artin stack (also called algebraic stack). If the stabilizers of the geometric points are finite and reduced, then it is a Deligne–Mumford stack.

(Totaro 2004) has shown: let X be a normal Noetherian algebraic stack whose stabilizer groups at closed points are affine. Then X is a quotient stack if and only if it has the resolution property; i.e., every coherent sheaf is a quotient of a vector bundle. Earlier, Thomason proved that a quotient stack has the resolution property.

Remark: It is possible to approach the construction from the point of view of simplicial sheaves.[2]

ExamplesEdit

If   with trivial action of G (often S is a point), then   is called the classifying stack of G (in analogy with the classifying space of G) and is usually denoted by BG. Borel's theorem describes the cohomology ring of the classifying stack.

Example:[3] Let L be the Lazard ring; i.e.,  . Then the quotient stack   by  ,

 ,

is called the moduli stack of formal group laws, denoted by  .

See alsoEdit

ReferencesEdit

  1. ^ The T-point is obtained by completing the diagram  .
  2. ^ Jardine, John F. (2015). Local homotopy theory. Springer Monographs in Mathematics. New York: Springer-Verlag. section 9.2. doi:10.1007/978-1-4939-2300-7. MR 3309296.
  3. ^ Taken from http://www.math.harvard.edu/~lurie/252xnotes/Lecture11.pdf

Some other references are