Dihedral group:D8

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Definition

Definition by presentation

The dihedral group D_8, sometimes called D_4, also called the dihedral group of order eight or the dihedral group of degree four (since its natural action is on four elements), is defined by the following presentation:

\langle x,a \mid a^4 = x^2 = e, xax^{-1} = a^{-1}\rangle

Here, the element a is termed the rotation or the generator of the cyclic piece and x is termed the reflection".

Geometric definition

The dihedral group D_8 (also called D_4) is defined as the group of all symmetries of the square (the regular 4-gon). This has a cyclic subgroup comprising rotations (which is the cyclic subgroup generated by a) and has four reflections each being an involution: reflections about lines joining midpoints of opposite sides, and reflections about diagonals.

Definition as a permutation group

Further information: D8 in S4

The group is (up to isomorphism) the subgroup of the symmetric group on \{ 1,2,3,4 \} given by:

\! \{ (), (1,2,3,4), (1,3)(2,4), (1,4,3,2), (1,3), (2,4), (1,4)(2,3), (1,2)(3,4) \}

This can be related to the geometric definition by thinking of 1,2,3,4 as the vertices of the square and considering an element of D_8 in terms of its induced action on the vertices. It relates to the presentation via setting a = (1,2,3,4) and x = (1,3).

Multiplication table

Here, e denotes the identity element, a is an element of order 4, and x is an element of order two that isn't equal to a^2, as in the above presentation.


Element \! e \! a \! a^2 \! a^3 \! x \! ax \! a^2x \! a^3x
\! e \! e \! a \! a^2 \! a^3 \! x \! ax \! a^2x \! a^3x
\! a \! a \! a^2 \! a^3 \! e \! ax \! a^2x \! a^3x \! x
\! a^2 \! a^2 \! a^3 \! e \! a \! a^2x \! a^3x \! x \! ax
\! a^3 \! a^3 \! e \! a \! a^2 \! a^3x \! x \! ax \! a^2x
\! x \! x \! a^3x \! a^2x \! ax \! e \! a^3 \! a^2 \! a
\! ax \! ax \! x \! a^3x \! a^2x \! a \! e \! a^3 \! a^2
\! a^2x \! a^2x \! ax \! x \! a^3x \! a^2 \! a \! e \! a^3
\! a^3x \! a^3x \! a^2x \! ax \! x \! a^3 \! a^2 \! a \! e


Other definitions

The dihedral group can be described in the following ways:

  1. The dihedral group of order eight.
  2. The generalized dihedral group corresponding to the cyclic group of order four.
  3. The holomorph of the cyclic group of order four.
  4. The external wreath product of the cyclic group of order two with the cyclic group of order two, acting via the regular action.
  5. The 2-Sylow subgroup of the symmetric group on four letters.
  6. The 2-Sylow subgroup of the symmetric group on five letters.
  7. The 2-Sylow subgroup of the alternating group on six letters.
  8. The 2-Sylow subgroup of PSL(3,2).

Position in classifications

Type of classification Name in that classification
GAP ID (8,3), i.e., the third among the groups of order 8
Hall-Senior number 4 among groups of order 8
Hall-Senior symbol 8\Gamma_2a_1

Elements

Further information: element structure of dihedral group:D8


Below, we list all the elements, also giving the interpretation of each element under the geometric description of the dihedral group as the symmetries of a 4-gon, and for the corresponding permutation representation (see D8 in S4). Note that for different conventions, one can obtain somewhat different correspondences, so this may not match up with other correspondences elsewhere. Note that the descriptions below assume the left action convention for functions and the corresponding convention for composition, and hence some of the entries may become different if you adopt the right action convention.:

Element in terms of a and x Geometric description Permutation on vertices Order of the element
e (identity element) does nothing, i.e., leaves the square invariant () 1
a rotation by angle of \pi/2 (i.e., 90\,^\circ) counterclockwise (1,2,3,4) 4
a^2 rotation by angle of \pi (i.e., 180\,^\circ), also called a half turn (1,3)(2,4) 2
a^3 rotation by angle of 3\pi/2 (i.e., 270\,^\circ) counter-clockwise, or equivalently, by \pi/2 (i.e., 90\,^\circ) clockwise (1,4,3,2) 4
x reflection about the diagonal joining vertices "2" and "4" (1,3) 2
ax = xa^3 reflection about the line joining midpoints of opposite sides "14" and "23" (1,4)(2,3) 2
a^2x reflection about the diagonal joining vertices "1" and "3" (2,4) 2
a^3x = xa reflection about the line joining midpoints of opposite sides "12" and "34" (1,2)(3,4) 2


Below is the conjugacy and automorphism class structure:


Conjugacy class in terms of a,x Geometric description of conjugacy class Conjugacy class as permutations Size of conjugacy class Order of elements in conjugacy class Centralizer of first element of class
\! \{ e \} identity element, does nothing \{ () \} 1 1 whole group
\! \{ a^2 \} half turn, rotation by \pi \{ (1,3)(2,4) \} 1 2 whole group
\! \{ x,a^2x \} reflections about diagonals \{ (1,3), (2,4) \} 2 2 \{ e, a^2, x, a^2x \} -- one of the Klein four-subgroups of dihedral group:D8
\! \{ ax, a^3x \} reflections about lines joining midpoints of opposite sides \{ (1,4)(2,3)\ , \ (1,2)(3,4) \} 2 2 \{ e, a^2, ax, a^3x \} -- one of the Klein four-subgroups of dihedral group:D8
\! \{ a, a^3 \} rotations by odd multiples of \pi/2 \{ (1,2,3,4) \ ,\  (1,4,3,2) \} 2 4 \{ e, a, a^2, a^3 \} -- the cyclic maximal subgroup of dihedral group:D8
Total (5) -- -- 8 -- --

The equivalence classes up to automorphisms are:

Equivalence class under automorphisms in terms of a,x Geometric description of equivalence class Equivalence class as permutations Size of equivalence class Number of conjugacy classes in it Size of each conjugacy class
\! \{ e \} identity element, does nothing \{ () \} 1 1 1
\! \{ a^2 \} half turn \{ (1,3)(2,4) \} 1 1 1
\! \{ x, ax, a^2x, a^3x \} reflections \{ (1,3)\ ,\  (2,4)\ , \ (1,4)(2,3)\ ,\  (1,2)(3,4) \} 4 2 2
\! \{ a, a^3 \} rotations by odd multiples of \pi/2 \{ (1,2,3,4)\ ,\  (1,4,3,2) \} 2 1 2
Total (4) -- -- 8 5 --


Arithmetic functions

Want to compare and contrast arithmetic function values with other groups of the same order? Check out groups of order 8#Arithmetic functions
Function Value Similar groups Explanation for function value
underlying prime of p-group 2
order (number of elements, equivalently, cardinality or size of underlying set) 8 groups with same order As a semidirect product of \Z_4 and \Z_2: the order is the product of the orders of \Z_4 and \Z_2, which is 4 \times 2 = 8

As a wreath product of \Z_2 and \Z_2: the order is 2^2 \cdot 2 = 8
prime-base logarithm of order 3 groups with same prime-base logarithm of order
max-length of a group 3 max-length of a group equals prime-base logarithm of order for group of prime power order
chief length 3 chief length equals prime-base logarithm of order for group of prime power order
composition length 3 composition length equals prime-base logarithm of order for group of prime power order
exponent 4 groups with same order and exponent | groups with same exponent As a dihedral group: the dihedral group of order 2n has exponent equal to \operatorname{lcm} \{ n,2 \}.
prime-base logarithm of exponent 2
nilpotency class 2 groups with same order and nilpotency class | groups with same prime-base logarithm of order and nilpotency class | groups with same nilpotency class
derived length 2 groups with same order and derived length | groups with same prime-base logarithm of order and derived length | groups with same derived length
Frattini length 2 groups with same order and Frattini length | groups with same prime-base logarithm of order and Frattini length | groups with same Frattini length
Fitting length 1 All groups of prime power order are nilpotent, hence have Fitting length 1.
minimum size of generating set 2 groups with same order and minimum size of generating set | groups with same prime-base logarithm of order and minimum size of generating set | groups with same minimum size of generating set Generator of cyclic subgroup of order four and element of order two outside.
subgroup rank of a group 2 groups with same order and subgroup rank of a group | groups with same prime-base logarithm of order and subgroup rank of a group | groups with same subgroup rank of a group All proper subgroups are cyclic or Klein four-groups.
rank of a p-group 2 groups with same order and rank of a p-group | groups with same prime-base logarithm of order and rank of a p-group | groups with same rank of a p-group There exist Klein four-subgroups.
normal rank of a p-group 2 groups with same order and normal rank of a p-group | groups with same prime-base logarithm of order and normal rank of a p-group | groups with same normal rank of a p-group There exist normal Klein four-subgroups.
characteristic rank of a p-group 1 groups with same order and characteristic rank of a p-group | groups with same prime-base logarithm of order and characteristic rank of a p-group | groups with same characteristic rank of a p-group All abelian characteristic subgroups are cyclic.

Arithmetic functions of a counting nature

Function Value Similar groups Explanation
number of subgroups 10 groups with same order and number of subgroups | groups with same prime-base logarithm of order and number of subgroups | groups with same number of subgroups As a dihedral group \! D_{2n}, n = 4 number of subgroups is \! d(n) + \sigma(n) = d(4) + \sigma(4) = 3 + 7 = 10, where d is the divisor count function and \sigma is the divisor sum function. See subgroup structure of dihedral group:D8, subgroup structure of dihedral groups
number of conjugacy classes 5 groups with same order and number of conjugacy classes | groups with same prime-base logarithm of order and number of conjugacy classes | groups with same number of conjugacy classes As a dihedral group D_{2n}, n even: number of conjugacy classes is \! (n + 6)/2 = (4 + 6)/2 = 5. See element structure of dihedral groups and element structure of dihedral group:D8
number of conjugacy classes of subgroups 7 groups with same order and number of conjugacy classes of subgroups | groups with same prime-base logarithm of order and number of conjugacy classes of subgroups | groups with same number of conjugacy classes of subgroups See subgroup structure of dihedral groups, subgroup structure of dihedral group:D8
number of equivalence classes under real conjugacy 5 groups with same order and number of equivalence classes under real conjugacy | groups with same prime-base logarithm of order and number of equivalence classes under real conjugacy | groups with same number of equivalence classes under real conjugacy Same as number of conjugacy classes, because the group is an ambivalent group. See dihedral groups are ambivalent
number of conjugacy classes of real elements 5 groups with same order and number of conjugacy classes of real elements | groups with same prime-base logarithm of order and number of conjugacy classes of real elements | groups with same number of conjugacy classes of real elements Same as number of conjugacy classes, because the group is an ambivalent group. See dihedral groups are ambivalent
number of equivalence classes under rational conjugacy 5 groups with same order and number of equivalence classes under rational conjugacy | groups with same prime-base logarithm of order and number of equivalence classes under rational conjugacy | groups with same number of equivalence classes under rational conjugacy Same as number of conjugacy classes, because the group is a rational group.
number of conjugacy classes of rational elements 5 groups with same order and number of conjugacy classes of rational elements | groups with same prime-base logarithm of order and number of conjugacy classes of rational elements | groups with same number of conjugacy classes of rational elements Same as number of conjugacy classes, because the group is a rational group.

Lists of numerical invariants

List Value Explanation/comment
conjugacy class sizes 1,1,2,2,2 Two central elements, all others in conjugacy classes of size two. See element structure of dihedral group:D8 and element structure of dihedral groups.
order statistics 1 \mapsto 1, 2 \mapsto 5, 4 \mapsto 2 Of the five elements of order two, one is central. The other four are automorphic to each other. See element structure of dihedral group:D8 and element structure of dihedral groups
degrees of irreducible representations 1,1,1,1,2 See linear representation theory of dihedral group:D8
orders of subgroups 1,2,2,2,2,2,4,4,4,8 See subgroup structure of dihedral group:D8

Numerical invariants arising from subgroup series-defining functions:

List Value Explanation/comment
prime-base logarithms of orders of successive quotient groups of upper central series 1,2
prime-base logarithms of orders of successive quotient groups of lower central series 2,1
prime-base logarithms of orders of successive quotient groups of derived series 2,1
prime-base logarithms of orders of successive quotient groups of Frattini series 2,1

Action-based/automorphism group realization invariants

Function Value Explanation
minimum degree of faithful representation 2
minimum degree of nontrivial irreducible representation 2
smallest size of set with faithful action 4
smallest size of set with faithful transitive action 4
symmetric genus  ?

Group properties

Want to compare and contrast group properties with other groups of the same order? Check out groups of order 8#Group properties
Property Satisfied? Explanation Comment
group of prime power order Yes
nilpotent group Yes prime power order implies nilpotent
supersolvable group Yes via nilpotent: finite nilpotent implies supersolvable
solvable group Yes via nilpotent: nilpotent implies solvable
abelian group No a and x don't commute Smallest non-abelian group of prime power order
T-group No \langle x \rangle \triangleleft \langle a^2,x \rangle, which is normal, but \langle x \rangle is not normal Smallest example for normality is not transitive.
monolithic group Yes Unique minimal normal subgroup of order two
one-headed group No Three distinct maximal normal subgroups of order four
SC-group No
ACIC-group Yes Every automorph-conjugate subgroup is characteristic
ambivalent group Yes dihedral groups are ambivalent Also see generalized dihedral groups are ambivalent
rational group Yes Any two elements that generate the same cyclic group are conjugate Thus, all characters are integer-valued.
rational-representation group Yes All representations over characteristic zero are realized over the rationals. Contrast with quaternion group, that is rational but not rational-representation.
extraspecial group Yes The center, derived subgroup, and Frattini subgroup all coincide and are cyclic of prime order
special group Yes (via extraspecial): the center, derived subgroup, and Frattini subgroup all coincide
Frattini-in-center group Yes (via extraspecial): the Frattini subgroup is contained in the center
group of nilpotency class two Yes (via special): the derived subgroup is contained in the center
UL-equivalent group Yes (via special): the upper central series and lower central series coincide
maximal class group Yes
Frobenius group No Frobenius groups are centerless, and this group isn't
Camina group Yes extraspecial implies Camina
Every element is automorphic to its inverse Yes Follows from being an ambivalent group
any two elements generating the same cyclic subgroup are automorphic Yes
every element is order-automorphic No
directly indecomposable group Yes
centrally indecomposable group Yes
splitting-simple group No

Subgroups

Further information: subgroup structure of dihedral group:D8

Lattice of subgroups of the dihedral group

The dihedral group has ten subgroups:

  1. The trivial subgroup (1)
  2. The center, which is the unique minimal normal subgroup, and is a two-element subgroup generated by a^2. This is isomorphic to the cyclic group of order two. (1) Further information: Center of dihedral group:D8
  3. The two-element subgroups generated by x, ax, a^2x and a^3x. All of these are isomorphic to the cyclic group of order two.These are in two conjugacy classes: the subgroups generated by x and by a^2x form one conjugacy class; the subgroups generated by ax and a^3x form another conjugacy class. (4) Further information: 2-subnormal subgroups of dihedral group:D8
  4. The four-element subgroups \langle a^2, x\rangle and \langle a^2,ax \rangle. Both are normal, related by an outer automorphism, and isomorphic to the Klein four-group. (2) Further information: Klein four-subgroups of dihedral group:D8
  5. The four-element subgroup generated by a. This is normal and is isomorphic to the cyclic group of order four. (1) Further information: Cyclic maximal subgroup of dihedral group:D8
  6. The whole group. (1)

Normal subgroups

All subgroups except those in header (3) above, are normal. The subgroups in header (3), which are two-element subgroups generated by some a^nx, are 2-subnormal, as each of these is contained in a Klein four-group. Of the subgroups in header (3), there are two conjugacy classes: one comprising the subgroups generated by x and by a^2x, and the other comprising the subgroups generated by ax and by a^3x. (These conjugacy classes are related by an outer automorphism).

Characteristic subgroups

The subgroups in headers (1), (2), (5) and (6) are characteristic. The subgroups in header (4) are normal but not characteristic, and in fact, the two subgroups are automorphs of each other.

Quotient groups

  1. The group itself: this is obtained as the quotient by the trivial subgroup. (1)
  2. The Klein four-group, which is obtained as the quotient by the center. (1)
  3. The cyclic group:Z2, which is obtained as the quotient by either of the two Klein four-subgroups. (2)
  4. The cyclic group:Z2, which is obtained as the quotient by the cyclic maximal subgroup. (1)
  5. The trivial group, which is obtained as the quotient by the group itself. (1)

Subgroup-defining functions

Subgroup-defining function Subgroup type in list Page on subgroup embedding Isomorphism class Comment
Center (2) Center of dihedral group:D8 Cyclic group:Z2 Prime power order implies not centerless
Derived subgroup (2) Center of dihedral group:D8 Cyclic group:Z2
Frattini subgroup (2) Center of dihedral group:D8 Cyclic group:Z2 The three maximal subgroups of order four intersect here.
Socle (2) Center of dihedral group:D8 Cyclic group:Z2 This subgroup is the unique minimal normal subgroup, i.e.,the monolith, and the group is monolithic. Also, minimal normal implies central in nilpotent.
Join of abelian subgroups of maximum order (6) -- whole group The group is generated by abelian subgroups of maximum order.
ZJ-subgroup (2) Center of dihedral group:D8 Cyclic group:Z2 Since the group equals the join of abelian subgroups of maximum order, the ZJ-subgroup equals the center.
Join of abelian subgroups of maximum rank (6) -- whole group The group is generated by abelian subgroups of maximum rank.
Join of elementary abelian subgroups of maximum order (6) -- whole group The group is generated by abelian subgroups of maximum rank.

Quotient-defining functions

Quotient-defining function Isomorphism class Comment
Inner automorphism group Klein four-group It is the quotient by the center, which is of order two.
Abelianization Klein four-group It is the quotient by the commutator subgroup, which is cyclic of order two.
Frattini quotient Klein four-group It is the quotient by the Frattini subgroup, which is cyclic of order two.

Other associated constructs

Associated construct Isomorphism class Comment
Automorphism group dihedral group:D8
Outer automorphism group cyclic group:Z2
Holomorph holomorph of D8
Inner holomorph inner holomorph of D8
Extended automorphism group direct product of D8 and Z2
Quasiautomorphism group direct product of D8 and Z2
1-automorphism group direct product of S4 and Z2

In larger groups

Occurrence as a subgroup

Further information: Supergroups of dihedral group:D8

The dihedral group of order eight occurs as a subgroup in bigger groups. Here are some examples:

  1. It is a subgroup in a dihedral group of order 2n where n is a multiple of 4.
  2. It occurs as a Sylow subgroup in a number of groups: for instance, in the symmetric group on four letters.

Occurrence as a quotient

The dihedral group of order eight also occurs as a quotient; for instance, it is a quotient of the dicyclic group of order 16, by its center (which has order two).

Distinguishing features

Smallest of its kind

  • This is the unique non-T-group of smallest order, i.e., the unique smallest example of a group in which normality is not transitive.
  • This is a non-abelian nilpotent group of smallest order, though not the only one. The other such group is the quaternion group.

Different from others of the same order

GAP implementation

Group ID

This finite group has order 8 and has ID 3 among the groups of order 8 in GAP's SmallGroup library. For context, there are 5 groups of order 8. It can thus be defined using GAP's SmallGroup function as:

SmallGroup(8,3)

For instance, we can use the following assignment in GAP to create the group and name it G:

gap> G := SmallGroup(8,3);

Conversely, to check whether a given group G is in fact the group we want, we can use GAP's IdGroup function:

IdGroup(G) = [8,3]

or just do:

IdGroup(G)

to have GAP output the group ID, that we can then compare to what we want.


Short descriptions

Description GAP functions used Mathematical translation of description
DihedralGroup(8) DihedralGroup dihedral group of order 8, degree 4
WreathProduct(CyclicGroup(2),CyclicGroup(2)) WreathProduct, CyclicGroup external wreath product of two copies of cyclic group of order two
ExtraspecialGroup(2^3,'+') ExtraspecialGroup extraspecial group of '+' type for the prime 2 and order 2^3
SylowSubgroup(SymmetricGroup(4),2) SylowSubgroup and SymmetricGroup The 2-Sylow subgroup of the symmetric group of degree four
SylowSubgroup(GL(3,2),2) SylowSubgroup, GL The 2-Sylow subgroup of GL(3,2)

Description using a presentation

Here is the code:

gap> F := FreeGroup(2);;
gap> G := F/[F.1^4, F.2^2, F.2 * F.1 * F.2 * F.1];
<fp group on the generators [ f1, f2 ]>

The group G constructed here is the dihedral group of order 8. The first generator F.1 maps to the rotation element of order four and the second generator F.2 maps to a reflection element of order two.

Long descriptions

It can be described as the holomorph of the cyclic group of order four. For this, first define C to be the cyclic group of order four (using CyclicGroup), and then use SemidirectProduct and AutomorphismGroup:

C := CyclicGroup(4);
G := SemidirectProduct(AutomorphismGroup(C),C);

Here, G is the dihedral group of order eight. We can also construct it as a semidirect product of the Klein four-group and an automorphism of order two.

K := DirectProduct(CyclicGroup(2),CyclicGroup(2));
A := AutomorphismGroup(K);
S := SylowSubgroup(A,2);
G := SemidirectProduct(S,K);

Then, G is isomorphic to the dihedral group of order eight.

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