Page 1
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 1
Subject: Mathematics
Lesson: Lagrange's Theorem and Homomorphism
Course Developer: Umesh Chand
Department / College: Assistant Professor, Department of
Mathematics, Kirorimal College
University of Delhi
Page 2
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 1
Subject: Mathematics
Lesson: Lagrange's Theorem and Homomorphism
Course Developer: Umesh Chand
Department / College: Assistant Professor, Department of
Mathematics, Kirorimal College
University of Delhi
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 2
Table of Contents:
Chapter: Lagrange's theorem and its consequences, Homomorphism,
Properties of Homomorphism
? 1. Learning outcomes
? 2. Introduction
? 3. Subgroup
? 4. Coset
? 5. Lagrange's theorem.
? 6. Normal subgroup
? 7. Quotient group
? 8. Homomorphism.
8.1 Kernel of homomorphism.
8.2 Properties of homomorphism.
8.3 Properties of subgroups under homomorphism.
? Exercise
? Summary
? References / Bibliography / Further Reading
Page 3
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 1
Subject: Mathematics
Lesson: Lagrange's Theorem and Homomorphism
Course Developer: Umesh Chand
Department / College: Assistant Professor, Department of
Mathematics, Kirorimal College
University of Delhi
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 2
Table of Contents:
Chapter: Lagrange's theorem and its consequences, Homomorphism,
Properties of Homomorphism
? 1. Learning outcomes
? 2. Introduction
? 3. Subgroup
? 4. Coset
? 5. Lagrange's theorem.
? 6. Normal subgroup
? 7. Quotient group
? 8. Homomorphism.
8.1 Kernel of homomorphism.
8.2 Properties of homomorphism.
8.3 Properties of subgroups under homomorphism.
? Exercise
? Summary
? References / Bibliography / Further Reading
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 3
1. Learning outcomes
After studying the whole content of this chapter, students will be able to
understand
? Lagrange's theorem.
? Consequences of Lagrange's theorem.
? Homomorphism and its properties
2. Introduction:
The Lagrange's theorem, Homomorphism are the most important
concept in Algebra. The consequences of Lagrenge's theorem is that the
order of any element of finite group, divides the order of group. This can
be used to prove any another theorem as Fermat's little theorem and
Euler's theorem.
The homomorphism is also very useful in modern algebra; the word
homomorphism come from the ancient Greek word; homo meaning
"same" and morphe meaning "shape".
3. Subgroup: Let G be a group. A non empty subset H of a group G
is said to be subgroup of G if H form the group under the binary operation
of G.
Example 1: Set of all even integers E with "+" is a subgroup of (Z, +).
Value addition : Note
1. A non empty subset H of a group G is a subgroup of G iff
(i) a, b ? H ? ab ? H
(ii) a ? H ? a
?1
?
H.
2. A non empty subset H of a group G is subgroup of G iff ab
?1
? H ?
Page 4
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 1
Subject: Mathematics
Lesson: Lagrange's Theorem and Homomorphism
Course Developer: Umesh Chand
Department / College: Assistant Professor, Department of
Mathematics, Kirorimal College
University of Delhi
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 2
Table of Contents:
Chapter: Lagrange's theorem and its consequences, Homomorphism,
Properties of Homomorphism
? 1. Learning outcomes
? 2. Introduction
? 3. Subgroup
? 4. Coset
? 5. Lagrange's theorem.
? 6. Normal subgroup
? 7. Quotient group
? 8. Homomorphism.
8.1 Kernel of homomorphism.
8.2 Properties of homomorphism.
8.3 Properties of subgroups under homomorphism.
? Exercise
? Summary
? References / Bibliography / Further Reading
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 3
1. Learning outcomes
After studying the whole content of this chapter, students will be able to
understand
? Lagrange's theorem.
? Consequences of Lagrange's theorem.
? Homomorphism and its properties
2. Introduction:
The Lagrange's theorem, Homomorphism are the most important
concept in Algebra. The consequences of Lagrenge's theorem is that the
order of any element of finite group, divides the order of group. This can
be used to prove any another theorem as Fermat's little theorem and
Euler's theorem.
The homomorphism is also very useful in modern algebra; the word
homomorphism come from the ancient Greek word; homo meaning
"same" and morphe meaning "shape".
3. Subgroup: Let G be a group. A non empty subset H of a group G
is said to be subgroup of G if H form the group under the binary operation
of G.
Example 1: Set of all even integers E with "+" is a subgroup of (Z, +).
Value addition : Note
1. A non empty subset H of a group G is a subgroup of G iff
(i) a, b ? H ? ab ? H
(ii) a ? H ? a
?1
?
H.
2. A non empty subset H of a group G is subgroup of G iff ab
?1
? H ?
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 4
a,b ? H.
4. Coset: Let G be a group. H is a subgroup of G. For any a ? G, the set
{ah | h ? H} is denoted by aH, is called left coset of H in G by a.
Similarly the set {ha | h ? H} is denoted by Ha, is called right coset of H
in G by a.
| Ha | denote the number of elements in the set Ha.
Example 2: Let G = S
3
and H = {(1), (13)}, then the all left-cosets of H
in G are
(1) H = H
(12) H = {(12), (12)(13)} = {(12), (132)} = (132)H
(13)H = {(13), {1)} = H
(2, 3)H = {(23), (23), (13)} = {(23), (123)} = (123)H
Value addition : Remember
Let H be a subgroup of a group G, and let a and b belongs to G.
Then
(i) a ? aH
(ii) aH = H iff a ? H
(iii) aH = bH or aH ? bH = ?
(iv) aH = bH iff a
?1
b ? H
(v) | aH | = | bH |
(vi) aH = Ha iff H = aHa
?1
(vii) aH is a subgroup of G iff a ?H.
Page 5
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 1
Subject: Mathematics
Lesson: Lagrange's Theorem and Homomorphism
Course Developer: Umesh Chand
Department / College: Assistant Professor, Department of
Mathematics, Kirorimal College
University of Delhi
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 2
Table of Contents:
Chapter: Lagrange's theorem and its consequences, Homomorphism,
Properties of Homomorphism
? 1. Learning outcomes
? 2. Introduction
? 3. Subgroup
? 4. Coset
? 5. Lagrange's theorem.
? 6. Normal subgroup
? 7. Quotient group
? 8. Homomorphism.
8.1 Kernel of homomorphism.
8.2 Properties of homomorphism.
8.3 Properties of subgroups under homomorphism.
? Exercise
? Summary
? References / Bibliography / Further Reading
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 3
1. Learning outcomes
After studying the whole content of this chapter, students will be able to
understand
? Lagrange's theorem.
? Consequences of Lagrange's theorem.
? Homomorphism and its properties
2. Introduction:
The Lagrange's theorem, Homomorphism are the most important
concept in Algebra. The consequences of Lagrenge's theorem is that the
order of any element of finite group, divides the order of group. This can
be used to prove any another theorem as Fermat's little theorem and
Euler's theorem.
The homomorphism is also very useful in modern algebra; the word
homomorphism come from the ancient Greek word; homo meaning
"same" and morphe meaning "shape".
3. Subgroup: Let G be a group. A non empty subset H of a group G
is said to be subgroup of G if H form the group under the binary operation
of G.
Example 1: Set of all even integers E with "+" is a subgroup of (Z, +).
Value addition : Note
1. A non empty subset H of a group G is a subgroup of G iff
(i) a, b ? H ? ab ? H
(ii) a ? H ? a
?1
?
H.
2. A non empty subset H of a group G is subgroup of G iff ab
?1
? H ?
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 4
a,b ? H.
4. Coset: Let G be a group. H is a subgroup of G. For any a ? G, the set
{ah | h ? H} is denoted by aH, is called left coset of H in G by a.
Similarly the set {ha | h ? H} is denoted by Ha, is called right coset of H
in G by a.
| Ha | denote the number of elements in the set Ha.
Example 2: Let G = S
3
and H = {(1), (13)}, then the all left-cosets of H
in G are
(1) H = H
(12) H = {(12), (12)(13)} = {(12), (132)} = (132)H
(13)H = {(13), {1)} = H
(2, 3)H = {(23), (23), (13)} = {(23), (123)} = (123)H
Value addition : Remember
Let H be a subgroup of a group G, and let a and b belongs to G.
Then
(i) a ? aH
(ii) aH = H iff a ? H
(iii) aH = bH or aH ? bH = ?
(iv) aH = bH iff a
?1
b ? H
(v) | aH | = | bH |
(vi) aH = Ha iff H = aHa
?1
(vii) aH is a subgroup of G iff a ?H.
Lagrange's Theorem and Homomorphism
Institute of Lifelong Learning, University of Delhi pg. 5
Value addition: Note
1. There is always a one-one onto mapping between any two right
coset of H in G.
2. The number of distinct left (or right) cosets of H in G is given by
G
O(G)
i (H)
O(H)
?? No. of distinct left cosets
It is called index of subgroup H of a group G.
5. Lagrange's theorem:
Theorem 1: Let G be a finite group and H is a subgroup of G then O(H) |
O(G).
Proof: Let
1 2 r
a H, a H,...a H denote the distinct left coset of H in G.
Let aG ? be any element of G, then a ? a
i
H for some i
?
1 2 r
a a H a H.... a H ? ? ?
?
1 2 r
G a H a H.... a H ? ? ?
Also
1 2 r
a H a H.... a H G ? ? ?
?
1 2 r
G a H a H.... a H ? ? ?
Since
1 2 r
G a H, a H....a H ? are distinct left coset
1 2 r
O(G) O(a H) (a H).... O(a H) ? ? ?
O(H) (H).... O(H) ? ? ? [ O(aH) O(H)] ? ?
? O(G) rO(H) ?
Hence O(H) | O(G).
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