2-group

A 2-group is a monoidal category G in which every morphism is invertible and every object has a weak inverse. (Here, a weak inverse of an object x is an object y such that xy and yx are both isomorphic to the unit object.)

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 (so that xy and yx are actually equal to the unit object).

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.

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 bicategory B and an object x of B, there is an automorphism 2-group of x in B, written Aut_B(x). 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 (so all objects and morphisms are weakly invertible) and x is its only object, then Aut_B(x) 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(G). (Note that even for a strict 2-group, the fundamental group will only be a quotient group of the underlying group.)

As a monoidal category, any 2-group G has a unit object I_G. The automorphism group of I_G is an abelian group by the Eckmann–Hilton argument, written Aut(I_G) or π₂(G).

The fundamental group of G acts on either side of π₂(G), and the associator of G (as a monoidal category) defines an element of the cohomology group H³(π₁(G),π₂(G)). 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(π₁,π₂), there is a unique (up to equivalence) 2-group G with π₁(G) isomorphic to π₁, π₂(G) isomorphic to π₂, and the other data corresponding.

The element of H³(π₁,π₂) associated to a 2-group is sometimes called its Sinh invariant, as it was developed by Grothendieck's student Hoàng Xuân Sính.

Given a topological space X and a point x in that space, there is a fundamental 2-group of X at x, written Π₂(X,x). 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 (up to weak homotopy equivalence) pointed connected space (X,x) whose fundamental 2-group is G and whose homotopy groups π_n 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,

${\displaystyle \pi _{1}(X,x)=\pi _{1}(\Pi _{2}(X,x)).\!}$

This fact is the origin of the term "fundamental" in both of its 2-group instances.

Similarly,

${\displaystyle \pi _{2}(X,x)=\pi _{2}(\Pi _{2}(X,x)).\!}$

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(X,x) on π₂(X,x) and an element of the cohomology group H³(π₁(X,x),π₂(X,x)), this is precisely the data needed to form the Postnikov tower of X if X is a pointed connected homotopy 2-type.

• N-group (category theory)
• Abelian 2-group

• John C. Baez and Aaron D. Lauda, Higher-dimensional algebra V: 2-groups, Theory and Applications of Categories 12 (2004), 423–491.
• John C. Baez and Danny Stevenson, The classifying space of a topological 2-group.
• R. Brown and P.J. Higgins, The classifying space of a crossed complex, Math. Proc. Camb. Phil. Soc. 110 (1991) 95-120.
• R. Brown, P.J. Higgins, R. Sivera, Nonabelian algebraic topology: filtered spaces, crossed complexes, cubical homotopy groupoids, EMS Tracts in Mathematics Vol. 15, 703 pages. (2011).
• Hendryk Pfeiffer, 2-Groups, trialgebras and their Hopf categories of representations, Adv. Math. 212 No. 1 (2007) 62–108.
• Hoàng Xuân Sính, Gr-catégories, thesis, 1975.

• Double groupoids and homotopy 2-types

• 2-group in nLab
• 2008 Workshop on Categorical Groups at the Centre de Recerca Matemàtica