Groupoid

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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:

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]

Definitions[edit]

A groupoid is an algebraic structure consisting of a non-empty set and a binary partial function '' defined on .

Algebraic[edit]

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 possible pairs of -elements. The precise conditions under which is defined are not articulated here and vary by situation.

and −1 have the following axiomatic properties. Let , , and be elements of . Then:

  1. Associativity: If and are defined, then and are defined and equal. Conversely, if either of these last two expressions is defined, then so is the other (and again they are equal).
  2. Inverse: and are always defined.
  3. Identity: If is defined, then , and . (The previous two axioms already show that these expressions are defined and unambiguous.)

From these axioms, two easy and convenient properties follow:

  • ;
  • If is defined, then .[3]

Category theoretic[edit]

A groupoid is a small category in which every morphism is an isomorphism, i.e. invertible.[1] More precisely, a groupoid G is:

  • A set G0 of objects;
  • For each pair of objects x and y in G0, there exists a (possibly empty) set G(x,y) of morphisms (or arrows) from x to y. We write f : xy to indicate that f is an element of G(x,y).
  • For every object x, a designated element of G(x,x);
  • For each triple of objects x, y, and z, a function ;
  • For each pair of objects x, y a function ;

satisfying, for any f : xy, g : yz, and h : zw:

  • and ;
  • ;
  • and .

If f is an element of G(x,y) then x is called the source of f, written s(f), and y the target of f (written t(f)).

More generally, one can consider a groupoid object in an arbitrary category admitting finite fiber products.

Comparing the definitions[edit]

The algebraic and category-theoretic definitions are equivalent, as follows. 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 partially defined operations on G, and will in fact be defined everywhere; so we define ∗ to be and −1 to be . Thus we have a groupoid in the algebraic sense. Explicit reference to G0 (and hence to ) can be dropped.

Conversely, given a groupoid G in the algebraic sense, let G0 be the set of all elements of the form xx−1 with x varying through G and define

as the set of all elements f such that

exists. Given

,

their composite is defined as

.

To see this is well defined, observe that since

exist, so does

.

The identity morphism on xx−1 is then xx−1 itself, 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.

Vertex groups[edit]

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.

Category of groupoids[edit]

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.

Fibrations and coverings[edit]

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.

Examples[edit]

Linear algebra[edit]

Given a field K, the corresponding general linear groupoid GL*(K) consists of all invertible matrices, of any size, whose entries range over K. Matrix multiplication interprets composition. If G = GL*(K), then the set of natural numbers is a proper subset of G0, since for each natural number n, there is a corresponding identity matrix of dimension n. G(m,n) is empty unless m=n, in which case it is the set of all nxn invertible matrices.

Topology[edit]

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 .

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.

Equivalence relation[edit]

If is a set with an equivalence relation denoted by infix , then a groupoid "representing" this equivalence relation can be formed as follows:

  • The objects of the groupoid are the elements of ;
  • For any two elements and in , there is a single morphism from to if and only if .

Group action[edit]

If the group acts on the set , then we can form the action groupoid (or transformation groupoid) representing this group action as follows:

  • The objects are the elements of ;
  • For any two elements and in , there is a morphism from to corresponding to every element of such that ;
  • Composition of morphisms interprets the binary operation of .

More explicitly, the action groupoid is the set 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 .

Fifteen puzzle[edit]

The transformations of the fifteen puzzle form a groupoid (not a group, as not all moves can be composed).[6][7][8] This groupoid acts on configurations.

Mathieu groupoid[edit]

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 M12.

Relation to groups[edit]

Group-like structures
Totalityα Associativity Identity Invertibility Commutativity
Semigroupoid Unneeded Required Unneeded Unneeded Unneeded
Category Unneeded Required Required Unneeded Unneeded
Groupoid Unneeded Required Required Required Unneeded
Magma Required Unneeded Unneeded Unneeded Unneeded
Quasigroup Required Unneeded Unneeded Required Unneeded
Loop Required Unneeded Required Required Unneeded
Semigroup Required Required Unneeded Unneeded Unneeded
Monoid Required Required Required Unneeded Unneeded
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.[9] 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 .

Consider the examples in the previous section. The general linear groupoid is both equivalent and isomorphic to the disjoint union of the various general linear groups . On the other hand:

  • The fundamental groupoid of is equivalent to the collection of the fundamental groups of each path-connected component of , but an isomorphism requires specifying the set of points in each component;
  • The set with the equivalence relation is equivalent (as a groupoid) to one copy of the trivial group for each equivalence class, but an isomorphism requires specifying what each equivalence class is:
  • The set equipped with an action of the group is equivalent (as a groupoid) to one copy of for each orbit of the action, but an isomorphism requires specifying what set each orbit is.

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 H 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.

Lie groupoids and Lie algebroids[edit]

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.

See also[edit]

Notes[edit]

  1. ^ a b Dicks & Ventura (1996). The Group Fixed by a Family of Injective Endomorphisms of a Free Group. p. 6. 
  2. ^ Brandt semigroup in Springer Encyclopaedia of Mathematics - ISBN 1-4020-0609-8
  3. ^ Proof of first property: from 2. and 3. we obtain . ✓
    Proof of second property: since a * b is defined, so is (a * b)−1 * a * b. Therefore (a * b)−1 * a * b * b−1 = (a * b)−1 * a is also defined. Moreover since a * b is defined, so is a * b * b−1 = a. Therefore a * b * b−1 * a−1 is also defined. From 3. we obtain (a * b)−1 = (a * b)−1 * a * a−1 = (a * b)−1 * a * b * b−1 * a−1 = b−1 * a−1. ✓
  4. ^ J.P. May, A Concise Course in Algebraic Topology, 1999, The University of Chicago Press ISBN 0-226-51183-9 (see chapter 2)
  5. ^ "fundamental groupoid in nLab". ncatlab.org. Retrieved 2017-09-17. 
  6. ^ Jim Belk (2008) Puzzles, Groups, and Groupoids, The Everything Seminar
  7. ^ The 15-puzzle groupoid (1), Never Ending Books
  8. ^ The 15-puzzle groupoid (2), Never Ending Books
  9. ^ Mapping a group to the corresponding groupoid with one object is sometimes called delooping, especially in the context of homotopy theory, see "delooping in nLab". ncatlab.org. Retrieved 2017-10-31. .

References[edit]