48 What's a groupoid? What's a good example of a groupoid? 2009-10-19T00:53:47.793

27 How many binary operations are associative? 2012-10-21T03:30:54.920

21 Toposes (topoi) as classifying toposes of groupoids 2011-12-24T00:12:21.227

20 How should one understand orbifold fundamental groups? 2012-08-29T20:13:59.237

18 Has any attempt been made to classify finite groupoids? 2012-10-06T03:42:25.887

17 What is the precise relationship between groupoid language and noncommutative algebra language? 2010-07-09T21:07:20.433

17 Brandt's definition of groupoids (1926) 2015-03-13T00:55:43.023

16 Characterizing Groupoids via Quotients? 2011-09-13T21:28:40.160

16 Convolution algebras for double groupoids? 2012-01-25T11:12:40.690

16 What's the big deal about $M_{13}$? 2014-07-16T20:24:43.703

15 How difficult is Morse theory on stacks? 2010-02-21T00:11:31.947

15 What is the 2-category whose 0-objects are Lie algebroids? 2010-07-30T19:35:24.903

13 The groupoid of algebraic expressions and proofs 2015-02-13T15:04:11.917

12 Groupoid of moves on trivalent fatgraph 2009-11-12T05:56:00.417

12 How do you define the strict infinity groupoids in Homotopy Type Theory? 2013-10-08T19:42:21.480

11 How to interpret topologically that the equalizer in Groupoids of ${\rm id}, {\rm id}: BG \rightrightarrows BG$ is $G/G$ (adjoint action)? 2011-01-15T18:36:34.757

11 Groupoid actions on spaces 2011-04-29T16:56:21.673

10 Representation of Groupoids 2010-02-26T06:34:10.090

10 What is the infinite-dimensional-manifold structure on the space of smooth paths mod thin homotopy? 2010-03-11T02:13:40.493

10 A quantity associated with a smooth groupoid 2017-10-28T20:51:35.140

9 Groupoid structure on G/H? 2010-03-11T01:13:37.153

9 Double Category of Topological Stacks 2010-06-01T11:48:15.637

9 Is a groupoid determined by its Hopfish algebra? 2010-07-13T18:27:21.533

9 Quotients of topological groupoids 2010-11-23T16:57:17.140

9 Groupoids vs Pseudogroups 2011-06-16T12:58:51.327

9 What's the link between topological spaces as locales and topological spaces as infinity-groupoids? 2013-09-18T22:31:59.260

8 2 questions on the groupoid algebra 2011-05-22T16:41:17.210

7 How equivalent are the theories of reduced and groupal $\infty$-groupoids? 2011-02-26T21:52:22.223

7 Groupoids and hypergroups 2012-06-05T13:06:56.970

7 Albrecht FrÃ¶hlich's text `Groupoids, groupoid spaces and cohomology' (1965) 2013-07-28T18:05:26.127

7 Relation between graphs and groupoid $C^*$-algebras 2013-11-02T08:37:35.287

7 How to construct a free 2-group on a groupoid? 2014-07-10T11:09:10.960

7 Groupoid cardinality and Egyptian fraction representations of 1 2016-05-31T22:42:48.623

6 Is there a theory of differential equations for smooth correspondences? 2009-11-29T03:14:32.740

6 Is there a "groupoid integral" with values in a groupoid? 2010-04-15T23:22:50.470

6 How can I understand the "groupoid" quotient of a group action as some sort of "product"? 2010-04-29T19:36:23.190

6 Geometric interpretation of the fundamental groupoid 2010-06-08T16:59:33.060

6 Is there a "geometric" language that describes the equivalence groupoid of a foliated manifold? 2011-01-13T06:00:09.163

6 Automorphism groups and etale topological stacks 2011-05-08T01:01:49.287

6 Continuous and smooth Lie groupoid cohomology 2012-02-18T09:19:38.817

6 Classification of weak 3-groups 2017-06-10T13:41:11.220

6 Category of representations of the path-groupoid 2017-09-27T17:53:56.307

5 Normal coordinates for a manifold with volume form 2009-12-04T22:26:06.950

5 Category = Groupoid x Poset? 2010-03-24T13:29:11.067

5 What is the local structure of a Lie groupoid? 2010-03-31T19:44:50.307

5 What condition on a "bibundle between categories" generalizes "right-principal bibundle between groupoids"? 2010-11-07T23:21:11.760

5 How to specify a finite group up to inner automorphism? 2013-02-15T00:58:48.000

5 Is the crossed product $\mathcal{K} \rtimes G$ a groupoid algebra? 2014-06-27T12:31:16.800

5 Group objects in $\infty$-categories 2018-02-23T22:41:52.323

5 moduli stack of double covers of $\mathbb{P}^1$ with one marked point 2018-03-03T16:02:06.353

4 Do Lie algebroids pull back (along submersions)? 2010-05-31T00:01:49.603

4 Colimits of topological groupoids 2010-10-15T22:39:37.793

4 $\infty-$groupoid of $A_{\infty}$ algebras 2010-11-17T16:07:02.540

4 Simplicity of reduced C*-algebras for non-Hausdorff etale groupoids 2012-09-05T15:10:38.970

4 Left adjoint to inclusion of Connected Groupoids into Groupoids 2016-11-06T14:52:53.587

4 Gluing together together differentiable stacks 2017-02-10T22:30:14.110

4 K-theory of a discrete groupoid crossed product 2017-12-09T12:57:08.970

3 What is an obviously coordinate-independent description of the Chevellay-Eilenberg complex for a Lie algebroid? 2010-05-28T17:18:58.750

3 Algebraic stacks as (etale) groupoid alg.spaces/schemes 2011-08-30T02:53:27.493

3 Cartesian cubes and groupoids 2012-03-12T23:49:52.930

3 reference request for essential equivalence of top. groupoids 2012-09-23T11:02:03.563

3 What are the symmetries of a principal homogeneous bundle? 2013-02-10T17:44:21.580

3 Connected groupoids and action groupoids 2013-04-16T18:17:18.590

3 Groupoids vs. action groupoids 2013-05-17T12:54:48.253

3 Is there a Fourier transform for principal r-discrete groupoids? 2013-07-23T16:25:32.693

3 How to detect if a simplicial set is the nerve of a groupoid? 2013-10-10T19:06:11.197

3 Classifying map of a principal bundle 2014-01-19T22:23:46.203

3 Homotopy category of groupoids 2014-01-22T20:14:06.373

3 Connection on 3-bundle given as triplet of forms 2014-04-27T15:24:59.650

3 Representable torsors on geometric groupoid 2014-06-04T16:35:47.633

3 Does the nerve functor (resp. fundamental groupoid functor) preserve homotopy colimits (resp. homotopy limits)? 2014-07-12T19:14:17.447

3 Groupoid isomorphism vs. group isomorphism 2016-01-22T18:52:46.233

3 A question on groupoids and measurable fields of Hilbert spaces 2017-03-22T06:15:28.737

3 Coverings of a space and coverings of a groupoid 2018-01-10T08:53:57.953

3 Relation between the Hochschild cohomology of group algebras and groupoids 2018-02-12T14:23:36.377

2 Is the cohomology of the corresponding Lie algebroid an invariant under equivalence of source-simply-connected Lie groupoids? 2010-09-16T01:19:28.027

2 Weak colimits of weak and strict presheaves in groupoids 2010-10-13T15:31:56.847

2 About properties of groupoid C*-algebras 2011-04-15T13:28:42.593

2 Examples of Simplicial Groupoids in Nature 2011-12-25T05:40:25.690

2 spaces of projections 2012-02-24T11:13:18.260

2 Semidirect product of groupoids 2013-02-14T19:44:18.987

2 Constructing a stack (gerbe) from a connected groupoid 2013-04-14T13:09:33.163

2 Bisections in Kan Complexes 2013-04-17T13:01:15.010

2 Equivalence and weak equivalence of groupoids 2013-05-15T12:58:08.210

2 Is the cotangent complexes of groupoids bounded above by degree $1$? 2014-09-07T20:59:40.593

2 Reference request for generalization of groups with out identity element? 2014-10-02T03:32:42.833

2 Local section of Lie Groupoids 2015-02-13T14:02:24.387

2 Are all locally compact anisotropic groupoids etale up to equivalence? 2015-07-21T12:32:49.610

2 Existence of enough local sections 2016-03-15T09:32:21.777

2 Group actions on principal groupoids 2016-05-18T14:58:23.613

2 Reference for the existence of bicolimits in groupoids and categories? 2016-06-11T11:02:30.057

2 The pair $(Gl(n,\mathbb{R}), O(n) )$ as a groupoid 2016-08-21T05:42:12.493

2 Certain groupoid and its $C^{*}$ algebra 2016-10-09T19:51:06.403

2 Twist of Groupoid: Surjection identity on (common) unit space? 2017-05-20T10:42:07.237

2 What is a "Twisted Groupoid" 2017-07-10T20:33:24.297

1 Given an object in a Lie groupoid, does there exist a subgroupoid for which the object has no automorphisms but retains its equivalence class? 2010-06-01T00:51:59.727

1 A category being self-dual vs. it being a groupoid 2011-10-07T10:12:01.580

1 When are $k$-sectors of a Lie groupoid a manifold? 2012-10-04T15:52:24.040

1 Reduced C*-algebras of locally compact etale Hausdorff groupoids 2013-06-18T11:05:52.950