A group with a system of generating elements such that any mapping from into an arbitrary group can be extended to a homomorphism from into . Such a system is called a system of free generators; its cardinality is called the rank of . The set is also called an alphabet. The elements of are words over the alphabet , that is, expressions of the form
where , for all , and also the empty word. A word is called irreducible if for every . The irreducible words are different elements of a free group , and each word is equal to a unique irreducible word. The number is called the length of the word if is irreducible.
The Nielsen transformations of a finite ordered set of elements of a group are: 1) permutations of two elements of this set; 2) the replacement of one of the by ; and 3) the replacement of one of the by , where . If a free group has finite rank, then the Nielsen transformations over the system of free generators lead to new systems of free generators, and any system of free generators can be obtained from any other by successive application of these transformations (Nielsen's theorem, see ). The importance of free groups lies in the fact that every group is isomorphic to a quotient group of a suitable free group. Every subgroup of a free group is also free (the Nielsen–Schreier theorem, see , ).
A free group in a variety of groups is defined analogously to a free group, but within . It is also called a -free group, or a relatively-free group (and also a reduced free group). If is defined by a set of identities , where , then a free group of with a system of generators is isomorphic to the quotient group of the free group with system of generators by the verbal subgroup defined by , i.e. the subgroup generated by all elements of obtained by inserting in words elements of . Free groups of certain varieties have special names, for example, free Abelian, free nilpotent, free solvable, free Burnside; they are free groups of the varieties , , , , respectively.
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Free group. A.L. Shmel'kin (originator), Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Free_group&oldid=18729