Contents

Definition

A groupoid $G$ is a category in which all morphisms are isomorphisms. Often the requirement is made that $G$ is small.

A groupoid is tame if its cardinality is finite.

Groupoids form a $2$-category (in fact a $(2,1)$-category) Grpd.

Notation

If $x,y$ are objects (also called vertices) of the groupoid $G$ then the set of morphisms (also called arrows) from $x$ to $y$ is written $G(x,y)$, or other notations for hom-sets. The set $G(x,x)$ (which is a group under composition) is also written $G(x)$ and called the vertex group of $G$ at $x$. It is also written ${\mathrm{Aut}}_G(x)$ and called the automorphism group of $x$ in $G$, or written ${\pi }_1(G,x)$ and called the fundamental group of $G$ at $x$ (especially if you think of a groupoid as giving a homotopy 1-type).

As in any category, there is a question of notation for the composition, and in particular of the order in which to write things. It can be more convenient to write the composition of $a:x\to y$ and $b:y\to z$ as $ab:x\to z$, although a more traditional notation would be $ba$. The two conventions can be distinguished by writing $a;b$ or $b\circ a$ (which is the most traditional notation for categories). See composition for further discussion.

A groupoid $G$ is connected, or transitive?, if $G(x,y)$ is nonempty for all $x,y\in \mathrm{Ob}(G)$; it is called inhabited?, or nonempty, if it has at least one object. A maximal inhabited connected subgroupoid? of $G$ is called a component of $G$, and $G$ is then the disjoint union (the coproduct in $Grpd$) of its connected components. The set of components of $G$ is written ${\Pi }_0(G)$ (especially if you think of a groupoid as giving a homotopy 1-type).

Examples

1. Any group $H$ gives rise to a groupoid, sometimes denoted $BH$ but often conflated with $H$ itself, which has exactly one object $*$ and with $BH(*,*)=H$. Any inhabited connected groupoid is equivalent to one arising in this way.

2. A disjoint union of (the one-object groupoids corresponding to) groups is naturally a groupoid, also called a bundle of groups. The axiom of choice is equivalent to the claim that any groupoid is equivalent to one of this form.

3. From any action of a group $H$ on a set $X$ we obtain an action groupoid or “weak quotient” $X//H$. If $X=\{*\}$ this gives the groupoid $BH$, above.

4. A setoid, that is a set $X$ equipped with an equivalence relation $E$, becomes a groupoid with the multiplication $(x,y);(y,z)=(x,z)$ for all $(x,y),(y,z)\in E$. (This gives one reason for the forward notation for composition.) Such a groupoid is equivalent to the discrete category on the quotient set $X/E$.

5. In particular, if $E$ is the universal relation $X\times X$, then we get the square groupoid? $X^2$, also called the trivial groupoid? on $X$. Despite the latter name, there is an important special case, namely the groupoid $I=\{0,1{\}}^2$. This groupoid has non-identity elements $\iota :0\to 1,{\iota }^{-1}:1\to 0$, and can be regarded as a groupoid model of the unit interval $[0,1]$ in topology.

6. Any topological space $X$ has a fundamental groupoid ${\Pi }_1(X)$ whose objects are the points of $X$ and whose arrows are (homotopy classes of) paths, with composition by concatenation of paths. Note that ${\Pi }_0({\Pi }_1(X))={\Pi }_0(X)$ is the set of path components? of $X$, and for any $x\in X$, ${\pi }_1({\Pi }_1(X),x)={\pi }_1(X,x)$ is the fundamental group of $X$ with basepoint $x$. In theory one can obtain the higher homotopy groups in this way as well, by generalizing from the fundamental groupoid to the fundamental infinity-groupoid.

7. More generally, if we choose some subset $S$ of the points of a space $X$, then we have a full subgroupoid of ${\Pi }_1(X)$ containing only those points in $S$, denoted ${\Pi }_1(X,S)$. This can result in much more manageable groupoids; for instance ${\Pi }_1([0,1],\{0,1\})$ is the groupoid $I$ considered above, while ${\Pi }_1([0,1])$ has uncountably many objects (but is equivalent to $I$).

8. If $\Gamma$ is a directed graph or quiver, then the free groupoid $F(\Gamma )$ is well defined. It is the left adjoint functor to the forgetful functor from groupoids to directed graphs. This shows an advantage of groupoids: the notion of free equivalence relation or free action groupoid does not make sense. But we can still talk of a presentation of an equivalence relation or action groupoid by generators and relations, by considering presentations of groupoids instead.

   Mike: It’s not clear to me that the notion of “free equivalence relation” doesn’t make sense. Can’t I talk about a left adjoint to the forgetful functor from equivalence relations to, say, directed graphs? Maybe sets-equipped-with-a-binary-relation would be more appropriate, but either one works fine.

   Ronnie: Are you sure this forgetful functor equivalence relations to directed graphs has a left adjoint? Suppose the directed graph $\Gamma$ has one vertex $x$ and one loop $u:x\to x$. The free groupoid on $\Gamma$ is the group of integers, which as a groupoid is not an equivalence relation.

   Toby: But there is still a free setoid (set equipped with an equivalence relation) on $\Gamma$; it is the point. As a groupoid, it is not the same as the free groupoid on $\Gamma$, although it is the same as the free setoid on the free groupoid on $\Gamma$. If there's an advantage to working with groupoids, perhaps it's that the free groupoid functor preserves distinctions that the free setoid functor forgets? (In this case, a distinction preserved or forgotten is that between $\Gamma$ and the point, which as a graph does not have $u$.)

9. A paper by Živaljević gives examples of groupoids used in combinatorics.

Properties

Characterization as 2-coskeletal Kan complexes

Groupoids $K$ are equivalent to 1-hypergroupoids, which are in particular 2-coskeletal Kan complexes $N(K)$ – their nerves.

The objects of the groupoids are the 0-simplices and the morphisms of the groupoid are the 1-simplices of the Kan complex. The composition operation $(f,g)\mapsto g\circ f$ in the grouopoid is encoded in the 2-simplices of the Kan complex

The associativity condition on the composition is exhibited by the 3-coskeleton-property of the Kan complex. This says that every simplicial 2-sphere in the Kan complex has unique filler. With the above identification of 2-simplices with composition operations, this means that the 2 ways

of composing a sequence of three composable morphisms are equal

For handling just groupoids exclusively their description in terms of Kan complexes may be a bit of an overkill, but the advantage is that it embeds groupoids naturally in the more general context of 2-groupoids, 3-groupoids and eventually ∞-groupoids. For instance a pseudo-functor out of an ordinary groupoid into a 2-groupoid is simply a homomorphism of the corresponding Kan complexes.

Related concepts

homotopy level	n-truncation	homotopy theory	higher category theory	higher topos theory	homotopy type theory
h-level 0	(-2)-truncated	contractible space	(-2)-groupoid		true/unit type/contractible type
h-level 1	(-1)-truncated		(-1)-groupoid/truth value		h-proposition
h-level 2	0-truncated	discrete space	0-groupoid/set	sheaf	h-set
h-level 3	1-truncated	homotopy 1-type	1-groupoid/groupoid	(2,1)-sheaf/stack	h-groupoid
h-level 4	2-truncated	homotopy 2-type	2-groupoid		h-2-groupoid
h-level 5	3-truncated	homotopy 3-type	3-groupoid		h-3-groupoid
h-level $n+2$	$n$-truncated	homotopy n-type	n-groupoid		h-$n$-groupoid
h-level $\mathrm{\infty }$	untruncated	homotopy type	∞-groupoid	(∞,1)-sheaf/∞-stack	h-$\mathrm{\infty }$-groupoid

• groupoid action, groupoid-principal bundle

• topological groupoid, Lie groupoid, smooth groupoid

• stack, geometric stack

• Hopf algebroid

References

A motivation and introduction of the concept of groupoid and a tour of examples (including the refinement to topological groupoids and Lie groupoids) is in

• Alan Weinstein, Groupoids: Unifying Internal and External Symmetry – A Tour through some Examples, Notices of the AMS volume 43, Number 7 (pdf)

A page Groupoids in Mathematics by Ronnie Brown includes the introductory text

• Ronnie Brown, From groups to groupoids: A brief survey, (pdf)

Technical discussion is for instance in the following references.

• Philip Higgins, 1971, Categories and Groupoids, van Nostrand, New York. Reprints in Theory and Applications of Categories, 7 (2005) pp 1–195.

• Philip Higgins, Presentations of Groupoids, with Applications to Groups, Proc. Camb. Phil. Soc., 60 (1964) 7–20.

• Ronnie Brown, Topology and groupoids, Booksurge, 2006.

• Rade T. Živaljević, Groupoids in combinatorics—applications of a theory of local symmetries. Algebraic and geometric combinatorics, 305–324, Contemp. Math., 423, Amer. Math. Soc., Providence, RI, 2006.