S3 in S4

This article is about a particular subgroup in a group, up to equivalence of subgroups (i.e., an isomorphism of groups that induces the corresponding isomorphism of subgroups). The subgroup is (up to isomorphism) symmetric group:S3 and the group is (up to isomorphism) symmetric group:S4 (see subgroup structure of symmetric group:S4).
VIEW: Group-subgroup pairs with the same subgroup part | Group-subgroup pairs with the same group part | All pages on particular subgroups in groups

We consider the subgroup $H$ in the group $G$ defined as follows.

$G$ is the symmetric group of degree four, which, for concreteness, we take as the symmetric group on the set $\{ 1,2,3,4 \}$.

$H$ is the subgroup of $G$ comprising those permutations that fix $\{ 4 \}$. In particular, $H$ is the symmetric group on $\{ 1, 2,3 \}$, embedded naturally in $G$. It is isomorphic to symmetric group:S3. $H$ has order $6$.

There are three other conjugate subgroups to $H$ in $G$ (so the total conjugacy class size of subgroups is 4). The other subgroups are the subgroups fixing $\{ 1 \}$, $\{ 2 \}$, and $\{ 3 \}$ respectively.

The four conjugates are:

$\! H = H_4 = \{ (), (1,2), (1,3), (2,3), (1,2,3), (1,3,2) \}$

$\! H_1 = \{ (), (2,3), (3,4), (2,4), (2,3,4), (2,4,3) \}$

$\! H_2 = \{ (), (1,3), (3,4), (1,4), (1,3,4), (1,4,3) \}$

$\! H_3 = \{ (), (1,2), (2,4), (1,4), (1,2,4), (1,4,2) \}$

See also subgroup structure of symmetric group:S4.

Cosets

There are four left cosets and four right cosets of each subgroup. Each left coset of a subgroup is a right coset of one of its conjugate subgroups. This gives a total of 16 subsets.

The cosets are parametrized by ordered pairs $(i,j) \in \{ 1,2,3,4 \} \times \{ 1,2,3,4 \}$. The coset parametrized by $(i,j)$ is the set of all elements that send $i$ to $j$. This is a left coset of $H_i$ and a right coset of $H_j$.

Complements

There is a unique normal complement that is common to all the subgroups. This is the subgroup normal Klein four-subgroup of symmetric group:S4:

$\! K := \{ (), (1,2)(3,4), (1,3)(2,4), (1,4)(2,3) \}$

There is also a conjugacy class of subgroups each of which is a permutable complement to each of the $H_i$s. These are cyclic four-subgroups of symmetric group:S4, and these are:

$\{ (), (1,2,3,4), (1,3)(2,4), (1,4,3,2) \}, \qquad \{ (), (1,3,2,4), (1,2)(3,4), (1,4,2,3) \}, \qquad \{ (), (1,2,4,3), (1,4)(2,3), (1,3,4,2) \}$

Note that the fact that these are permutable complements can be understood as a special case of Cayley's theorem. See also every group of given order is a permutable complement for symmetric groups, which says that any finite group of order $n$ is, via the Cayley embedding, a permutable complement to $S_{n-1}$ in $S_n$.

Apart from these, each of the $H_i$s has a number of lattice complements:

• Any subgroup generated by double transposition in S4 is a lattice complement to each $H_i$ in the whole group. Thus, each $H_i$ has three such lattice complements.
• For each $H_i$, a subgroup of order three not contained in that $H_i$ is a lattice complement to it. Thus, each $H_i$ has three such lattice complements, because one of the four subgroups of order three is contained in that $H_i$.

Properties related to complementation

Property	Meaning	Satisfied?	Explanation	Comment
retract	has a normal complement	Yes	subgroup $K$ above is a normal complement
permutably complemented subgroup	has a permutable complement	Yes	normal complement is permutable complement too
lattice-complemented subgroup	has a lattice complement	Yes	normal complement is lattice complement too
complemented normal subgroup	normal subgroup with permutable complement	No	not normal itself

Arithmetic functions

Function	Value	Explanation
order of whole group	24
order of subgroup	6
index of subgroup	4
size of conjugacy class of subgroup (=index of normalizer)	4	see above for list of conjugates
number of conjugacy classes in automorphism class of subgroup	1	the whole group is a complete group, so the conjugation actions are precisely the automorphisms.
size of automorphism class of subgroup	4	same as size of conjugacy class