Tensor product of groups
Contents
History
The concept of tensor product of (not necessarily abelian) groups was introduced by Loday and Brown in their paper on van Kampen's theorem.
Definition
Direct definition
Suppose and are (not necessarily abelian) groups with a compatible pair of actions and . The tensor product (sometimes called the nonabelian tensor product) of and for this pair of actions, denoted , is defined as the quotient group of the free group on the set of symbols by the following relations:
 , one such relation for all
 , one such relation for all
If we use to denote and the actions by conjugation of the groups on themselves, this becomes:
Definition in terms of crossed pairings
Suppose and are (not necessarily abelian) groups with a compatible pair of actions and . The tensor product (sometimes called the nonabelian tensor product) of and for this pair of actions, denoted , is defined as the universal (initial) object among crossed pairings of and for the compatible pair of actions. Explicitly, is a group along with a set map such that, for any group , if we consider the mapping:
Then this mapping establishes a bijection from the set of group homomorphisms to the set of crossed pairings from to .
This latter definition is very useful in terms of showing that certain maps from the tensor product, defined on generators, are indeed welldefined homomorphisms. The idea is to show that the rule used to do the map on generators defines a crossed pairing.
Facts
Maps and constructions
For the statements in these facts, we will use the same notation as in the definition above: and are groups with a compatible pair of actions and .
Name  The kind of map or construction  Explicit description using named actions  Explicit description using 

Tensor product of groups is commutative up to natural isomorphism  A natural isomorphism . If the isomorphism is applied twice, it gives the identity mapping.  
Tensor product of groups maps to both groups  Homomorphisms: 


Group acts naturally on its tensor product with any group  Homomorphisms 
The action of on gives The action of on gives 

Other facts
 Tensor product of groups has a crossed module structure with respect to each group: This says that the homomorphism and the group action of on together make a crossed module over .
 Tensor product of finite groups is finite
 Tensor product of pgroups is pgroup
Particular cases
 In case that both are subgroups in some big group and they normalize each other, we can take the actions on each other as action by conjugation. These actions form a compatible pair of actions, hence it makes sense to take the tensor product of the two groups.
 In case that both the groups are abelian groups and both the maps are trivial maps (i.e., each group acts trivially on the other), this becomes the usual tensor product of abelian groups.
 In case that the actions of the groups on each other are trivial, but the groups are not necessarily abelian, the tensor product is the tensor product of abelian groups of the abelianizations of the two groups. In other words, with trivial actions, the tensor product cares only about the abelianizations.
References
Journal references
Original use
 Van Kampen theorems for diagrams of spaces by Ronald Brown and JeanLouis Loday, Topology, Volume 26,Number 3, Page 311  335(Year 1987): ^{ungated copy (PDF)}^{More info}: Definition introduced in Section 2, on Page 4 (Page 314 in the original print version).
Subsequent uses
 The nonabelian tensor product of groups: computations and structural results, the Ph.D. thesis of Aidan McDermott.^{More info}
 On the nilpotent multipliers of a group by John Burns and Graham Ellis, Math. Zeitschr., Volume 226, Page 405  428(Year 1997): ^{Official page (PDF downloadable)}^{More info}