This article is about a basic definition in group theory. The article text may, however, contain advanced material.
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For a finite set
Let be a finite set. The alternating group on is defined in the following equivalent ways:
- It is the group of all even permutations on under composition. An even permutation is a permutation whose cycle decomposition has an even number of cycles of even size. Specifically, the alternating group on is the subgroup of the symmetric group on comprising the even permutations.
- It is the kernel of the sign homomorphism from the symmetric group on to the group .
For having size zero or one, the alternating group on equals the whole symmetric group on . For having size at least two, the alternating group on is the unique subgroup of index two in the symmetric group on .
The alternating group on a set of size is denoted and is termed the alternating group of degree .
For an infinite set
Let be an infinite set. The finitary alternating group on is defined in the following equivalent ways:
- It is the group of all even permutations on under composition.
- It is the kernel of the sign homomorphism on the finitary symmetric group on .
- The alternating group on a set of size five or more is simple. Also, the finitary alternating group on an infinite set is simple. For full proof, refer: A5 is simple, alternating groups are simple
- Projective special linear group equals alternating group in only finitely many cases
Here, is the degree of the alternating group, i.e., the size of the set it acts on.
|order||for , for||It has index two in the symmetric group of degree .|
|exponent||(skipping over ) if is odd, if is even.||A -cycle can occur as a cycle in the cycle decomposition of an even permutation if either is odd with (in which case we can use that cycle itself as the even permutation) or is even with (in which case we can take the even permutation as that cycle times a disjoint 2-cycle). The exponent is the lcm of possible cycle sizes that can occur, so the above method gives that the lcm is over all the odd numbers less than or equal to , and all the even numbers less than or equal to . Working separately the cases of odd and even gives the conclusion.|
|nilpotency class||for , undefined for||abelian group for , nilpotent group for .|
|derived length||for , for , undefined for||abelian for , simple for .|
|Frattini length||for , for||Frattini-free group: intersection of maximal subgroups is trivial.|
|Fitting length||for , for , undefined for||abelian for , simple for .|
|minimum size of generating set||2|
|subgroup rank||At most|
|chief length||for , for , for .|
|composition length||for , for , for .|
|Abelian group||Yes for , no for||and don't commute.|
|Nilpotent group||Yes for , no for||is centerless.|
|Solvable group||Yes for , no for||alternating groups are simple for degree five or more.|
|Supersolvable group||Yes for , no for|
|Simple group||Yes for and , no for .|
|Rational group||No for all .|
|Ambivalent group||Yes for , no otherwise||See classification of ambivalent alternating groups.|