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A Lie groupoid is said to be effective if its morphisms act locally freely on germs, in a sense.
(Beware that this use of the term is entirely independent of “effective” in the sense of Giraud's axioms, as discussed are groupoid object in an (infinity,1)-category.)
Let be a Lie groupoid. Equivalently, let be a differentiable stack equipped with an atlas .
Then given any element in , hence given a morphism , it induces a germ of a local diffeomorphism as follows:
choose to be any neighbourhood of small enough such that the restricted source and target maps
are diffeomorphisms. Then define to be the germ .
The Lie groupoid is called effective if this assignment of morphisms to germs of local diffeomorphisms is injective.
Similarly a differentiable stack is called an effective étale stack if it is represented by an effective étale Lie groupoid.
This means that the action of the automorphism group at any point on the germ at is faithful.
The following are equivalent
the canonical smooth functor (see here) to the Haefliger groupoid of the manifold of objects is faithful, i.e. gives a representable morphism of stacks;
under the equivalence (here) between smooth étale stacks and stacks on the site of smooth manifolds with local diffeomorphisms between them, corresponds to a sheaf (i.e. to a 0-truncated stack) on .
A standard textbook aacount is in
Brief survey is in
Discussion in a more general context of étale stacks is in
Last revised on September 16, 2020 at 20:44:52. See the history of this page for a list of all contributions to it.