# Quotient stack

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.

## Definition

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 $[X/G]$ be the category over the category of S-schemes:

• an object over T is a principal G-bundle $P\to T$ together with equivariant map $P\to X$ ;
• an arrow from $P\to T$ to $P'\to T'$ is a bundle map (i.e., forms a cartesian diagram) that is compatible with the equivariant maps $P\to X$ and $P'\to X$ .

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

$[X/G]\to X/G$ ,

that sends a bundle P over T to a corresponding T-point, 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 $X/G$ exists.)

In general, $[X/G]$ 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.

## Examples

An effective quotient orbifold, e.g. $[M/G]$ where the $G$ action has only finite stabilizers on the smooth space $M$ , is an example of a quotient stack.

If $X=S$ with trivial action of G (often S is a point), then $[S/G]$ 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: Let L be the Lazard ring; i.e., $L=\pi _{*}\operatorname {MU}$ . Then the quotient stack $[\operatorname {Spec} L/G]$ by $G$ ,

$G(R)=\{g\in R[\![t]\!]|g(t)=b_{0}t+b_{1}t^{2}+\cdots ,b_{0}\in R^{\times }\}$ ,

is called the moduli stack of formal group laws, denoted by ${\mathcal {M}}_{\text{FG}}$ .