Group abstract algebra

सुस्वागतम ्Ë

Pratap College Amalner
S. Y. B. Sc.
Subject :- Mathematics
Groups
Prof. Nalini S. Patil
(HOD)
Mob. 9420941034, 9075881034

Groups
1. Introduction.
2. Normal subgroups, quotien groups.
3. Homomorphism.

1. Introduction
1.1. Binary Operations
1.2. Definition of Groups
1.3. Examples of Groups
1.4. Subgroups

1.1. Binary Operations
A binary operation on a set is a rule
for combining two elements of the set. More
precisely, if S iz a nonempty set, a binary
operation on S iz a mapping f : S  S  S.
Thus f associates with each ordered pair
(x, y) of element of S an element f(x, y) of S.
It is better notation to write x*y for f(x, y),
refering to as the binary operation.

1.2.Definition of Groups
A group (G, ・) is a set G together with a binary operation
・ satisfying the following axioms.
(i) The operation ・ is associative; that is,
(a ・ b) ・ c = a ・ (b ・ c) for all a, b, c ∈ G.
(ii) There is an identity element e ∈ G such that
e ・ a = a ・ e = a for all a ∈ G.
(iii) Each element a ∈ G has an inverse element a−1 ∈ G such that a-1
・ a = a ・ a−1 = e.

If the operation is commutative,
that is,
if a ・ b = b ・ a for all a, b ∈ G,
the group is called commutative or
abelian, in honor of the mathematician
Niels Abel.

1.3.Examples of Groups :
Example 1.3.1.
Let G be the set of complex numbers {1,−1,
i,−i} and let ・ be the standard multiplication of complex
numbers. Then (G, ・) is an abelian group. The product
of any two of these elements is an element of G; thus G
is closed under the operation. Multiplication is
associative and commutative in G because multiplication
of complex numbers is always associative and
commutative. The identity element is 1, and the inverse
of each element a is the element 1/a. Hence
1−1 = 1, (−1)−1 = −1, i−1 = −i, and (−i)−1 = i.

Example 1.3.2.
The set of all rational numbers, Q, forms an abelian
group (Q,+) under addition.The identity is 0, and the inverse
of each element is its negative. Similarly,
(Z,+), (R,+), and (C,+) are all abelian groups under
addition.
Example1. 3.3.
If Q∗, R∗, and C∗ denote the set of nonzero rational, real,
and complex numbers, respectively, (Q∗,・),
(R∗,・), and (C∗, ・) are all abelian groups under
multiplication.

Example 1.3.4.
A translation of the plane R2 in the direction of
the vector (a, b) is a function f :R2 → R2 defined by f (x, y)
= (x + a, y + b). The composition of this translation with a
translation g in the direction of (c, d) is the function
f g:R2 → R2, where
f g(x, y) = f (g(x, y))= f (x + c, y + d)= (x + c + a, y + d
+ b).
This is a translation in the direction of (c + a, d + b). It
can easily be verified that the set of all translations in R2
forms an abelian group, under composition. The identity
is the identity transformation 1R
2 :R2 → R2, and the inverse
of the translation in the direction (a, b) is the translation in
the opposite direction (−a,−b).

Example1.3.5.
If S(X) is the set of bijections from any set X to
itself, then (S(X), ) is a group under composition. This
group is called the symmetric group or permutation group
of X.
Proposition 1.3.1.
If a, b, and c are elements of a group G, then
(i) (a−1)−1 = a.
(ii) (ab)−1 = b−1a−1.
(iii) ab = ac or ba = ca implies that b = c. (cancellation
law)

1.4. Subgroups :
It often happens that some subset of a group will
also form a group under the same operation.Such
a group is called a subgroup. If (G, ・) is a
group and H is a nonempty subset of G, then
(H, ・) is called a subgroup of (G, ・) if the
following conditions hold:
(i) a ・ b ∈ H for all a, b ∈ H. (closure)
(ii) a−1 ∈ H for all a ∈ H. (existence of inverses)
Conditions (i) and (ii) are equivalent to the single
condition:
(iii) a ・ b−1 ∈ H for all a, b ∈ H.

Proposition 1.4.2.
If H is a nonempty finite subset of a group G and ab
∈ H for all a, b ∈ H, then H is a subgroup of G.
Example 1.4.1.
In the group ({1,−1, i,−i}, ・), the subset {1,−1}
forms a subgroup because this subset is closed under
multiplication

Example 1.4.2.
The group Z is a subgroup of Q,Q is a subgroup of
R, and R is a subgroup of C. (Remember that addition is
the operation in all these groups.)
However, the set N = {0, 1, 2, . . .} of nonnegative
integers is a subset of Z but not a subgroup, because the
inverse of 1, namely, −1, is not in N. This example shows
that Proposition 1.4.2 is false if we drop the condition that
H be finite.
The relation of being a subgroup is transitive. In
fact, for any group G, the inclusion relation between the
subgroups of G is a partial order relation.

Definition. :
Let G be a group and let a  G. If ak = 1 for some k
 1, then the smallest such exponent k  1 is called the
order of a; if no such power exists, then one says that a has
infinite order.
Proposition 1.4.3. :
Let G be a group and assume that a G has finite
order k. If an = 1, then k | n. In fact, {n Z : an = 1} is the
set of all the multiples of k.

Definition. :
If G is a group and a  G, write
<a > = {an : n Z} = {all powers of a } .
It is easy to see that <a > is a subgroup of G .
< a > is called the cyclic subgroup of G generated by a.
A group G is called cyclic if there is some a  G with G =
< a >; in this case a is called a generator of G.
If G= <a > is a cyclic group of order n, then ak is a
generator of G if and only if gcd(k; n)= 1.
Corollary 1.4.5. :
The number of generators of a cyclic group of order n is
(n).

Let G be a finite group and let a  G. Then the
order of a is the number of elements in <a >.
Definition. :
If G is a finite group, then the number of elements in
G, denoted by G, is called the order of G.

Group abstract algebra

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