D8 in D16

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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) dihedral group:D8 and the group is (up to isomorphism) dihedral group:D16 (see subgroup structure of dihedral group:D16).
The subgroup is a normal subgroup and the quotient group is isomorphic to cyclic group:Z2.
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Here, G is the dihedral group:D16, the dihedral group of order sixteen (and hence, degree eight). We use here the presentation:

G := \langle a,x \mid a^8 = x^2 = e, xax = a^{-1} \rangle

G has 16 elements:

\! \{ e,a,a^2,a^3,a^4,a^5,a^6,a^7,x,ax,a^2x,a^3x,a^4x,a^5x,a^6x,a^7x \}

The subgroups H_1 and H_2 of interest are:

\!H_1 = \{ e,a^2,a^4,a^6,x,a^2x,a^4x,a^6x \}, \qquad H_2 = \{ e, a^2, a^4, a^6, ax, a^3x, a^5x, a^7x \}

Both subgroups are normal but are related by the outer automorphism class of a \mapsto a, x \mapsto ax.


Both subgroups has index two and are normal subgroup (See index two implies normal), so left cosets coincide with right cosets.

The cosets of H_1 are:

\! H_1 = \{ e,a^2,a^4,a^6,x,a^2x,a^4x,a^6x \}, G \setminus H_1 = \{ a, a^3, a^5, a^7, ax, a^3x, a^5x, a^7x \}

The cosets of H_2 are:

H_2 = \{ e, a^2, a^4, a^6, ax, a^3x, a^5x, a^7x \}, G \setminus H_2 = \{ a, a^3, a^5, a^7, x,a^2x,a^4x,a^6x \}

Arithmetic functions

Function Value Explanation
order of whole group 16
order of subgroup 8
index of subgroup 2
size of conjugacy class (=index of normalizer) 1
number of conjugacy classes in automorphism class 2

Subgroup properties

Invariance under automorphisms and endomorphisms

Property Meaning Satisfied? Explanation
normal subgroup invariant under inner automorphisms Yes index two implies normal
characteristic subgroup invariant under all automorphisms No The automorphism a \mapsto a, x \mapsto ax interchanges H_1 and H_2.
fully invariant subgroup invariant under all endomorphisms No (follows from not being characteristic)
isomorph-free subgroup no other isomorphic subgroup No The two subgroups H_1, H_2 are isomorphic.
isomorph-automorphic subgroup all isomorphic subgroups are automorphic subgroups Yes H_1,H_2 are the only subgroups isomorphic to dihedral group:D8.
potentially fully invariant subgroup fully invariant subgroup inside a possibly bigger group No See D8 is not potentially fully invariant in D16.