Linear representation theory of alternating groups
This article gives specific information, namely, linear representation theory, about a family of groups, namely: alternating group.
View linear representation theory of group families | View other specific information about alternating group
This article discusses the linear representation theory of the alternating group of finite degree
. Note that for
, the alternating group coincides with the symmetric group, and for
, it is trivial, so the interesting behavior begins from
.
The article builds heavily on the linear representation theory of symmetric groups.
See also element structure of alternating groups.
Particular cases
![]() |
Alternating group | Order | Degrees of irreducible representations | Number of irreducible representations | Linear representation theory page |
---|---|---|---|---|---|
3 | cyclic group:Z3 | 3 | 1,1,1 | 3 | linear representation theory of cyclic group:Z3 |
4 | alternating group:A4 | 12 | 1,1,1,3 | 4 | linear representation theory of alternating group:A4 |
5 | alternating group:A5 | 60 | 1,3,3,4,5 | 5 | linear representation theory of alternating group:A5 |
6 | alternating group:A6 | 360 | 1,5,5,8,8,9,10 | 7 | linear representation theory of alternating group:A6 |
7 | alternating group:A7 | 2520 | 1,6,10,10,14,14,15,21,35 | 9 | linear representation theory of alternating group:A7 |
8 | alternating group:A8 | 20160 | 1,7,14,20,21,21,21,28,35,45,45,56,64,70 | 14 | linear representation theory of alternating group:A8 |
9 | alternating group:A9 | 181440 | [SHOW MORE] | 18 | linear representation theory of alternating group:A9 |
10 | alternating group:A10 | 1814400 | [SHOW MORE] | 24 | linear representation theory of alternating group:A10 |
Combinatorics
Split conjugacy classes
We have canonical bijections:
Conjugacy classes from that split in
Partitions of
into distinct odd parts (via splitting criterion)
Self-conjugate unordered integer partitions of
Irreducible representations of
that split in
For more on these bijections, see set of self-conjugate unordered integer partitions
Non-split conjugacy classes
We have equalities:
Number of conjugacy classes of even permutations in that don't split in
= Number of conjugacy classes of odd permutations in
= Number of (conjugate) pairs of non-self-conjugate partitions of
= Number of pairs (in the sense of restricting to the same thing on
) of irreducible representations of
that don't split in
.
Totals
If equals the number of conjugacy classes of even permutations from
that split in
, and
equals the number of conjugacy classes of even permutations in
that don't split i
, then:
- The number of conjugacy classes in
is
- The number of conjugacy classes in
is
![]() |
Alternating group ![]() |
Order | ![]() ![]() ![]() |
![]() ![]() ![]() |
![]() ![]() |
![]() ![]() |
![]() ![]() |
![]() ![]() |
![]() ![]() |
---|---|---|---|---|---|---|---|---|---|
3 | cyclic group:Z3 | 3 | 1 | 1 | 3 | 3 | 0 | 1 | 2 |
4 | alternating group:A4 | 12 | 1 | 2 | 4 | 5 | 0 | 2 | 3 |
5 | alternating group:A5 | 60 | 1 | 3 | 5 | 7 | 1 | 5 | 5 |
6 | alternating group:A6 | 360 | 1 | 5 | 7 | 11 | 1 | 7 | 7 |
7 | alternating group:A7 | 2520 | 1 | 7 | 9 | 15 | 0 | 7 | 8 |
8 | alternating group:A8 | 20160 | 2 | 10 | 14 | 22 | 0 | 10 | 12 |
9 | alternating group:A9 | 181440 | 2 | 14 | 18 | 30 | 1 | 16 | 17 |
10 | alternating group:A10 | 1814400 | 2 | 20 | 24 | 42 | 2 | 24 | 24 |
Splitting field
TERMINOLOGY AND FACTS TO CHECK AGAINST:
Terminology: field generated by character values (unique, cyclotomic) | minimal splitting field (not necessarily unique or cyclotomic) | splitting field | sufficiently large field (minimal choice is unique and cyclotomic)
Facts: sufficiently large implies splitting | splitting not implies sufficiently large | field generated by character values need not be a splitting field | field generated by character values is splitting field implies it is the unique minimal splitting field | minimal splitting field need not be unique | minimal splitting field need not be cyclotomic
Field generated by character values
Below is information on the field generated by character values for the alternating group of degree for small values of
, all in characteristic zero. It is the same as the unique minimal splitting field, because all the irreducible representations have Schur index one.
Note that for each irreducible representation, the extension over the rationals for its character values, if nontrivial, is a quadratic extension. The overall field generated by character values is thus a Galois extension of the rationals with Galois group an elementary abelian 2-group and extension degree a power of 2.
![]() |
Alternating group ![]() |
Field generated by character values =unique minimal splitting field | Degree of extension over rationals | Smallest ![]() ![]() |
Degree of cyclotomic extension over rationals | Galois group of extension over rationals |
---|---|---|---|---|---|---|
3 | cyclic group:Z3 | ![]() ![]() |
2 | 3 | 2 | cyclic group:Z2 |
4 | alternating group:A4 | ![]() ![]() |
2 | 3 | 2 | cyclic group:Z2 |
5 | alternating group:A5 | ![]() ![]() |
2 | 5 | 4 | cyclic group:Z2 |
6 | alternating group:A6 | ![]() ![]() |
2 | 5 | 4 | Klein four-group |
7 | alternating group:A7 | ![]() ![]() Same as ![]() |
2 | 7 | 6 | cyclic group:Z2 |
8 | alternating group:A8 | ![]() ![]() ![]() Same as ![]() |
4 | 105 | 48 | Klein four-group |
9 | alternating group:A9 | ![]() ![]() Same as ![]() |
2 | 15 | 8 | cyclic group:Z2 |
Criterion for the field generated by character values to be real
For a finite group, the field generated by character values is real if and only if the group is an ambivalent group, i.e., every element is conjugate to its inverse. For alternating groups, using the criterion for element of alternating group to be real, we can obtain a classification of ambivalent alternating groups, which basically gives us that the alternating group of degree is ambivalent iff
.