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A stack on a site is geometric if, roughly, it is represented by a suitably well-behaved groupoid object internal to , i.e. if to an object the stack assigns the (ordinary) groupoid
A crucial difference between the groupoid object in and the geometric stack is that the equivalence class of the stack in general contains more (geometric) stacks than there are groupoid objects internally equivalent to : two groupoid objects with equivalent geometric stacks are called Morita equivalent groupoid objects.
Geometric stacks for the following choices of sites are called
for Top – topological stack;
for Diff – differentiable stack;
for CRing – algebraic stack;
for CplxMfd – complex analytic stack;
for Loc – localic stack;
There are slight variations in the literature on what precisely is required of a stack on a site with subcanonical topology in order that it qualifies as geometric.
A general requirement is that
the diagonal morphism is a representable morphism of stacks
there exists an atlas for the stack, in that there is a representable and a surjective morphism
This is necessarily itself representable, precisely if is.
Further conditions are the following
for the site of schemes with the etale topology
is required to be quasicompact and separated?
for Deligne-Mumford stacks is moreover required to be etale
for Artin stacks is required to be smooth.
The groupoid object associated to a geometric stack with atlas is the Cech groupoid of (this is simply the Cech groupoid of seen as a singleton cover) defined by and , where the latter is the 2-categorical pullback
geometric stack
A good discussion of topological and differentiable stacks is around definition 2.3 in
Differentiable stacks are discussed in
Specifically for the relation to groupoid objects see
3.1 and 3.3 in
paragraphs 2.4.3, 3.4.3, 3.8, 4.3 in
paragraph 4.4 in
See also
Geometric stacks over the site of schemes modeled on smooth loci is in section 8 of
Last revised on April 15, 2023 at 19:33:00. See the history of this page for a list of all contributions to it.