nLab algebraic stack (Rev #1)

An algebraic stack denotes either a Deligne-Mumford stack or more general Artin stack in the traditional setup of algebraic spaces: they are (Grothendieck) 1-stacks of groupoids on the étale site satisfying additional representability conditions, namely an algebraic stack $X$ is required to have an atlas $A\to X$ by an algebraic space or in a stronger version by a scheme $A$; the morphisms of the cover are required to be

• surjective representable and étale (Deligne-Mumford stack);

• surjective representable and smooth (Artin stack).

Here we used the following definition. A map $f:X\to Y$ of 1-stacks on the étale site is representable if for any scheme $S$ the fiber product $S\times_Y X$ is a stack associated to a scheme. Let $P$ be a property stable under composition and pullback. A representable map $f:X\to Y$ of stacks is said to have a property $P$ if for any scheme $S$ and a morphism of 1-stacks $S\to Y$ the pullback $S\times_Y X\to X$ has property $P$.

Nowdays people talk about differentiable stacks and topological stacks meaning analogues of Deligne-Mumford (DM) or Artin stacks defined in the setup of the category of manifolds or some convenient category of topological spaces. Orbifolds are an example of an Artin stack. For orbifolds stabilizer groups are finite, while for Artin stacks in general they are algebraic groups. An important source of DM-stacks are moduli problems, resulting often in moduli stacks (or their derived versions). DM-stacks are introduced in

• P. Deligne, D. Mumford, “The irreducibility of the space of curves of given genus”. Publications Mathématiques de l’IHÉS (Paris) 36: 75–109 (1969) numdam

A noncommutative generalization for Q-categories instead of Grothendieck topologies, hence applicable in noncommutative geometry of Deligne-Mumford and Artin stacks can be found in

• M. Kontsevich, A. Rosenberg, Noncommutative stacks, preprint MPIM2004-37 dvi ps

For topological variant see topological stacks and references therein and for differentiable stacks

• Kai Behrend, Ping Xu, Differentiable Stacks and Gerbes