In mathematics, especially in category theory and homotopy theory, a groupoid (less often Brandt groupoid or virtual group) generalises the notion of group in several equivalent ways. A groupoid can be seen as a:
In the presence of dependent typing, a category in general can be viewed as a typed monoid, and similarly, a groupoid can be viewed as simply a typed group. The morphisms take one from one object to another, and form a dependent family of types, thus morphisms might be typed , , say. Composition is then a total function: , so that .
Special cases include:
Groupoids are often used to reason about geometrical objects such as manifolds. Heinrich Brandt (1927) introduced groupoids implicitly via Brandt semigroups.^{[2]}
A groupoid is an algebraic structure consisting of a non-empty set and a binary partial function '' defined on .
A groupoid is a set with a unary operation and a partial function . Here * is not a binary operation because it is not necessarily defined for all pairs of elements of . The precise conditions under which is defined are not articulated here and vary by situation.
and ^{-1} have the following axiomatic properties: For all , , and in ,
Two easy and convenient properties follow from these axioms:
A groupoid is a small category in which every morphism is an isomorphism, i.e. invertible.^{[1]} More precisely, a groupoid G is:
satisfying, for any f : x -> y, g : y -> z, and h : z -> w:
If f is an element of G(x,y) then x is called the source of f, written s(f), and y is called the target of f, written t(f).
More generally, one can consider a groupoid object in an arbitrary category admitting finite fiber products.
The algebraic and category-theoretic definitions are equivalent, as we now show. Given a groupoid in the category-theoretic sense, let G be the disjoint union of all of the sets G(x,y) (i.e. the sets of morphisms from x to y). Then and become partial operations on G, and will in fact be defined everywhere. We define * to be and ^{-1} to be , which gives a groupoid in the algebraic sense. Explicit reference to G_{0} (and hence to ) can be dropped.
Conversely, given a groupoid G in the algebraic sense, define an equivalence relation on its elements by iff a * a^{-1} = b * b^{-1}. Let G_{0} be the set of equivalence classes of , i.e. . Denote a * a^{-1} by if with .
Now define as the set of all elements f such that exists. Given and their composite is defined as . To see that this is well defined, observe that since and exist, so does . The identity morphism on x is then , and the category-theoretic inverse of f is f^{-1}.
Sets in the definitions above may be replaced with classes, as is generally the case in category theory.
Given a groupoid G, the vertex groups or isotropy groups or object groups in G are the subsets of the form G(x,x), where x is any object of G. It follows easily from the axioms above that these are indeed groups, as every pair of elements is composable and inverses are in the same vertex group.
A subgroupoid is a subcategory that is itself a groupoid. A groupoid morphism is simply a functor between two (category-theoretic) groupoids. The category whose objects are groupoids and whose morphisms are groupoid morphisms is called the groupoid category, or the category of groupoids, denoted Grpd.
It is useful that this category is, like the category of small categories, Cartesian closed. That is, we can construct for any groupoids a groupoid whose objects are the morphisms and whose arrows are the natural equivalences of morphisms. Thus if are just groups, then such arrows are the conjugacies of morphisms. The main result is that for any groupoids there is a natural bijection
This result is of interest even if all the groupoids are just groups.
Particular kinds of morphisms of groupoids are of interest. A morphism of groupoids is called a fibration if for each object of and each morphism of starting at there is a morphism of starting at such that . A fibration is called a covering morphism or covering of groupoids if further such an is unique. The covering morphisms of groupoids are especially useful because they can be used to model covering maps of spaces.^{[4]}
It is also true that the category of covering morphisms of a given groupoid is equivalent to the category of actions of the groupoid on sets.
Given a topological space , let be the set . The morphisms from the point to the point are equivalence classes of continuous paths from to , with two paths being equivalent if they are homotopic. Two such morphisms are composed by first following the first path, then the second; the homotopy equivalence guarantees that this composition is associative. This groupoid is called the fundamental groupoid of , denoted (or sometimes, ).^{[5]} The usual fundamental group is then the vertex group for the point . For a path-connected space, fundamental groupoid and the fundamental group coincide, and the composition operation is defined for all pairs of equivalence classes.
An important extension of this idea is to consider the fundamental groupoid where is a chosen set of "base points". Here, one considers only paths whose endpoints belong to . is a sub-groupoid of . The set may be chosen according to the geometry of the situation at hand.
If is a set with an equivalence relation denoted by infix , then a groupoid "representing" this equivalence relation can be formed as follows:
If the group acts on the set , then we can form the action groupoid (or transformation groupoid) representing this group action as follows:
More explicitly, the action groupoid is a small category with and with source and target maps and . It is often denoted (or ). Multiplication (or composition) in the groupoid is then which is defined provided .
For in , the vertex group consists of those with , which is just the isotropy subgroup at for the given action (which is why vertex groups are also called isotropy groups).
Another way to describe -sets is the functor category , where is the groupoid (category) with one element and isomorphic to the group . Indeed, every functor of this category defines a set and for every in (i.e. for every morphism in ) induces a bijection : . The categorical structure of the functor assures us that defines a -action on the set . The (unique) representable functor : -> is the Cayley representation of . In fact, this functor is isomorphic to and so sends to the set which is by definition the "set" and the morphism of (i.e. the element of ) to the permutation of the set . We deduce from the Yoneda embedding that the group is isomorphic to the group , a subgroup of the group of permutations of .
Consider the finite set , we can form the group action acting on by taking each number to its negative, so and . The quotient groupoid is the set of equivalence classes from this group action , and has a group action of on it.
On , any finite group which maps to give a group action on (since this is the group of automorphisms). Then, a quotient groupoid can be forms , which has one point with stabilizer at the origin.
Given a diagram of groupoids with groupoid morphisms
where and , we can form the groupoid whose objects are triples , where , , and in . Morphisms can be defined as a pair of morphisms where and such that for triples , there is a commutative diagram in of , and the .^{[6]}
A two term complex
of objects in a concrete Abelian category can be used to form a groupoid. It has as objects the set and arrows where the source morphism is just the projection onto while the target morphism is the addition of projection onto composed with and projection onto . That is, given we have
Of course, if the abelian category is the category of coherent sheaves on a scheme, then this construction can be used to form a presheaf of groupoids.
While puzzles such as the Rubik's Cube can be modeled using group theory (see Rubik's Cube group), certain puzzles are better modeled as groupoids.^{[7]}
The transformations of the fifteen puzzle form a groupoid (not a group, as not all moves can be composed).^{[8]}^{[9]}^{[10]} This groupoid acts on configurations.
The Mathieu groupoid is a groupoid introduced by John Horton Conway acting on 13 points such that the elements fixing a point form a copy of the Mathieu group M_{12}.
Group-like structures | |||||
---|---|---|---|---|---|
Totality^{?} | Associativity | Identity | Invertibility | Commutativity | |
Semigroupoid | Unneeded | Required | Unneeded | Unneeded | Unneeded |
Small Category | Unneeded | Required | Required | Unneeded | Unneeded |
Groupoid | Unneeded | Required | Required | Required | Unneeded |
Magma | Required | Unneeded | Unneeded | Unneeded | Unneeded |
Quasigroup | Required | Unneeded | Unneeded | Required | Unneeded |
Unital Magma | Required | Unneeded | Required | Unneeded | Unneeded |
Loop | Required | Unneeded | Required | Required | Unneeded |
Semigroup | Required | Required | Unneeded | Unneeded | Unneeded |
Inverse Semigroup | Required | Required | Unneeded | Required | Unneeded |
Monoid | Required | Required | Required | Unneeded | Unneeded |
Commutative monoid | Required | Required | Required | Unneeded | Required |
Group | Required | Required | Required | Required | Unneeded |
Abelian group | Required | Required | Required | Required | Required |
^? Closure, which is used in many sources, is an equivalent axiom to totality, though defined differently. |
If a groupoid has only one object, then the set of its morphisms forms a group. Using the algebraic definition, such a groupoid is literally just a group.^{[11]} Many concepts of group theory generalize to groupoids, with the notion of functor replacing that of group homomorphism.
If is an object of the groupoid , then the set of all morphisms from to forms a group (called the vertex group, defined above). If there is a morphism from to , then the groups and are isomorphic, with an isomorphism given by the mapping .
Every connected groupoid - that is, one in which any two objects are connected by at least one morphism - is isomorphic to an action groupoid (as defined above) . By connectedness, there will only be one orbit under the action. If the groupoid is not connected, then it is isomorphic to a disjoint union of groupoids of the above type (possibly with different groups and sets for each connected component).
Note that the isomorphism described above is not unique, and there is no natural choice. Choosing such an isomorphism for a connected groupoid essentially amounts to picking one object , a group isomorphism from to , and for each other than , a morphism in from to .
In category-theoretic terms, each connected component of a groupoid is equivalent (but not isomorphic) to a groupoid with a single object, that is, a single group. Thus any groupoid is equivalent to a multiset of unrelated groups. In other words, for equivalence instead of isomorphism, one need not specify the sets , only the groups For example,
The collapse of a groupoid into a mere collection of groups loses some information, even from a category-theoretic point of view, because it is not natural. Thus when groupoids arise in terms of other structures, as in the above examples, it can be helpful to maintain the full groupoid. Otherwise, one must choose a way to view each in terms of a single group, and this choice can be arbitrary. In our example from topology, you would have to make a coherent choice of paths (or equivalence classes of paths) from each point to each point in the same path-connected component.
As a more illuminating example, the classification of groupoids with one endomorphism does not reduce to purely group theoretic considerations. This is analogous to the fact that the classification of vector spaces with one endomorphism is nontrivial.
Morphisms of groupoids come in more kinds than those of groups: we have, for example, fibrations, covering morphisms, universal morphisms, and quotient morphisms. Thus a subgroup of a group yields an action of on the set of cosets of in and hence a covering morphism from, say, to , where is a groupoid with vertex groups isomorphic to . In this way, presentations of the group can be "lifted" to presentations of the groupoid , and this is a useful way of obtaining information about presentations of the subgroup . For further information, see the books by Higgins and by Brown in the References.
The inclusion has both a left and a right adjoint:
Here, denotes the localization of a category that inverts every morphism, and denotes the subcategory of all isomorphisms.
The nerve functor embeds Grpd as a full subcategory of the category of simplicial sets. The nerve of a groupoid is always Kan complex.
The nerve has a left adjoint
Here, denotes the fundamental groupoid of the simplicial set X.
When studying geometrical objects, the arising groupoids often carry some differentiable structure, turning them into Lie groupoids. These can be studied in terms of Lie algebroids, in analogy to the relation between Lie groups and Lie algebras.