In mathematics, a 2-group, or 2-dimensional higher group, is a certain combination of group and groupoid. The 2-groups are part of a larger hierarchy of n-groups. In some of the literature, 2-groups are also called gr-categories or groupal groupoids.
Much of the literature focuses on strict 2-groups. A strict 2-group is a strict monoidal category in which every morphism is invertible and every object has a strict inverse. A strict 2-group is a group object in a category of categories; as such, they are also called groupal categories. Conversely, a strict 2-group is a category object in the category of groups; as such, they are also called categorical groups. They can also be identified with crossed modules, and are most often studied in that form. Thus, 2-groups in general can be seen as a weakening of crossed modules. Every 2-group is equivalent to a strict 2-group, although this can't be done coherently: it doesn't extend to 2-group homomorphisms.
Properties
Weak inverses can always be assigned coherently: one can define a functor on any 2-group G that assigns a weak inverse to each object and makes that object an adjoint equivalence in the monoidal category G. Given a bicategoryB and an object x of B, there is an automorphism 2-group of x in B, written AutB. The objects are the automorphisms of x, with multiplication given by composition, and the morphisms are the invertible 2-morphisms between these. If B is a 2-groupoid and x is its only object, then AutB is the only data left in B. Thus, 2-groups may be identified with one-object 2-groupoids, much as groups may be identified with one-object groupoids and monoidal categories may be identified with one-object bicategories. If G is a strict 2-group, then the objects of G form a group, called the underlying group of G and written G0. This will not work for arbitrary 2-groups; however, if one identifies isomorphic objects, then the equivalence classes form a group, called the fundamental group of G and written π1. As a monoidal category, any 2-group G has a unit objectIG. The automorphism group of IG is an abelian group by the Eckmann–Hilton argument, written Aut or π2. The fundamental group of G acts on either side of π2, and the associator of G defines an element of the cohomology group H3,π2). In fact, 2-groups are classified in this way: given a group π1, an abelian group π2, a group action of π1 on π2, and an element of H3, there is a unique 2-group G with π1 isomorphic to π1, π2 isomorphic to π2, and the other data corresponding. The element of H3 associated to a 2-group is sometimes called its Sinh invariant, as it was developed by Grothendieck's student Hoàng Xuân Sính.
Fundamental 2-group
Given a topological spaceX and a point x in that space, there is a fundamental 2-group of X at x, written Π2. As a monoidal category, the objects are loops at x, with multiplication given by concatenation, and the morphisms are basepoint-preserving homotopies between loops, with these morphisms identified if they are themselves homotopic. Conversely, given any 2-group G, one can find a unique pointedconnected space whose fundamental 2-group is G and whose homotopy groups πn are trivial for n>; 2. In this way, 2-groups classify pointed connected weak homotopy 2-types. This is a generalisation of the construction of Eilenberg–Mac Lane spaces. If X is a topological space with basepoint x, then the fundamental group of X at x is the same as the fundamental group of the fundamental 2-group of X at x; that is, This fact is the origin of the term "fundamental" in both of its 2-group instances. Similarly, Thus, both the first and second homotopy groups of a space are contained within its fundamental 2-group. As this 2-group also defines an action of π1 on π2 and an element of the cohomology group H3,π2), this is precisely the data needed to form the Postnikov tower of X if X is a pointed connected homotopy 2-type.
