§3.6 Permutation Groups

Shaoyun Yi

MATH 546/701I

University of South Carolina

June 3-4, 2020

Yi Permutation Groups June 3-4, 2020 1 / 23

Review from Section 3.5

Every subgroup of a cyclic group G is cyclic.

Let G be a cyclic group.

{If G is infinite, then G ∼= Z.

If |G | = n, then G ∼= Zn.(a) Any two infinite cyclic groups are isomorphic to each other.(b) Two finite cyclic groups are isomorphic ⇔ they have the same order.

Subgroups of Z : For any m ∈ Z, mZ = 〈m〉 ∼= Z = 〈1〉 = 〈−1〉.• mZ ⊆ nZ⇔ n|m. • mZ = nZ⇔ m = ±n.

Subgroups of Zn : For any d |n, dZn = 〈[d ]n〉 subgroup diagram(a) Let d = gcd(m, n) : 〈[m]n〉 = 〈[d ]n〉 & |〈[m]n〉| = |〈[d ]n〉| = n/d .

(i) 〈[k]n〉 = Zn ⇔ gcd(k, n) = 1, i.e., [k]n ∈ Z×n .

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Let G be a finite abelian group. Let N be the exponent of G .(a) N = max{o(a) | a ∈ G}.(b) The group G is cyclic ⇔ N = |G |.

For small n, check Z×n cyclic or not without using primitive root thm.

If G is infinite, then G ∼= Z.

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

If |G | = n, then G ∼= Zn.

(a) Any two infinite cyclic groups are isomorphic to each other.(b) Two finite cyclic groups are isomorphic ⇔ they have the same order.

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

If |G | = n, then G ∼= Zn.(a) Any two infinite cyclic groups are isomorphic to each other.

(b) Two finite cyclic groups are isomorphic ⇔ they have the same order.

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Subgroups of Z :

For any m ∈ Z, mZ = 〈m〉 ∼= Z = 〈1〉 = 〈−1〉.• mZ ⊆ nZ⇔ n|m. • mZ = nZ⇔ m = ±n.

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Subgroups of Z : For any m ∈ Z, mZ = 〈m〉 ∼= Z = 〈1〉 = 〈−1〉.

• mZ ⊆ nZ⇔ n|m. • mZ = nZ⇔ m = ±n.Subgroups of Zn : For any d |n, dZn = 〈[d ]n〉 subgroup diagram(a) Let d = gcd(m, n) : 〈[m]n〉 = 〈[d ]n〉 & |〈[m]n〉| = |〈[d ]n〉| = n/d .

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Subgroups of Zn :

For any d |n, dZn = 〈[d ]n〉 subgroup diagram(a) Let d = gcd(m, n) : 〈[m]n〉 = 〈[d ]n〉 & |〈[m]n〉| = |〈[d ]n〉| = n/d .

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Subgroups of Zn : For any d |n, dZn = 〈[d ]n〉 subgroup diagram

(a) Let d = gcd(m, n) : 〈[m]n〉 = 〈[d ]n〉 & |〈[m]n〉| = |〈[d ]n〉| = n/d .(i) 〈[k]n〉 = Zn ⇔ gcd(k, n) = 1, i.e., [k]n ∈ Z×

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

(ii) If d1|n and d2|n, then 〈[d1]n〉 ⊆ 〈[d2]n〉 ⇔ d2|d1.

(iii) If d1|n and d2|n and d1 6= d2, then 〈[d1]n〉 6= 〈[d2]n〉.Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Let G be a finite abelian group. Let N be the exponent of G .

(a) N = max{o(a) | a ∈ G}.(b) The group G is cyclic ⇔ N = |G |.

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Let G be a finite abelian group. Let N be the exponent of G .(a) N = max{o(a) | a ∈ G}.

(b) The group G is cyclic ⇔ N = |G |.For small n, check Z×

n cyclic or not without using primitive root thm.

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

Zn∼= Zp

α11× Zp

α22× · · · × Zpαm

m ϕ(n) = n(1− 1

p1) · · · (1− 1

A permutation σ of a set S is a function from S to S that is bothone-to-one and onto.

Notation:

Sym(S) = {σ | σ : S → S} or write Sn if S = {1, 2, . . . , n}.Sym(S) is a group under ◦. Sn is the symmetric group of degree n.

|Sn| = n!

Let σ ∈ Sym(S). Then σ = (a1a2 · · · ak) is a cycle of length k .

Disjoint cycles are commutative

σ ∈ Sn can be written as a (unique) product of disjoint cycles.

A cycle σ of length m has order m, i.e., o(σ) = m.

The order of σ is the lcm of the lengths (orders) of its disjoint cycles.

A transposition is a cycle (a1a2) of length two.

σ ∈ Sn can be written as a (NOT unique) product of transpositions.

Product of transpositions: Even permutation vs. Odd permutation

A cycle of odd length is even. & A cycle of even length is odd.

Notation: Sym(S) = {σ | σ : S → S} or write Sn if S = {1, 2, . . . , n}.

Sym(S) is a group under ◦. Sn is the symmetric group of degree n.

|Sn| = n!

Notation: Sym(S) = {σ | σ : S → S} or write Sn if S = {1, 2, . . . , n}.Sym(S) is a group under ◦. Sn is the symmetric group of degree n.

|Sn| = n!

Definition

Definition 1

Any subgroup of the symmetric group Sym(S) on a set S is called apermutation group.

Note 1 (Let G be a finite group.)

As we have observed, each row in the multiplication table represents apermutation of the group elements. Furthermore, each row corresponds tomultiplication by a given element, and so there is a natural way to assign apermutation to each element a ∈ G .In fact, this natural way will be important in the proof of Cayley’s theorem.

Definition

Definition 1

As we have observed, each row in the multiplication table represents apermutation of the group elements.

Furthermore, each row corresponds tomultiplication by a given element, and so there is a natural way to assign apermutation to each element a ∈ G .In fact, this natural way will be important in the proof of Cayley’s theorem.

Definition

Definition 1

As we have observed, each row in the multiplication table represents apermutation of the group elements. Furthermore, each row corresponds tomultiplication by a given element,

and so there is a natural way to assign apermutation to each element a ∈ G .In fact, this natural way will be important in the proof of Cayley’s theorem.

Definition

Definition 1

As we have observed, each row in the multiplication table represents apermutation of the group elements. Furthermore, each row corresponds tomultiplication by a given element, and so there is a natural way to assign apermutation to each element a ∈ G .

In fact, this natural way will be important in the proof of Cayley’s theorem.

Definition

Definition 1

As we have observed, each row in the multiplication table represents apermutation of the group elements. Furthermore, each row corresponds tomultiplication by a given element, and so there is a natural way to assign apermutation to each element a ∈ G .In fact, this natural way will be important in the proof of Cayley’s theorem.

Cayley’s Theorem

Theorem 2 (Cayley’s Theorem)

Every group G is isomorphic to a permutation group.

Given a ∈ G , define λa : G → G by λa(x) = ax , for all x ∈ G .

λa is one-to-one: if λa(x1) = λa(x2)⇒ ax1 = ax2 ⇒ x1 = x2. (Why?)

λa is onto: For any x ∈ G , we have λa(a−1x) = a(a−1x) = x . (Why?)

Thus, λa is a permutation of G . This shows that φ : G → Sym(G ) definedby φ(a) = λa is well-defined. Claim: Gλ = φ(G ) is a subgroup of Sym(G ).

(i) Closure: For any λa, λb ∈ Gλ with a, b ∈ G , to show λaλb ∈ Gλ.

λaλb(x) = λa(λb(x)) = λa(bx) = a(bx) = (ab)x = λab(x),

for all x ∈ G . This implies that λaλb = λab ∈ Gλ. (Why?)

(ii) Identity: λe . For any λa ∈ Gλ, λaλe = λae = λa & λeλa = λea = λa.

(iii) Inverses: λa−1 . It is easy to see that (λa)−1 = λa−1 . (Check it!)

Thus, Gλ = φ(G ) is a subgroup of Sym(G ).

Cayley’s Theorem

λa is one-to-one:

if λa(x1) = λa(x2)⇒ ax1 = ax2 ⇒ x1 = x2. (Why?)

Cayley’s Theorem

λa is onto:

For any x ∈ G , we have λa(a−1x) = a(a−1x) = x . (Why?)

Cayley’s Theorem

Thus, λa is a permutation of G .

This shows that φ : G → Sym(G ) definedby φ(a) = λa is well-defined. Claim: Gλ = φ(G ) is a subgroup of Sym(G ).

Cayley’s Theorem

Thus, λa is a permutation of G . This shows that φ : G → Sym(G ) definedby φ(a) = λa is well-defined.

Claim: Gλ = φ(G ) is a subgroup of Sym(G ).

Cayley’s Theorem

(i) Closure:

For any λa, λb ∈ Gλ with a, b ∈ G , to show λaλb ∈ Gλ.

Cayley’s Theorem

for all x ∈ G .

This implies that λaλb = λab ∈ Gλ. (Why?)

Cayley’s Theorem

(ii) Identity:

λe . For any λa ∈ Gλ, λaλe = λae = λa & λeλa = λea = λa.

Cayley’s Theorem

(iii) Inverses:

λa−1 . It is easy to see that (λa)−1 = λa−1 . (Check it!)

Cayley’s Theorem

Thus, Gλ = φ(G ) is a subgroup of Sym(G ).Yi Permutation Groups June 3-4, 2020 5 / 23

Proof of Cayley’s Theorem cont.: To show G ∼= Gλ.

Define φ : G → Gλ by φ(a) = λa.

To show φ is a group isomorphism.

well-defined: Trivial. X

φ preserves products: For any a, b ∈ G , to show φ(ab) = φ(a)φ(b).

φ(ab) = λab = λaλb = φ(a)φ(b).

φ is one-to-one: If φ(a) = φ(b) for a, b ∈ G , then it is to show a = b.

For all x ∈ G , φ(a) = φ(b)⇒ λa(x) = λb(x)⇒ ax = bx ⇒ a = b.

φ is onto: Trivial. By the definition of Gλ = φ(G ).

Thus, φ is a group isomorphism.

So, G ∼= Gλ, where Gλ is a subgroup of Sym(G ), i.e., a permutation group.

Define φ : G → Gλ by φ(a) = λa. To show φ is a group isomorphism.

well-defined:

Trivial. X

φ preserves products:

For any a, b ∈ G , to show φ(ab) = φ(a)φ(b).

φ is one-to-one:

If φ(a) = φ(b) for a, b ∈ G , then it is to show a = b.

For all x ∈ G , φ(a) = φ(b)⇒

λa(x) = λb(x)⇒ ax = bx ⇒ a = b.

For all x ∈ G , φ(a) = φ(b)⇒ λa(x) = λb(x)⇒

ax = bx ⇒ a = b.

For all x ∈ G , φ(a) = φ(b)⇒ λa(x) = λb(x)⇒ ax = bx ⇒

a = b.

φ is onto:

Trivial. By the definition of Gλ = φ(G ).

Example: Rigid motions of a square

Definition 3 (Rigid Motion:)

a change in position where the distance between points is preserved andfigures remain congruent (having the same size and shape). It may be

a translation (slide)

a reflection (flip)

a rotation (turn)

or a combination of these.

Each of the rigid motions determines a permutation of the vertices of the square,

and the permutation notation gives a convenient way to describe these motions.

There are a total of eight rigid motions of a square. (Why?)

There are four choices of a position in which to place first vertex A,

and then two choices for second vertex since it must be adjacent to A.

a reflection (flip)

a rotation (turn)

a reflection (flip)

a rotation (turn)

a reflection (flip)

a rotation (turn)

a reflection (flip)

a rotation (turn)

a reflection (flip)

a rotation (turn)

Example cont.: Rigid motions of a square

(1234) counterclockwise rotation through 90◦

(13)(24) counterclockwise rotation through 180◦

(24) flip about vertical axis

(13) flip about horizontal axis

(12)(34) flip about diagonal

Note 2

Note that we do not obtain all elements of S4 as rigid motion, since,for example, (12) would represent an impossible configuration.

Question 1

What is the order of each rigid motion?

Note 2

Question 1

Note 2

Question 1

Note 2

Question 1

Note 2

Question 1

Note 2

Question 1

Note 2

Question 1

Note 2

Question 1

Note 2

Question 1

Note 2

Question 1

Note 2

Note that we do not obtain all elements of S4 as rigid motion, since,for example,

(12) would represent an impossible configuration.

Question 1

Note 2

Question 1

Note 2

Question 1

Rigid motions of a square: Multiplication table

(1) (1234) (13)(24) (1432) (24) (12)(34) (13) (14)(23)

(1) (1) (1234) (13)(24) (1432) (24) (12)(34) (13) (14)(23)

(1234) (1234) (13)(24) (1432) (1) (12)(34) (13) (14)(23) (24)

(13)(24) (13)(24) (1432) (1) (1234) (13) (14)(23) (24) (12)(34)

(1432) (1432) (1) (1234) (13)(24) (14)(23) (24) (12)(34) (13)

(24) (24) (14)(23) (13) (12)(34) (1) (1432) (13)(24) (1234)

(12)(34) (12)(34) (24) (14)(23) (13) (1234) (1) (1432) (13)(24)

(13) (13) (12)(34) (24) (14)(23) (13)(24) (1234) (1) (1432)

(14)(23) (14)(23) (13) (12)(34) (24) (1432) (13)(24) (1234) (1)

Rigid motions of a square: Multiplication table

(1) (1234) (13)(24) (1432) (24) (12)(34) (13) (14)(23)

(1) (1) (1234) (13)(24) (1432) (24) (12)(34) (13) (14)(23)

(1234) (1234) (13)(24) (1432) (1) (12)(34) (13) (14)(23) (24)

(13)(24) (13)(24) (1432) (1) (1234) (13) (14)(23) (24) (12)(34)

(1432) (1432) (1) (1234) (13)(24) (14)(23) (24) (12)(34) (13)

(24) (24) (14)(23) (13) (12)(34) (1) (1432) (13)(24) (1234)

(12)(34) (12)(34) (24) (14)(23) (13) (1234) (1) (1432) (13)(24)

(13) (13) (12)(34) (24) (14)(23) (13)(24) (1234) (1) (1432)

(14)(23) (14)(23) (13) (12)(34) (24) (1432) (13)(24) (1234) (1)

Example: Rigid motions of an equilateral triangle

Proposition 1

The rigid motions of an equilateral triangle yield the group S3.

(13) flip about angle bisector

Note 3 (Another notion for describing S3 in §3.3)

S3 = {e, a, a2, b, ab, a2b}, where a3 = e, b2 = e, ba = a2b = a−1b.

Note 4 (Another notion for describing Rigid Motions of a Square)

Let a = (1234) and b = (24). It can be shown that ba = a3b. The groupG = {e, a, a2, a3, b, ab, a2b, a3b}, where a4 = e, b2 = e, ba = a3b = a−1b.

Proposition 1

Let a = (1234) and b = (24). It can be shown that ba = a3b.

The groupG = {e, a, a2, a3, b, ab, a2b, a3b}, where a4 = e, b2 = e, ba = a3b = a−1b.

Proposition 1

Example: Rigid motions of a regular polygon (n-gon)

Proposition 2

There are 2n rigid motions of a regular n-gon.

i) There are n choices of a position in which to place first vertex A,ii) and then two choices for second vertex since it must be adjacent to A.

i) Let a be a counterclockwise rotation about the center, through 360/n degrees.

Thus a is a the cycle (123 · · · n) of length n and has order n.

ii) Let b be a flip about the line of symmetry through position number 1.

Thus b has order 2 and is given by the product of transpositions (2n)(3 n − 1) · · · .

Proposition 2

i) There are n choices of a position in which to place first vertex A,

ii) and then two choices for second vertex since it must be adjacent to A.

Proposition 2

Let a be a counterclockwise rotation about the center, through 360/n degrees.

Proposition 2

Let b be a flip about the line of symmetry through position number 1.

Proposition 2

Thus b has order 2 and is given by the product of transpositions (2n)(3 n − 1) · · · .Yi Permutation Groups June 3-4, 2020 11 / 23

Example cont.: Rigid motions of a regular polygon (n-gon)

Consider the set S = {ak , akb | 0 ≤ k < n} of rigid motions.

The elements ak for 0 ≤ k < n are all distinct. (Why?) [o(a) = n]The elements akb for 0 ≤ k < n are all distinct. (Why?) [o(a) = n]ak 6= ajb for all 0 ≤ k , j < n. (Why?) [ak does NOT flip the n-gon]

Thus, |S | = 2n, and so G = S .

Note 5 (Notion for describing Rigid Motions of a regular n-gon)

G = {ak , akb | 0 ≤ k < n}, where an = e, b2 = e, ba = an−1b = a−1b.

Goal: To show ba = a−1b. Note: a−1 = an−1 (Why?)& b−1 = b (Why?)

That is, to show bab = a−1.

The elements ak for 0 ≤ k < n are all distinct. (Why?) [

o(a) = n]The elements akb for 0 ≤ k < n are all distinct. (Why?) [o(a) = n]ak 6= ajb for all 0 ≤ k , j < n. (Why?) [ak does NOT flip the n-gon]

The elements ak for 0 ≤ k < n are all distinct. (Why?) [o(a) = n]

The elements akb for 0 ≤ k < n are all distinct. (Why?) [o(a) = n]ak 6= ajb for all 0 ≤ k , j < n. (Why?) [ak does NOT flip the n-gon]

The elements ak for 0 ≤ k < n are all distinct. (Why?) [o(a) = n]The elements akb for 0 ≤ k < n are all distinct. (Why?) [

o(a) = n]ak 6= ajb for all 0 ≤ k , j < n. (Why?) [ak does NOT flip the n-gon]

The elements ak for 0 ≤ k < n are all distinct. (Why?) [o(a) = n]The elements akb for 0 ≤ k < n are all distinct. (Why?) [o(a) = n]

ak 6= ajb for all 0 ≤ k , j < n. (Why?) [ak does NOT flip the n-gon]

The elements ak for 0 ≤ k < n are all distinct. (Why?) [o(a) = n]The elements akb for 0 ≤ k < n are all distinct. (Why?) [o(a) = n]ak 6= ajb for all 0 ≤ k , j < n. (Why?) [

ak does NOT flip the n-gon]

Goal: To show ba = a−1b.

Note: a−1 = an−1 (Why?)& b−1 = b (Why?)

Goal: To show ba = a−1b. Note: a−1 = an−1 (Why?)&

b−1 = b (Why?)

Dihedral group Dn

Definition 4

Let n ≥3 be an integer. The group of rigid motions of a regular n-gon iscalled the nth dihedral group, denoted by Dn. Note that |Dn| = 2n.

Proposition 3 (Note 5)

Dn = {ak , akb | 0 ≤ k < n}, where an = e, b2 = e, ba = a−1b.

Remark 1 (Let n ≥ 4.)

We will not list all the subgroups of Sn. (Why?) [there are too many!!]

The “simple” subgroups of Sn: cyclic subgroup generated by σ ∈ Sn.

The dihedral group Dn is one important example of subgroups of Sn.

The alternating group An is another one important example. (soon!)

Dihedral group Dn

Definition 4

Dihedral group Dn

Definition 4

Dihedral group Dn

Definition 4

We will not list all the subgroups of Sn. (Why?) [

there are too many!!]

Dihedral group Dn

Definition 4

Dihedral group Dn

Definition 4

Dihedral group Dn

Definition 4

Dihedral group Dn

Definition 4

Example: Subgroups of D3 = S3

Proposition 4

If G = S3, then every proper subgroup of S3 is cyclic. (Why?)

By Lagrange’s theorem, a proper subgroup of S3 must have order 1, 2, or 3.And subgroups of order 2 or 3 must be cyclic. (Why?) & {e} is trivial.XThe subgroup diagram of S3 :

{e, a2b}{e, ab}{e, b} {e, a, a2}

Note that D3 = S3 = {e, a, a2, b, ab, a2b}, where a3 = e, b2 = e, ba = a2b.

Proposition 4

{e, a2b}{e, ab}{e, b} {e, a, a2}

Proposition 4

By Lagrange’s theorem, a proper subgroup of S3 must have order 1, 2, or 3.

And subgroups of order 2 or 3 must be cyclic. (Why?) & {e} is trivial.XThe subgroup diagram of S3 :

{e, a2b}{e, ab}{e, b} {e, a, a2}

Proposition 4

By Lagrange’s theorem, a proper subgroup of S3 must have order 1, 2, or 3.And subgroups of order 2 or 3 must be cyclic. (Why?) &

{e} is trivial.XThe subgroup diagram of S3 :

{e, a2b}{e, ab}{e, b} {e, a, a2}

Proposition 4

{e, a2b}{e, ab}{e, b} {e, a, a2}

Proposition 4

{e, a2b}{e, ab}{e, b} {e, a, a2}

Proposition 4

{e, a2b}{e, ab}{e, b} {e, a, a2}

Note that D3 = S3 = {e, a, a2, b, ab, a2b}, where a3 = e, b2 = e, ba = a2b.Yi Permutation Groups June 3-4, 2020 14 / 23

Subgroups of D4

D4 = {e, a, a2, a3, b, ab, a2b, a3b}, where a4 = e, b2 = e, ba = a3b.The possible orders of proper subgroups of D4 are 1, 2, or 4. (Why?)

I. The trivial subgroups: {e},D4.

II. The cyclic (proper) subgroups:

(a) a has order 4. In fact, 〈a〉 = 〈a3〉 = {e, a, a2, a3}. (Why?)(b) Each of the elements a2, b, ab, a2b, a3b has order 2. (Check it!)

III. Are there (proper) subgroups of D4 that are not cyclic? Yes! (How?)

(i) If H is a non-cyclic (proper) subgroup, then |H| = 4. (Why?)(ii) Any non-identity element of H has order 2. (Why?)(iii) H ∼= Z2 × Z2: Say, H = {e, x , y , xy}, and so yx = xy . (Why?)(iv) Consider all possible pairs of elements of order 2 to find all such H’s.(v) In fact, there are two such groups. (Check it!)

(1) H1 = {e, a2, b, a2b}: ba2 = (ba)a = a3(ba) = a3(a3b) = a2b X(2) H2 = {e, a2, ab, a3b}: (ab)a2 = a(ba)a = a(a3b)a = ba = a3b X

(vi) A cyclic subgroup is the smallest subgroup containing the generator;these subgroups H’s are the smallest ones containing the two elementsused to construct it.

Subgroups of D4

Note: D4 = {e, a, a2, a3, b, ab, a2b, a3b}, where a4 = e, b2 = e, ba = a3b.

The possible orders of proper subgroups of D4 are 1, 2, or 4. (Why?)

Subgroups of D4

Note: D4 = {e, a, a2, a3, b, ab, a2b, a3b}, where a4 = e, b2 = e, ba = a3b.The possible orders of proper subgroups of D4 are 1, 2, or 4. (Why?)

Subgroups of D4

(a) a has order 4. In fact,

〈a〉 = 〈a3〉 = {e, a, a2, a3}. (Why?)(b) Each of the elements a2, b, ab, a2b, a3b has order 2. (Check it!)

Subgroups of D4

(a) a has order 4. In fact, 〈a〉 = 〈a3〉 = {e, a, a2, a3}. (Why?)

(b) Each of the elements a2, b, ab, a2b, a3b has order 2. (Check it!)

Subgroups of D4

III. Are there (proper) subgroups of D4 that are not cyclic?

Yes! (How?)

Subgroups of D4

(i) If H is a non-cyclic (proper) subgroup, then |H| = 4. (Why?)

(ii) Any non-identity element of H has order 2. (Why?)(iii) H ∼= Z2 × Z2: Say, H = {e, x , y , xy}, and so yx = xy . (Why?)(iv) Consider all possible pairs of elements of order 2 to find all such H’s.(v) In fact, there are two such groups. (Check it!)

Subgroups of D4

(i) If H is a non-cyclic (proper) subgroup, then |H| = 4. (Why?)(ii) Any non-identity element of H has order 2. (Why?)

(iii) H ∼= Z2 × Z2: Say, H = {e, x , y , xy}, and so yx = xy . (Why?)(iv) Consider all possible pairs of elements of order 2 to find all such H’s.(v) In fact, there are two such groups. (Check it!)

Subgroups of D4

(i) If H is a non-cyclic (proper) subgroup, then |H| = 4. (Why?)(ii) Any non-identity element of H has order 2. (Why?)(iii) H ∼= Z2 × Z2: Say, H = {e, x , y , xy}, and so yx = xy . (Why?)

(iv) Consider all possible pairs of elements of order 2 to find all such H’s.(v) In fact, there are two such groups. (Check it!)

Subgroups of D4

(i) If H is a non-cyclic (proper) subgroup, then |H| = 4. (Why?)(ii) Any non-identity element of H has order 2. (Why?)(iii) H ∼= Z2 × Z2: Say, H = {e, x , y , xy}, and so yx = xy . (Why?)(iv) Consider all possible pairs of elements of order 2 to find all such H’s.(v) In fact,

there are two such groups. (Check it!)

Subgroups of D4

(1) H1 = {e, a2, b, a2b}:

ba2 = (ba)a = a3(ba) = a3(a3b) = a2b X(2) H2 = {e, a2, ab, a3b}: (ab)a2 = a(ba)a = a(a3b)a = ba = a3b X

Subgroups of D4

(1) H1 = {e, a2, b, a2b}: ba2 = (ba)a = a3(ba) = a3(a3b) = a2b X

(2) H2 = {e, a2, ab, a3b}: (ab)a2 = a(ba)a = a(a3b)a = ba = a3b X

Subgroups of D4

(1) H1 = {e, a2, b, a2b}: ba2 = (ba)a = a3(ba) = a3(a3b) = a2b X(2) H2 = {e, a2, ab, a3b}:

(ab)a2 = a(ba)a = a(a3b)a = ba = a3b X

Subgroups of D4

(vi) A cyclic subgroup is the smallest subgroup containing the generator;

these subgroups H’s are the smallest ones containing the two elementsused to construct it.

Subgroups of D4

Subgroups of D4 cont.: Subgroup diagram

{e, a2, b, a2b} {e, a, a2, a3} {e, a2, ab, a3b}

{e, a2}{e, b} {e, a2b} {e, ab} {e, a3b}

Subgroups of D4 cont.: Subgroup diagram

{e, a2, b, a2b} {e, a, a2, a3} {e, a2, ab, a3b}

{e, a2}{e, b} {e, a2b} {e, ab} {e, a3b}

{e}Yi Permutation Groups June 3-4, 2020 16 / 23

Alternating group An

Recall that a permutation is called even if it can be expressed as an evennumber of transpositions, and odd otherwise.

Proposition 5

The set of all even permutations of Sn is a subgroup of Sn.

Since Sn is a finite set, we can apply Corollary 8 in §3.2. In particular,

(i) Nonempty: The identity permutation is even. (Why?) [(1) = (12)(21)]

(ii) Closure: If σ and τ are even permutations, so is τσ. (Why?)

Definition 5

The set of all even permutations of Sn is called the alternating group onn elements, and will be denoted by An.

Theorem 6 (Let n > 1.)

|An| =|Sn|

2. This is the largest possible cardinality for a proper subgroup.

Proposition 5

Definition 5

|An| =|Sn|

Proposition 5

Definition 5

|An| =|Sn|

Proposition 5

(i) Nonempty:

The identity permutation is even. (Why?) [(1) = (12)(21)]

Definition 5

|An| =|Sn|

Proposition 5

(i) Nonempty: The identity permutation is even. (Why?) [

(1) = (12)(21)]

Definition 5

|An| =|Sn|

Proposition 5

(ii) Closure:

If σ and τ are even permutations, so is τσ. (Why?)

Definition 5

|An| =|Sn|

Proposition 5

Definition 5

|An| =|Sn|

Proposition 5

Definition 5

|An| =|Sn|

Proposition 5

Definition 5

|An| =|Sn|

This is the largest possible cardinality for a proper subgroup.

Proposition 5

Definition 5

|An| =|Sn|

Proof of Theorem 6: |An| =|Sn|

2for n > 1.

Let On be the set of odd permutations in Sn. Note: On is not a subgroup. (Why?)

We have Sn = An⊔On (Why?), so |Sn| = |An|+ |On|.

I. For each odd permutation σ, the permutation (12)σ is even. (Why?)If σ and τ are two distinct odd permutations, then (12)σ 6= (12)τ .

Proof by contradiction: Suppose (12)σ = (12)τ , then σ!

=τ . (Why?)

Thus, |An| ≥ |On|. (Why?)

II. Similarly, we can show that |On| ≥ |An|.

III. Therefore, |An| = |On| =|Sn|

2. (Why?)

2for n > 1.

Let On be the set of odd permutations in Sn. Note:

On is not a subgroup. (Why?)

=τ . (Why?)

2. (Why?)

2for n > 1.

=τ . (Why?)

2. (Why?)

2for n > 1.

We have Sn = An⊔On (Why?),

so |Sn| = |An|+ |On|.

=τ . (Why?)

2. (Why?)

2for n > 1.

=τ . (Why?)

2. (Why?)

2for n > 1.

I. For each odd permutation σ, the permutation (12)σ is even. (Why?)

If σ and τ are two distinct odd permutations, then (12)σ 6= (12)τ .

=τ . (Why?)

2. (Why?)

2for n > 1.

Proof by contradiction:

Suppose (12)σ = (12)τ , then σ!

=τ . (Why?)

2. (Why?)

2for n > 1.

=τ . (Why?)

2. (Why?)

2for n > 1.

=τ . (Why?)

II. Similarly,

we can show that |On| ≥ |An|.

2. (Why?)

2for n > 1.

=τ . (Why?)

2. (Why?)

2for n > 1.

=τ . (Why?)

2. (Why?)

Example: List all the elements of A3 and A4.

Recall that

S3 = {(1), (12), (13), (23), (123), (132)}, then we have

A3 = {(1), (123), (132)}. (Why?)

|S4| = 4! = 24: List all the possible decomposition types of elements.

Definition 7

The decomposition type of a permutation σ in Sn is the list of all thecycle lengths involved in a decomposition of σ into disjoint cycles.

Upshot: Possible decomposition types of permutations of S4: (Check it!)

(i) a single cycle of length 1, 2, 3 or 4

(ii) two disjoint cycles of length 2

Question 2

Which of these are even permutations?

(a) single cycles of length 1 and 3

(b) two disjoint cycles of length 2

Recall that S3 = {(1), (12), (13), (23), (123), (132)}, then

we have

A3 = {(1), (123), (132)}. (Why?)

Definition 7

Question 2

Recall that S3 = {(1), (12), (13), (23), (123), (132)}, then we have

A3 = {(1), (123), (132)}. (Why?)

Definition 7

Question 2

A3 = {(1), (123), (132)}. (Why?)

|S4| = 4! = 24:

List all the possible decomposition types of elements.

Definition 7

Question 2

A3 = {(1), (123), (132)}. (Why?)

Definition 7

Question 2

A3 = {(1), (123), (132)}. (Why?)

Definition 7

Question 2

A3 = {(1), (123), (132)}. (Why?)

Definition 7

Question 2

A3 = {(1), (123), (132)}. (Why?)

Definition 7

Question 2

A3 = {(1), (123), (132)}. (Why?)

Definition 7

Question 2

A3 = {(1), (123), (132)}. (Why?)

Definition 7

Question 2

A3 = {(1), (123), (132)}. (Why?)

Definition 7

Question 2

Example cont.: List all the elements of A4, |A4| = 12

single cycle of length 1: the identity permutation (1)single cycles of length 3: Choose any 3 of the numbers 1, 2, 3, 4:(

)= Four choices: 123 124 134 234

For each choice, there are two ways to make a cycle. (Why?)The following is the list of all cycles of length 3 in S4:

(123), (132), (124), (142), (134), (143), (234), (243)

two disjoint cycles of length 2: Choose any two of the #s 1, 2, 3, 4(4

)= Six choices: 12 13 14 23 24 34

Each pair of two numbers listed above gives rise to a transposition.The other two numbers form another transposition, which is disjointfrom the first one. The order doesn’t matter.(Why?) This impliesthat there are three different products of two disjoint transpositions:Pick any pair of two numbers: 6 choices; the other pair is determined.(Why?) The order doesn’t matter ⇒ 3 different products. (Why?)

(12)(34), (13)(24), (14)(23)

single cycle of length 1:

the identity permutation (1)single cycles of length 3: Choose any 3 of the numbers 1, 2, 3, 4:(

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)

single cycle of length 1: the identity permutation (1)single cycles of length 3:

Choose any 3 of the numbers 1, 2, 3, 4:(4

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)

single cycle of length 1: the identity permutation (1)single cycles of length 3: Choose any 3 of the numbers 1, 2, 3, 4:

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)

)= Four choices:

123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

For each choice, there are two ways to make a cycle. (Why?)

The following is the list of all cycles of length 3 in S4:

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

two disjoint cycles of length 2:

Choose any two of the #s 1, 2, 3, 4(4

)= Six choices: 12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

two disjoint cycles of length 2: Choose any two of the #s 1, 2, 3, 4

)= Six choices: 12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices:

12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

Each pair of two numbers listed above gives rise to a transposition.

The other two numbers form another transposition, which is disjointfrom the first one. The order doesn’t matter.(Why?) This impliesthat there are three different products of two disjoint transpositions:Pick any pair of two numbers: 6 choices; the other pair is determined.(Why?) The order doesn’t matter ⇒ 3 different products. (Why?)

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

Each pair of two numbers listed above gives rise to a transposition.The other two numbers form another transposition, which is disjointfrom the first one.

The order doesn’t matter.(Why?) This impliesthat there are three different products of two disjoint transpositions:Pick any pair of two numbers: 6 choices; the other pair is determined.(Why?) The order doesn’t matter ⇒ 3 different products. (Why?)

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

Each pair of two numbers listed above gives rise to a transposition.The other two numbers form another transposition, which is disjointfrom the first one. The order doesn’t matter.(Why?)

This impliesthat there are three different products of two disjoint transpositions:Pick any pair of two numbers: 6 choices; the other pair is determined.(Why?) The order doesn’t matter ⇒ 3 different products. (Why?)

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

Each pair of two numbers listed above gives rise to a transposition.The other two numbers form another transposition, which is disjointfrom the first one. The order doesn’t matter.(Why?) This impliesthat there are three different products of two disjoint transpositions:

Pick any pair of two numbers: 6 choices; the other pair is determined.(Why?) The order doesn’t matter ⇒ 3 different products. (Why?)

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

Each pair of two numbers listed above gives rise to a transposition.The other two numbers form another transposition, which is disjointfrom the first one. The order doesn’t matter.(Why?) This impliesthat there are three different products of two disjoint transpositions:Pick any pair of two numbers: 6 choices;

the other pair is determined.(Why?) The order doesn’t matter ⇒ 3 different products. (Why?)

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

Each pair of two numbers listed above gives rise to a transposition.The other two numbers form another transposition, which is disjointfrom the first one. The order doesn’t matter.(Why?) This impliesthat there are three different products of two disjoint transpositions:Pick any pair of two numbers: 6 choices; the other pair is determined.(Why?)

The order doesn’t matter ⇒ 3 different products. (Why?)

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)

)= Four choices: 123 124 134 234

(123), (132), (124), (142), (134), (143), (234), (243)

)= Six choices: 12 13 14 23 24 34

(12)(34), (13)(24), (14)(23)Yi Permutation Groups June 3-4, 2020 20 / 23

Example: The converse of Lagrange’s theorem is false

Upshot:

The following is the list of elements in A4: (1), (123), (132),(124), (142), (134), (143), (234), (243), (12)(34), (13)(24), (14)(23).

Proposition 6

Although 6 is a divisor of |A4| = 12, A4 has no subgroup of order 6.

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [since |H| = 6 is even]• It must contain an element of order 3. [Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

This implies that H must contain an element of the form (abc) and anelement of the form (ab)(cd). Then H contains (abc)(ab)(cd) = (acd)and (ab)(cd)(abc) = (bdc). H has six elements of order 3. (Why?)

Upshot: The following is the list of elements in A4: (1), (123), (132),(124), (142), (134), (143), (234), (243), (12)(34), (13)(24), (14)(23).

Proposition 6

In A4,

all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [since |H| = 6 is even]• It must contain an element of order 3. [Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .

Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [since |H| = 6 is even]• It must contain an element of order 3. [Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction:

Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [since |H| = 6 is even]• It must contain an element of order 3. [Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.•

It must contain an element of order 2. (Why?) [since |H| = 6 is even]• It must contain an element of order 3. [Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [

since |H| = 6 is even]• It must contain an element of order 3. [Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [since |H| = 6 is even]•

It must contain an element of order 3. [Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [since |H| = 6 is even]• It must contain an element of order 3. [

Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [since |H| = 6 is even]• It must contain an element of order 3. [Proof by contradiction:]

Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [since |H| = 6 is even]• It must contain an element of order 3. [Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2.

So o(xy) = 2. (Why?)And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [since |H| = 6 is even]• It must contain an element of order 3. [Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)

And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [since |H| = 6 is even]• It must contain an element of order 3. [Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)And then xy = yx . (Why?) [

xy = (xy)−1 = y−1x−1 = yx .]Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

In A4, all elements different from the identity have the form (abc) or(ab)(cd) for distinct a, b, c , d .Proof by contradiction: Suppose that H is a subgroup of order 6 in A4.• It must contain an element of order 2. (Why?) [since |H| = 6 is even]• It must contain an element of order 3. [Proof by contradiction:]Assume every non-identity element of H has order 2.Let x , y ∈ H with x 6= y and o(x) = o(y) = 2. So o(xy) = 2. (Why?)And then xy = yx . (Why?) [xy = (xy)−1 = y−1x−1 = yx .]

Hence {e, x , y , xy} is a subgroup of H of order 4, a contradiction. (Why?)

Proposition 6

This implies that H must contain an element of the form (abc) and anelement of the form (ab)(cd).

Then H contains (abc)(ab)(cd) = (acd)and (ab)(cd)(abc) = (bdc). H has six elements of order 3. (Why?)

Proposition 6

This implies that H must contain an element of the form (abc) and anelement of the form (ab)(cd). Then H contains (abc)(ab)(cd) = (acd)and

(ab)(cd)(abc) = (bdc). H has six elements of order 3. (Why?)

Proposition 6

This implies that H must contain an element of the form (abc) and anelement of the form (ab)(cd). Then H contains (abc)(ab)(cd) = (acd)and (ab)(cd)(abc) = (bdc).

H has six elements of order 3. (Why?)

Proposition 6

Standard approach to the definition of even and odd permutations

Definition 8

Let ∆n be the polynomial in n variables x1, x2, . . . , xn defined by

∆n =∏

1≤i<j≤n

(xi − xj).

Any permutation σ ∈ Sn acts on ∆n by permuting the subscripts, and we write

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

If i < j and σ(i) < σ(j), then the factors xi − xj and xσ(i) − xσ(j) have thesame sign, but if σ(i) > σ(j) then xσ(i) − xσ(j) = −(xσ(j) − xσ(i)). Becauseof such sign changes, we either have σ(∆n) = ∆n or σ(∆n) = −∆n.

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Let σ = (123) acts on ∆3: σ(∆3) = (x2 − x3)(x2 − x1)(x3 − x1)= ∆3.Let τ = (12) acts on ∆3: τ(∆3) = (x2 − x1)(x2 − x3)(x1 − x3)= −∆3.

Definition 8

∆n =∏

1≤i<j≤n

(xi − xj).

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Definition 8

∆n =∏

1≤i<j≤n

(xi − xj).

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Definition 8

∆n =∏

1≤i<j≤n

(xi − xj).

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

If i < j and σ(i) < σ(j), then the factors xi − xj and xσ(i) − xσ(j) have thesame sign, but

if σ(i) > σ(j) then xσ(i) − xσ(j) = −(xσ(j) − xσ(i)). Becauseof such sign changes, we either have σ(∆n) = ∆n or σ(∆n) = −∆n.

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Definition 8

∆n =∏

1≤i<j≤n

(xi − xj).

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

If i < j and σ(i) < σ(j), then the factors xi − xj and xσ(i) − xσ(j) have thesame sign, but if σ(i) > σ(j) then xσ(i) − xσ(j) = −(xσ(j) − xσ(i)).

Becauseof such sign changes, we either have σ(∆n) = ∆n or σ(∆n) = −∆n.

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Definition 8

∆n =∏

1≤i<j≤n

(xi − xj).

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Definition 8

∆n =∏

1≤i<j≤n

(xi − xj).

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Let σ = (123) acts on ∆3:

σ(∆3) = (x2 − x3)(x2 − x1)(x3 − x1)= ∆3.Let τ = (12) acts on ∆3: τ(∆3) = (x2 − x1)(x2 − x3)(x1 − x3)= −∆3.

Definition 8

∆n =∏

1≤i<j≤n

(xi − xj).

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Let σ = (123) acts on ∆3: σ(∆3) = (x2 − x3)(x2 − x1)(x3 − x1)=

∆3.Let τ = (12) acts on ∆3: τ(∆3) = (x2 − x1)(x2 − x3)(x1 − x3)= −∆3.

Definition 8

∆n =∏

1≤i<j≤n

(xi − xj).

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Let σ = (123) acts on ∆3: σ(∆3) = (x2 − x3)(x2 − x1)(x3 − x1)= ∆3.

Let τ = (12) acts on ∆3: τ(∆3) = (x2 − x1)(x2 − x3)(x1 − x3)= −∆3.

Definition 8

∆n =∏

1≤i<j≤n

(xi − xj).

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Let σ = (123) acts on ∆3: σ(∆3) = (x2 − x3)(x2 − x1)(x3 − x1)= ∆3.Let τ = (12) acts on ∆3:

τ(∆3) = (x2 − x1)(x2 − x3)(x1 − x3)= −∆3.

Definition 8

∆n =∏

1≤i<j≤n

(xi − xj).

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Let σ = (123) acts on ∆3: σ(∆3) = (x2 − x3)(x2 − x1)(x3 − x1)= ∆3.Let τ = (12) acts on ∆3: τ(∆3) = (x2 − x1)(x2 − x3)(x1 − x3)=

−∆3.

Definition 8

∆n =∏

1≤i<j≤n

(xi − xj).

σ(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)).

Example 9 (∆3 = (x1 − x2)(x1 − x3)(x2 − x3))

Theorem 10

A permutation σ in Sn is even (i.e., σ ∈ An) if and only if σ(∆n) = ∆n.

Set X = {∆n,−∆n}. For σ ∈ Sn, we define σ̂ : X → X by

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

It is easy to check that σ̂τ(∆n) = σ̂(τ̂(∆n)) for any two σ, τ ∈ Sn.Let ρ = (rs) be any transposition. Claim: ρ̂(∆n) = −∆n.

Assume that r < s. By definition, ρ̂(∆n) =∏

1≤i<j≤n

(xρ(i) − xρ(j)). We have

xρ(r)− xρ(s) = xs − xr = −(xr − xs) and xρ(i)− xρ(j) = xi − xj for i , j 6= r , s.

(1) if i > s : (xρ(r) − xi )(xρ(s) − xi ) = (xs − xi )(xr − xi ) = (xr − xi )(xs − xi ).

(2) if r < i < s : (xρ(r) − xi )(xi − xρ(s)) = (xs − xi )(xi − xr ) = (xr − xi )(xi − xs).

(3) if i < r : (xi − xρ(r))(xi − xρ(s)) = (xi − xs)(xi − xr ) = (xi − xr )(xi − xs).

Thus ρ̂(∆n) = −∆n. Given any σ ∈ Sn, we can write σ = ρ1ρ2 · · · ρk .Then σ̂(∆n) = (−1)k∆n. (Why?) This completes the proof. (Why?)

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

Theorem 10

Set X = {∆n,−∆n}.

For σ ∈ Sn, we define σ̂ : X → X by

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

It is easy to check that σ̂τ(∆n) = σ̂(τ̂(∆n)) for any two σ, τ ∈ Sn.

Let ρ = (rs) be any transposition. Claim: ρ̂(∆n) = −∆n.

1≤i<j≤n

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

It is easy to check that σ̂τ(∆n) = σ̂(τ̂(∆n)) for any two σ, τ ∈ Sn.Let ρ = (rs) be any transposition. Claim:

ρ̂(∆n) = −∆n.

1≤i<j≤n

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

Assume that r < s.

By definition, ρ̂(∆n) =∏

1≤i<j≤n

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

xρ(r)− xρ(s) = xs − xr = −(xr − xs) and

xρ(i)− xρ(j) = xi − xj for i , j 6= r , s.

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

ρ̂(∆n) = −∆n. Given any σ ∈ Sn, we can write σ = ρ1ρ2 · · · ρk .Then σ̂(∆n) = (−1)k∆n. (Why?) This completes the proof. (Why?)

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

Thus ρ̂(∆n) = −∆n.

Given any σ ∈ Sn, we can write σ = ρ1ρ2 · · · ρk .Then σ̂(∆n) = (−1)k∆n. (Why?) This completes the proof. (Why?)

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

Thus ρ̂(∆n) = −∆n. Given any σ ∈ Sn, we can write σ = ρ1ρ2 · · · ρk .Then

σ̂(∆n) = (−1)k∆n. (Why?) This completes the proof. (Why?)

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

Thus ρ̂(∆n) = −∆n. Given any σ ∈ Sn, we can write σ = ρ1ρ2 · · · ρk .Then σ̂(∆n) = (−1)k∆n. (Why?)

This completes the proof. (Why?)

Theorem 10

σ̂(∆n) =∏

1≤i<j≤n

(xσ(i) − xσ(j)) and σ̂(−∆n) = −∏

1≤i<j≤n

(xσ(i) − xσ(j)).

1≤i<j≤n

§3.6 Permutation Groups

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