Element structure of alternating group:A4

This article gives specific information, namely, element structure, about a particular group, namely: alternating group:A4.
View element structure of particular groups | View other specific information about alternating group:A4

This article gives information on the element structure of alternating group:A4.

See also element structure of alternating groups and element structure of symmetric group:S4.

The multiplication table (to be completed) is:

Element	$()$	$(1,2)(3,4)$	$(1,3)(2,4)$	$(1,4)(2,3)$	$(1,2,3)$	$(1,3,2)$	$(1,2,4)$	$(1,4,2)$	$(1,3,4)$	$(1,4,3)$	$(2,3,4)$	$(2,4,3)$
$()$	$()$	$(1,2)(3,4)$	$(1,3)(2,4)$	$(1,4)(2,3)$	$(1,2,3)$	$(1,3,2)$	$(1,2,4)$	$(1,4,2)$	$(1,3,4)$	$(1,4,3)$	$(2,3,4)$	$(2,4,3)$
$(1,2)(3,4)$	$(1,2)(3,4)$	$()$	$(1,4)(2,3)$	$(1,3)(2,4)$	$(2,4,3)$	$(1,4,3)$	$(2,3,4)$	$(1,3,4)$	$(1,4,2)$	$(1,3,2)$	$(1,2,4)$	$(1,2,3)$
$(1,3)(2,4)$	$(1,3)(2,4)$	$(1,4)(2,3)$	$()$	$(1,2)(3,4)$	$(1,4,2)$	$(2,3,4)$	$(1,4,3)$	$(1,2,3)$	$(2,4,3)$	$(1,2,4)$	$(1,3,2)$	$(1,3,4)$
$(1,4)(2,3)$	$(1,4)(2,3)$	$(1,2)(3,4)$	$(1,2)(3,4)$	$()$	$(1,3,4)$	$(1,2,4)$	$(1,3,2)$	$(2,4,3)$	$(1,2,3)$	$(2,3,4)$	$(1,4,3)$	$(1,4,2)$

Family contexts

Family name	Parameter values	General discussion of element structure of family
alternating group	degree $n=4$	element structure of alternating groups
projective special linear group of degree two over a finite field	field:F3, i.e., the group is $PSL(2,3)$	element structure of projective special linear group of degree two over a finite field
general affine group of degree one over a finite field	field:F4, i.e., the group is $GA(1,4)$	element structure of general affine group of degree one over a finite field

COMPARE AND CONTRAST: View element structure of groups of order 12 to compare and contrast the element structure with other groups of order 12.

Elements

Multiple ways of describing permutations

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Order computation

The alternating group of degree four has order 12, with prime factorization $12=2^2\cdot 3^1=4\cdot 3$. Below are listed various methods that can be used to compute the order, all of which should give the answer 12:

Family	Parameter values	Formula for order of a group in the family	Proof or justification of formula	Evaluation at parameter values	Full interpretation of conjugacy class structure
alternating group $A_n$ of degree $n$	degree $n=4$	$n!/2$	See alternating group, element structure of alternating groups	$4!/2=3\cdot 2\cdot 1/2=12$	#Interpretation as alternating group
projective special linear group of degree two over a finite field of size $q$	$(q^3-q)/2=q(q-1)(q+1)/2$ for $q$ odd $q^3-q=q(q-1)(q+1)$ for $q$ a power of 2	See order formulas for linear groups of degree two, order formulas for linear groups, and projective special linear group of degree two	$q=3$, i.e., field:F3, so the group is $PSL(2,3)$	$(3^3-3)/2=(27-3)/2=12$ Factored version: $3(3-1)(3+1)/2=3(2)(4)/2=12$	#Interpretation as projective special linear group of degree two
general affine group of degree one over a finite field of size $q$	See general affine group of degree one	$q(q-1)$	field size $q=4$, i.e., field:F4, so the group is $GA(1,4)$	$4(4-1)=4(3)=12$	#Interpretation as general affine group of degree one

Conjugacy class structure

FACTS TO CHECK AGAINST FOR CONJUGACY CLASS SIZES AND STRUCTURE:
Divisibility facts: size of conjugacy class divides order of group | size of conjugacy class divides index of center | size of conjugacy class equals index of centralizer
Bounding facts: size of conjugacy class is bounded by order of derived subgroup
Counting facts: number of conjugacy classes equals number of irreducible representations | class equation of a group

Interpretation as alternating group

FACTS TO CHECK AGAINST SPECIFICALLY FOR SYMMETRIC GROUPS AND ALTERNATING GROUPS:
Please read element structure of symmetric groups for a summary description.
Conjugacy class parametrization: cycle type determines conjugacy class (in symmetric group)
Conjugacy class sizes: conjugacy class size formula in symmetric group
Other facts: even permutation (definition) -- the alternating group is the set of even permutations | splitting criterion for conjugacy classes in the alternating group (from symmetric group)| criterion for element of alternating group to be real

For a symmetric group, cycle type determines conjugacy class. The statement is almost true for the alternating group, except for the fact that some conjugacy classes of even permutations in the symmetric group split into two in the alternating group, as per the splitting criterion for conjugacy classes in the alternating group, which says that a conjugacy class of even permutations splits in the alternating group if and only if its cycle decomposition comprises odd cycles of distinct length.

Here are the unsplit conjugacy classes:

Partition	Verbal description of cycle type	Elements with the cycle type	Size of conjugacy class	Formula for size	Element order
1 + 1 + 1 + 1	four cycles of size one each, i.e., four fixed points	$()$ -- the identity element	1	$\frac{4!}{(1{)}^4(4!)}$	1
2 + 2	double transposition: two cycles of size two	$(1,2)(3,4)$, $(1,3)(2,4)$, $(1,4)(2,3)$	3	$\frac{4!}{(2{)}^2(2!)}$	2
Total	--	$()$, $(1,2)(3,4)$, $(1,3)(2,4)$ and $(1,4)(2,3)$	4	NA	NA

In this case, the union of the unsplit conjugacy classes is a proper normal subgroup isomorphic to the Klein four-group. Note that this phenomenon is unique to the case $n=4$.

Here is the split conjugacy class:

Partition	Verbal description of cycle type	Elements with the cycle type	Combined size of conjugacy classes	Formula for combined size	Size of each half	First split half	Second split half	Real?	Rational?	Element order
3 + 1	one 3-cycle, one fixed point	$(1,2,3)$, $(1,3,2)$, $(2,3,4)$, $(2,4,3)$, $(3,4,1)$, $(3,1,4)$, $(4,1,2)$, $(4,2,1)$	8	$\frac{4!}{(3)(1)}$	4	$(1,2,3)$, $(4,2,1)$, $(2,4,3)$, $(3,4,1)$	$(1,3,2)$, $(4,1,2)$, $(2,3,4)$, $(3,1,4)$	No	No	3

Interpretation as projective special linear group of degree two

Compare with element structure of projective special linear group of degree two over a finite field#Conjugacy class structure

We consider the group as $PSL(2,q)$, $q=3$. We use the letter $q$ to denote the generic case of $q\equiv 3(\mathrm{mod}4)$.

Nature of conjugacy class upstairs in $S{L}_2$	Eigenvalues	Characteristic polynomial	Minimal polynomial	Size of conjugacy class (generic $q$ that is 3 mod 4)	Size of conjugacy class ($q=3$)	Number of such conjugacy classes (generic $q$ that is 3 mod 4)	Number of such conjugacy classes ($q=3$)	Total number of elements (generic $q$ that is 3 mod 4)	Total number of elements ($q=3$)	Representatives as permutations
Diagonalizable over ${\mathbb{F}}_q$ with equal diagonal entries, hence a scalar	$\{1,1\}$ or $\{-1,-1\}$, both correspond to the same element	$(x-a{)}^2$ where $a\in \{-1,1\}$	$x-a$ where $a\in \{-1,1\}$	1	1	1	1	1	1	$()$
Diagonalizable over ${\mathbb{F}}_{q^2}$, not over ${\mathbb{F}}_q$, eigenvalues square roots of $-1$	Square roots of $-1$	$x^2+1$	$x^2+1$	$q(q-1)/2$	3	1	1	$q(q-1)/2$	3	$(1,2)(3,4)$
Not diagonal, has Jordan block of size two	$1$ (multiplicity 2) or $-1$ (multiplicity 2). Each conjugacy class has one representative of each type.	$(x-a{)}^2$ where $a\in \{-1,1\}$	$x-a$ where $a\in \{-1,1\}$	$(q^2-1)/2$	4	2	2	$q^2-1$	8	$(1,2,3)$ and $(1,3,2)$
Diagonalizable over ${\mathbb{F}}_{q^2}$, not over ${\mathbb{F}}_q$. Must necessarily have no repeated eigenvalues. Eigenvalues not square roots of $-1$.	Pair of conjugate elements of ${\mathbb{F}}_{q^2}$ of norm 1, not square roots of -1. Each pair identified with its negative pair.	$x^2-ax+1$, $a\ne 0$ irreducible; note that $x^2-ax+1$'s pair and $x^2+ax+1$'s pair get identified.	Same as characteristic polynomial	$q(q-1)$	6	$(q-3)/4$	0	$q(q-1)(q-3)/4$	0	--
Diagonalizable over ${\mathbb{F}}_q$ with distinct (and hence mutually inverse) diagonal entries	$\lambda ,1/\lambda$ where $\lambda \in {\mathbb{F}}_q\setminus \{0,1,-1,i,-i\}$ where $i,-i$ are square roots of $-1$. Note that the representative pairs $\{\lambda ,1/\lambda \}$ and $\{-\lambda ,-1/\lambda \}$ get identified.	$x^2-(\lambda +1/\lambda )x+1$, again with identification.	$x^2-(\lambda +1/\lambda )x+1$, again with identification.	$q(q+1)$	12	$(q-3)/4$	0	$q(q+1)(q-3)/4$	0	--
Total	NA	NA	NA	NA	NA	$(q+5)/2$	4	$(q^3-q)/2$	12	NA

Interpretation as general affine group of degree one

Compare with element structure of general affine group of degree one over a finite field#Conjugacy class structure

The alternating group of degree four is isomorphic to the general affine group of degree one over field:F4. All the elements of this group are of the form:

$x\mapsto ax+v,a\in {\mathbb{F}}_q^{\ast },v\in {\mathbb{F}}_q$

where $q=4$. Below, we interpret the conjugacy classes of the group in these terms:

Nature of conjugacy class	Size of conjugacy class (generic $q$)	Size of conjugacy class ($q=4$)	Number of such conjugacy classes (generic $q$)	Number of such conjugacy classes ($q=4$)	Total number of elements (generic $q$)	Total number of elements ($q=4$)	Representatives of conjugacy classes as permutations
$a=1,v=0$	1	1	1	1	1	1	$()$
$a=1,v\ne 0$ (conjugacy class is independent of choice of $v$)	$q-1$	3	1	1	$q-1$	3	$(1,2)(3,4)$
$a\ne 1$ (conjugacy class is determined completely by choice of $a$ and is independent of choice of $v$; in other words, each conjugacy class is a coset of the subgroup of translations)	$q$	4	$q-2$	2	$q(q-2)$	8	$(1,2,3)$ and $(1,3,2)$
Total (--)	--	--	$q$	4	$q(q-1)$	12	--