1. Chapter 6 Abstract algebra
Groups 
Rings 
Field 
Lattics and Boolean algebra

2. 6.1 Operations on the set
Definition 1：An unary operation on a nonempty set S is an everywhere function f from S into S; A binary operation on a nonempty set S is an everywhere function f from S×S into S; A n-ary operation on a nonempty set S is an everywhere function f from Sn into S.
closed

3. Associative law: Let  be a binary operation on a set S
Associative law: Let  be a binary operation on a set S. a(bc)=(ab)c for a,b,cS
Commutative law: Let  be a binary operation on a set S. ab=ba for a,bS
Identity element: Let  be a binary operation on a set S. An element e of S is an identity element if ae=ea=a for all a S.
Theorem 6.1: If  has an identity element, then it is unique.

4. Inverse element: Let  be a binary operation on a set S with identity element e. Let a S. Then b is an inverse of a if ab = ba = e.
Theorem 6.2: Let  be a binary operation on a set A with identity element e. If the operation is Associative, then inverse element of a is unique when a has its inverse

5. a(bc)=(ab)(ac), (bc)a=(ba)(ca)
Distributive laws: Let  and  be two binary operations on nonempty S. For a,b,cS,
a(bc)=(ab)(ac), (bc)a=(ba)(ca)
Associative law
commutative law
Identity elements
Inverse element + √
-a for a  1
1/a for a0

6. Definition 2: An algebraic system is a nonempty set S in which at least one or more operations Q1,…,Qk(k1), are defined. We denoted by [S;Q1,…,Qk].
[Z;+]
[Z;+,*]
[N;-] is not an algebraic system

7. Definition 3: Let [S;*] and [T;] are two algebraic system with a binary operation. An everywhere function  from S to T is called a homomorphism from [S;*] to [T;] if (a*b)=(a)(b) for a,bS.

8. Theorem 6. 3 Let  be a homomorphism from [S;. ] to [T;]
Theorem 6.3 Let  be a homomorphism from [S;*] to [T;]. If  is onto, then the following results hold.
(1)If * is Associative on S, then  is also Associative on T.
(2)If * is commutative on S, then  is also commutation on T
(3)If there exist identity element e in [S;*],then (e) is identity element of [T;]
(4) Let e be identity element of [S;*]. If there is the inverse element a-1 of aS, then (a-1) is the inverse element (a).

9. Definition 4: Let  be a homomorphism from [S;. ] to [T;]
Definition 4: Let  be a homomorphism from [S;*] to [T;].  is called an isomorphism if  is also one-to-one correspondence. We say that two algebraic systems [S;*] and [T;] are isomorphism, if there exists an isomorphic function. We denoted by [S;*][T;](ST)

10. 6.2 Semigroups,monoids and groups
Definition 5: A semigroup [S;] is a nonempty set together with a binary operation  satisfying associative law.
Definition 6: A monoid is a semigroup [S; ] that has an identity.

11. Let P be the set of all nonnegative real numbers. Define & on P by
a&b=(a+b)/(1+a  b)
Prove［P;&］is a monoid.

12. 6.2.2 Groups
Definition 7: A group [S; ] is a monoid, and there exists inverse element for aS.
(1)for a,b,cS,a (b  c)=(a  b)  c;
(2)eS,for aS,a  e=e  a=a;
(3)for aS, a-1S, a  a-1=a-1  a=e

13. Abelian (or commutative) group
[R-{0},] is a group
[R,] is a monoid, but is not a group
[R-{0},], for a,bR-{0},ab=ba
Abelian (or commutative) group
Definition 8: We say that a group [G;]is an Abelian (or commutative) group if ab=ba for a,bG.
[R-{0},],[Z;+],[R;+],[C;+] are Abelian (or commutative) group .
Example: Let [G; ] be a group with identity e. If x  x=e for xG, then [G; ] is an Abelian group.

14. Example: Let G={1,-1,i,-i}.

15. G={1,-1,i,-i}, finite group
[R-{0},],[Z;+],[R;+],[C;+]，infinite group
|G|=n is called an order of the group G
Let G ={ (x; y)| x,yR with x 0} , and consider the binary operation ● introduced by (x, y) ● (z,w) = (xz, xw + y) for (x, y), (z, w) G.
Prove that (G; ●) is a group.
Is (G;●) an Abelian group?

16. [R-{0},] , [R;+]
a+b+c+d+e+f+…=(a+b)+c+d+(e+f)+…,
abcdef…=(ab)cd(ef)…,
Theorem 6.4: If a1,…,an(n3), are arbitrary elements of a semigroup, then all products of the elements a1,…,an that can be formed by inserting meaningful parentheses arbitrarily are equal.

17. a1  a2  …  an
If ai=aj=a(i,j=1,…,n), then a1  a2  …  an=an。 na

18. Theorem 6.5: Let [G;] be a group and let aiG(i=1…,n). Then
(a1…an)-1=an-1…a1-1

19. Exercise p333 9,11,18,19,22,23; Prove Theorem 6.3 (2)(4)
Next: Permutation groups and cyclic groups
Exercise
p333 9,11,18,19,22,23;
P340 5—7,13,14,19—21
P357 2,6-9,
Prove Theorem 6.3 (2)(4)