ABSTRACT ALGEBRA ON LINE: Groups

3.1.9. Definition. A group G is said to be a finite group if the set G has a finite number of elements. In this case, the number of elements is called the order of G, denoted by |G|.

3.2.7. Definition. Let a be an element of the group G. If there exists a positive integer n such that aⁿ = e, then a is said to have finite order, and the smallest such positive integer is called the order of a, denoted by o(a).
If there does not exist a positive integer n such that aⁿ = e, then a is said to have infinite order.

3.2.1. Definition. Let G be a group, and let H be a subset of G. Then H is called a subgroup of G if H is itself a group, under the operation induced by G.

3.2.2. Proposition. Let G be a group with identity element e, and let H be a subset of G. Then H is a subgroup of G if and only if the following conditions hold:

(i) ab

H for all a,b

(ii) e

(iii) a^-1

H for all a

3.2.10. Theorem. (Lagrange) If H is a subgroup of the finite group G, then the order of H is a divisor of the order of G.

3.2.11. Corollary. Let G be a finite group of order n.

(a) For any a

G, o(a) is a divisor of n.

(b) For any a

G, aⁿ = e.

Example 3.2.12. (Euler's theorem) Let G be the multiplicative group of congruence classes modulo n. The order of G is given by

(n), and so by Corollary 3.2.11, raising any congruence class to the power

(n) must give the identity element.

3.2.12. Corollary. Any group of prime order is cyclic.

3.4.1. Definition. Let G₁ and G₂ be groups, and let : G₁ -> G₂ be a function. Then is said to be a group isomorphism if

(i)

is one-to-one and onto and

(ii)

(ab) =

(a)

(b) for all a,b

G₁.

In this case, G₁ is said to be isomorphic to G₂, and this is denoted by G₁

G₂.

3.4.3. Proposition. Let : G₁ -> G₂ be an isomorphism of groups.

(a) If a has order n in G₁, then

(a) has order n in G₂.

(b) If G₁ is abelian, then so is G₂.

(c) If G₁ is cyclic, then so is G₂.

Cyclic groups

3.2.5 Definition. Let G be a group, and let a be any element of G. The set

<a> = { x G | x = aⁿ for some n Z }

is called the cyclic subgroup generated by a.
The group G is called a cyclic group if there exists an element a

G such that G=<a>. In this case a is called a generator of G.

3.2.6 Proposition. Let G be a group, and let a G.

(a) The set <a> is a subgroup of G.

(b) If K is any subgroup of G such that a

K, then <a>

3.2.8. Proposition. Let a be an element of the group G.

(a) If a has infinite order, and a^k = a^m for integers k,m, them k=m.

(b) If a has finite order and k is any integer, then a^k = e if and only if o(a) | k.

(c) If a has finite order o(a)=n, then for all integers k, m, we have

a^k = a^m if and only if k m (mod n).

Furthermore, |<a>|=o(a).

Corollaries to Lagrange's Theorem (restated):

(a) For any a

G, o(a) is a divisor of |G|.

(b) For any a

G, aⁿ = e, for n = |G|.

3.5.1. Theorem. Every subgroup of a cyclic group is cyclic.

3.5.2 Theorem. Let G cyclic group.

(a) If G is infinite, then G

(b) If |G| = n, then G

Z_n.

3.5.3. Proposition. Let G = <a> be a cyclic group with |G| = n.

(a) If m

Z, then <a^m> = <a^d>, where d=gcd(m,n), and a^m has order n/d.

(b) The element a^k generates G if and only if gcd(k,n)=1.

(c) The subgroups of G are in one-to-one correspondence with the positive divisors of n.

(d) If m and k are divisors of n, then <a^m> <a^k> if and only if k | m.

3.5.6. Definition. Let G be a group. If there exists a positive integer N such that a^N=e for all a

G, then the smallest such positive integer is called the exponent of G.

3.5.7. Lemma. Let G be a group, and let a,b G be elements such that ab = ba. If the orders of a and b are relatively prime, then o(ab) = o(a)o(b).

3.5.8. Proposition. Let G be a finite abelian group.

(a) The exponent of G is equal to the order of any element of G of maximal order.

(b) The group G is cyclic if and only if its exponent is equal to its order.

Permutation groups

3.1.4. Definition. The set of all permutations of a set S is denoted by Sym(S).
The set of all permutations of the set {1,2,...,n} is denoted by S_n.

3.1.5. Proposition. If S is any nonempty set, then Sym(S) is a group under the operation of composition of functions.

2.3.5. Theorem. Every permutation in S_n can be written as a product of disjoint cycles. The cycles that appear in the product are unique.

2.3.8 Proposition. If a permutation in S_n is written as a product of disjoint cycles, then its order is the least common multiple of the lengths of its cycles.

3.6.1. Definition. Any subgroup of the symmetric group Sym(S) on a set S is called a permutation group or group of permutations.

3.6.2. Theorem. (Cayley) Every group is isomorphic to a permutation group.

3.6.3. Definition. Let n > 2 be an integer. The group of rigid motions of a regular n-gon is called the nth dihedral group, denoted by D_n.

We can describe the nth dihedral group as

D_n= { a^k, a^kb | 0 k < n },

subject to the relations o(a) = n, o(b) = 2, and ba = a^-1b.

2.3.11. Theorem. If a permutation is written as a product of transpositions in two ways, then the number of transpositions is either even in both cases or odd in both cases.

2.3.12. Definition. A permutation is called even if it can be written as a product of an even number of transpositions, and odd if it can be written as a product of an odd number of transpositions.

3.6.4. Proposition. The set of all even permutations of S_n is a subgroup of S_n.

3.6.5. Definition. The set of all even permutations of S_n is called the alternating group on n elements, and will be denoted by A_n.

Other examples

Example 3.1.4. (Group of units modulo n) Let n be a positive integer. The set of units modulo n, denoted by Z_n^×, is an abelian group under multiplication of congruence classes. Its order is given by the value

(n) of Euler's phi-function.

3.1.10. Definition. The set of all invertible n × n matrices with entries in R is called the general linear group of degree n over the real numbers, and is denoted by GL_n(R).

3.1.11. Proposition. The set GL_n(R) forms a group under matrix multiplication.

3.3.3. Definition. Let G₁ and G₂ be groups. The set of all ordered pairs (x₁,x₂) such that x₁ G₁ and x₂ G₂ is called the direct product of G₁ and G₂, denoted by G₁ × G₂.

3.3.4. Proposition. Let G₁ and G₂ be groups.

(a) The direct product G₁ × G₂ is a group under the multiplication defined for all
(a₁,a₂), (b₁,b₂)

G₁ × G₂ by

(a₁,a₂) (b₁,b₂) = (a₁b₁,a₂b₂).

(b) If the elements a₁

G₁ and a₂

G₂ have orders n and m, respectively, then in
G₁ × G₂ the element (a₁,a₂) has order lcm[n,m].

3.3.5. Definition. Let F be a set with two binary operations + and · with respective identity elements 0 and 1, where 1 is distinct from 0. Then F is called a field if

(i) the set of all elements of F is an abelian group under +;

(ii) the set of all nonzero elements of F is an abelian group under ·;

(iii) a · (b+c) = a · b + a · c for all a,b,c in F.

3.3.6. Definition. Let F be a field. The set of all invertible n × n matrices with entries in F is called the general linear group of degree n over F, and is denoted by GL_n(F).

3.3.7. Proposition. Let F be a field. Then GL_n(F) is a group under matrix multiplication.

3.4.5. Proposition. If m,n are positive integers such that gcd(m,n)=1, then

Z_m × Z_n Z_mn.

Example. 3.3.7. (Quaternion group)
Consider the following set of invertible 2 × 2 matrices with entries in the field of complex numbers.

± , ± , ± , ± .

If we let

1 = , i = , j = , k =

then we have the identities

i² = j² = k² = -1;

ij = k, jk = i, ki = j;

ji = -k, kj = -i, ik = -j.

These elements form a nonabelian group Q of order 8 called the quaternion group, or group of quaternion units.

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