Free group
This article is about a standard (though not very rudimentary) definition in group theory. The article text may, however, contain more than just the basic definition
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Definition
No. | Shorthand | A group is termed free if ... | A group is termed free if ... |
---|---|---|---|
1 | free product of copies of integers | it is the internal free product of (possibly infinitely many) groups, each of which is isomorphic to the group of integers. Hence, it is also isomorphic to the external free product of copies of the group of integers. | there are subgroups of such that is the internal free product of the s and each is isomorphic to , the group of integers. |
2 | freely generating set, in terms of unique reduced words | there is a generating set for the group such that every element of the group can uniquely be expressed as a reduced word in terms of the elements of the generating set (and their inverses), with the multiplication being by concatenation of words. | there is a generating set for such that any can be uniquely expressed as a reduced word in (that is, a product of elements from and their inverses, with no letter occurring adjacent to its inverse). |
3 | freely generating set, in terms of universal property | there is a subset of the group, such that any set-theoretic map from that subset to any target group, lifts uniquely to a group homomorphism from the whole group to the target group. | there is a subset of such that given any set-theoretic map from to a group , there is a unique group homomorphism from to whose restriction to is . |
4 | projective object in the category of groups | any surjective homomorphism from another group to it splits, i.e., there is an injective homomorphism backward such that the one-way composite is the identity. | for any surjective homomorphism , there is an injective homomorphism such that for all . |
Note that the notions of freely generating set described in formulations (2) and (3) of the definition are equivalent.
Equivalence of definitions
Further information: Equivalence of definitions of free group
Formalisms
Category-theoretic formulation
We can consider the free group functor: the functor that associates to any set, the group generated freely by that set. This is a functor because any map of sets gives rise to a map of the corresponding free groups.
The free group functor can be defined as the left adjoint to the forgetful functor from groups to sets. In other words, if denotes the forgetful functor from groups to sets (that sends a group to its underlying set) and denotes the free group functor, then for any set and group , there is a natural isomorphism of sets:
where the left side is the set of group homomorphisms and the right set is the set of set homomorphisms (i.e., all the set-theoretic maps).
Examples
- The free group on the empty set is the trivial group (this isn't typically considered a free group).
- The free group on a set of size one is isomorphic to the group of integers , i.e., it is infinite cyclic. it is the only Abelian nontrivial free group).
- The free group on a set of size two is an important free group. It is non-Abelian, finitely generated, and is SQ-universal: every finitely generated group is a subquotient of this group
Relation with other properties
Stronger properties
Group properties stronger than the property of being free are:
Property | Meaning | Proof of implication | Proof of strictness (reverse implication failure) | Intermediate notions |
---|---|---|---|---|
Finitely generated free group | both free and a finitely generated group, or equivalently, free on a finite generating set | |FULL LIST, MORE INFO |
Weaker properties
Incomparable properties
Property | Meaning | Proof of one non-implication | Proof of other non-implication | Properties stronger than both | Properties weaker than both |
---|---|---|---|---|---|
Free abelian group | free object in variety of abelian groups | free groups of rank 2 or more are not abelian, hence definitely not free abelian. | free abelian groups that are not cyclic cannot be free because if the group is free and abelian, it must have rank at most one and hence be cyclic | Only two such groups: trivial group and group of integers | Group in which every fully invariant subgroup is verbal, Reduced free group|FULL LIST, MORE INFO |
Complete group | centerless, every automorphism is inner; equivalently, injective in the category of groups | Only the trivial group has both properties. | |FULL LIST, MORE INFO |
Facts
The cardinalities of any two freely generating sets of the same free group are equal. This result actually follows from the fact that the corresponding result is true for free Abelian groups.
For full proof, refer: Free groups satisfy IBN
This cardinality is termed the rank of the free group. It is further clear that any two free groups of the same rank are isomorphic.
Metaproperties
Metaproperty name | Satisfied? | Proof | Statement with symbols |
---|---|---|---|
Subgroup-closed group property | Yes | freeness is subgroup-closed | If is a free group, and is a subgroup of , then is also a free group. |
Quotient-closed group property | No | every group is a quotient of a free group | A quotient of a free group need not be free. In fact, for any group , there is a free group having as a quotient. |
Finite-direct product-closed group property | No | We can have free groups and such that is not free (in fact, if both are nontrivial, the direct product is definitely not free). | |
Free product-closed group property | Yes | free product of free groups is free | If are free groups, so is their free product. |