Non-normal subgroups of dihedral group:D8

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) cyclic group:Z2 and the group is (up to isomorphism) dihedral group:D8 (see subgroup structure of dihedral group:D8).
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Definition

Suppose $G$ is the dihedral group of order eight (degree four) given by the presentation below, where $e$ denotes the identity element of $G$ :

$G : = ⟨ a, x ∣ a^{4} = x^{2} = e, x a x = a^{- 1} ⟩$ .

Then, we are interested in the following four subgroups:

$A_{0} : = ⟨ x ⟩ = {x, e}, A_{1} : = ⟨ a x ⟩ = {a x, e}, A_{2} : = ⟨ a^{2} x ⟩ = {a^{2} x, e}, A_{3} = ⟨ a^{3} x ⟩ = {a^{3} x, e}$ .

$A_{0}$ and $A_{2}$ are conjugate subgroups (via $a$ , for instance). $A_{1}$ and $A_{3}$ are conjugate subgroups (via $a$ , for instance). $A_{0}$ and $A_{1}$ are not conjugate but are related by an outer automorphism that fixes $a$ and sends $x$ to $a x$ . Thus, all four subgroups are automorphic subgroups. These are the only non-normal subgroups of $G$ and they are all 2-subnormal subgroups.

Arithmetic functions

Function	Value	Explanation
order of whole group	8
order of subgroup	2
index	2
size of conjugacy class	2
number of conjugacy classes in automorphism class	2
size of automorphism class	2
subnormal depth	2
hypernormalized depth	2

Effect of subgroup operators

Specific values (in the second column) are for $A_{0} = ⟨ x ⟩$ .

Function	Value as subgroup (descriptive)	Value as subgroup (link)	Value as group
normalizer	$⟨ a^{2}, x ⟩$	Klein four-subgroups of dihedral group:D8	Klein four-group
centralizer	$⟨ a^{2}, x ⟩$	Klein four-subgroups of dihedral group:D8	Klein four-group
normal core	${e}$	--	trivial group
normal closure	$⟨ a^{2}, x ⟩$	Klein four-subgroups of dihedral group:D8	Klein four-group
characteristic core	${e}$	--	trivial group
characteristic closure	$G$ , i.e., $⟨ a, x ⟩$	--	dihedral group:D8

Related subgroups

Intermediate subgroups

We use $A_{0} = ⟨ x ⟩$ here.

Value of intermediate subgroup (descriptive)	Isomorphism class of intermediate subgroup	Small subgroup in intermediate subgroup	Intermediate subgroup in big group
$⟨ a^{2}, x ⟩$	Klein four-group	Z2 in V4	Klein four-subgroups of dihedral group:D8

Subgroup properties

Invariance under automorphisms and endomorphisms

Suppose $c_{a}$ and $c_{x}$ denote conjugation by $a$ and $x$ respectively. Let $σ$ denote the automorphism that sends $a$ to $a^{3}$ and $x$ to $a x$ . Then, $⟨ c_{a}, c_{x} ⟩$ is the inner automorphism group and $⟨ c_{a}, c_{x}, σ ⟩$ is the automorphism group.

The automorphism $c_{x}$ fixes $A_{0}$ and $A_{2}$ while interchanging $A_{1}$ and $A_{3}$ . The automorphism $c_{a}$ interchanges $A_{0}$ and $A_{2}$ while also interchanging $A_{1}$ and $A_{3}$ . The automorphism $c_{a x} = c_{a} \circ c_{x}$ fixes $A_{1}$ and $A_{3}$ while interchanging $A_{0}$ and $A_{2}$ . The automorphism $σ$ interchanges $A_{0}$ and $A_{1}$ and also interchanges $A_{2}$ and $A_{3}$ .

Property	Meaning	Satisfied?	Explanation
normal subgroup	invariant under inner automorphisms	No	See above description of conjugation automorphisms that permute the subgroups
coprime automorphism-invariant subgroup	invariant under automorphisms of coprime order to group	Yes	there are no nontrivial automorphisms of coprime order
cofactorial automorphism-invariant subgroup	invariant under all automorphisms whose order has prime factors only among those of the group	No	follows from not being normal
2-subnormal subgroup	normal subgroup of normal subgroup	Yes	normal inside Klein four-subgroups of dihedral group:D8 (of the form $⟨ a^{2}, x ⟩$ and $⟨ a^{2}, a x ⟩$ ) that are normal in the whole group.
subnormal subgroup		Yes	follows from being 2-subnormal, also from being subgroup of nilpotent group.