effective Lie groupoid


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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 X =(X 1Y)X_\bullet = (X_1 \stackrel{\longrightarrow}{\longrightarrow} Y) be a Lie groupoid. Equivalently, let XX be a differentiable stack equipped with an atlas X 0XX_0 \to X.

Then given any element ff in X 1X_1, hence given a morphism f:xyf \colon x \to y, it induces a germ of a local diffeomorphism f˜:(X 0,x)(X 0,y)\tilde f \colon (X_0,x) \to (X_0,y) as follows:

choose U xX 0U_x \subset X_0 to be any neighbourhood of xx small enough such that the restricted source and target maps

X 1× X 0U x X 1 s| U,t| U s,t U x X 0 \array{ X_1 \times_{X_0} U_x &\to& X_1 \\ {}^{\mathllap{s|_U, t|_U}}\downarrow && \downarrow^{\mathrlap{s,t}} \\ U_x &\stackrel{}{\hookrightarrow} & X_0 }

are diffeomorphisms. Then define f˜\tilde f to be the germ t(s| U) 1t \circ (s|_U)^{-1}.


The Lie groupoid X X_\bullet 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 xx on the germ at xx is faithful.


Equivalent characterizations


The following are equivalent

  1. X X_\bullet is an effective Lie groupoid, def. 1;

  2. the canonical smooth functor X (X 0)X_\bullet \to \mathbb{H}(X_0) (see here) to the Haefliger groupoid of the manifold of objects is faithful, i.e. gives a representable morphism of stacks;

  3. under the equivalence (here) between smooth étale stacks and stacks on the site SmthMfd etSmthMfd^{et} of smooth manifolds with local diffeomorphisms between them, XX corresponds to a sheaf (i.e. to a 0-truncated stack) on SmthMfd etSmthMfd^{et}.

(Carchedi 12, theorem 4.1).


A standard textbook aacount is in

  • Ieke Moerdijk, Janez Mrčun Introduction to Foliations and Lie Groupoids ,Cambridge Studies in Advanced Mathematics 91, Cambridge University Press, Cambridge, (2003)

Brief survey is in

Discussion in a more general context of étale stacks is in

Last revised on December 27, 2014 at 17:27:44. See the history of this page for a list of all contributions to it.