discrete mathematics by meri dedania assistant professor mca department
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Discrete Mathematics by Meri Dedania Assistant Professor MCA department Atmiya Institute of Technology & Science Yogidham Gurukul Rajkot. Group Theory. Definition of Group A Group < G , > is an algebraic system in which on G satisfies four condition Closure Property - PowerPoint PPT PresentationTRANSCRIPT
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Discrete Mathematicsby
Meri DedaniaAssistant ProfessorMCA department
Atmiya Institute of Technology & ScienceYogidham Gurukul
Rajkot
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Group Theory
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Definition of Group A Group < G , > is an algebraic system
in which on G satisfies four condition Closure Property For all x , y G x y G Associative Property For all x , y , z G x (y z) = (x y) z Existence of Identity element There exists an element e G such that for any a
G x e = x = e x Existence of Inverse Element For every x G ,there exists an element denoted by
a-1 G such that x-1 x = x x-1 = e
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Definition of abelian Group A Group < G , >in which the
operation is commutative is called abelian Group i.e. a,b G , a b = b a
Example1. < Z , + > is Abelian Group2. < Q , + > is abelian Group
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Theorem 1 : Let e be an identity element in group < G , > , Then e is unique
Proof : Let e and e` are two identity in G e e` = e if e` is identity e e` = e` if e is identity since ee` is unique element in G e = e`
Properties of Group
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Theorem 2 : Inverse of each element of a group < G , > is unique
Proof : Let a be any element of G and e the
identity of GSuppose b and c are two different inverse of
a in G. a b = e = b a (if b is an inverse of a) a c = e = c a (if c is an inverse of a) Now , b = b e = b ( a c) = (b a) c = e c = cThus a has unique inverse
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Theorem 3 : if a-1 is the inverse of an element a of group < G , > then (a-1)-1=a
Proof : Let e be the identity of Group < G
, > a-1 a = e (a-1)-1 (a-1 a) = (a-1)-1 e ((a-1)-1 a-1) a = (a-1)-1
e a = (a-1)-1
(a-1)-1 = a
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Theorem 4 : If < G , > be a group then for any two elements a and b of < G , > prove that ( a b )-1 = b-1 a-1 rule of reversal
Proof : Let a-1 and b-1 are inverse of a and b respectively
and e be the identity a a-1 = e = a-1 a b b-1 = e = b-1 b (a b) (b-1 a-1) = [(a b) b-1] a-1 = [a (b b-1)] a-1 = [a e] a-1 = a a-1
= e Similarly , (b-1 a-1) (a b) = e This show that b-1 and a-1 is inverse of b and a Hence , ( a b )-1 = b-1 a-1
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Cancellation Property : if a , b and cbe any three elements of a group < G , > then
ab = ac b = c left cancellation ba = ca b = c right cancellationProof : Let a G and also a-1 G aa-1 = e = a-1a where e is identity of G Now , ab = ac a-1(ab) = a-1 (ac) (a-1 a) b = (a-1 a)c e . b = e . c b =c similarly , ba = ca b = c
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• Definition of Permutation A permutation is one to one mapping of non empty set P , say onto itself Example : Let S = {1,2,3} Then function f : S S f(1) = 2 f(2) = 3 f(3) = 1 Then permutation P1 = P2 =
Permutation Group
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P3 = P4 =
P5 = P6 =
There are n! of pattern of expressing Permutation .
So if Set has 3 elements then pattern of expressing permutation is 3! = 6
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Equality of Permutations : Let f and g be two permutations
defined on a non empty set P. Then f = g if and only if f(x) = g(x) x P
Example 1) Let S = {1,2,3,4} and let
permutation f and g are equal or not..
f = g = 2) Let S = {1,2,3,4} and let
permutation f and g are equal or not..
f = g =
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Permutation Identity An Identity permutation on S ,
denoted by I , is defined as I(a) = a a S
For example :
f =
Note : In identity permutation the image of element is element itself
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Composition of Permutation ( Product of Permutation)
Let f and g be two arbitrary permutations of like degree , given by,
f = g = on non empty set A. Then the
composition (or Product) of f and g is defined as
Continue…
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f g = = Example Let P1 = P2= P3 =
Check P1 (P2 P3) = (P1 P2) P3
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• Inverse Permutation Every permutation f on set P =
{a1,a2,a3,…,an} Possesses a unique inverse permutation , denoted by f-1 thus if
f = Then f-1 =
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Cyclic PermutationLet t1,t2,…..,tr be r distinct
elements of the set P = {t1,t2,…., tn}.Then the permutation p : P P is defined by
p(t1) = t2 , p(t2) = t3,….,p(tr-1)= tr, p(tr)=t1 is called a cyclic permutation of length r.
Example : The permutation P = is written as
(1,2) , (3,4,6) , (5).. The cycle (1,2) has length 2 , The cycle length 3,The cycle 1.
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Definition of Cyclic Group If there exists an element a G
for some group < G , > such that every element of G can be written as some power of a , that is an for some integer n. then a Group
< G , > is said to be cyclic Group
Every Cyclic Group is abelian
Example for set A = { , , ,} and binary operation
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I. If < G , > is an abelian group , then for all a , b G show that ( a b )n = an bn
Solution ( a b )n = an bn
( a b )n+1=an+1 bn+1
( a b )n+2=an+2 bn+2
Now , ( an bn ) ( a b ) = ( a b )n+1
= ( an+1 bn+1 ) (bn a )=(a bn)
By cancellation , similarly bn+1 a = a bn+1
Again bn+1 a = b(bn a) = b(abn) i.e., abn+1 = b(abn )
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III. Show that in a Group < G , > , if for any a, b G , ( a b )2 = a2 b2, then <G , > must be abelian
Solution : Let < G , > be a Group and let a , b
G ( a b )2 = a2 b2
( a b ) ( a b ) = ( a a) ( b b ) a ( b a) b = a ( a b) b By left and right cancellation property b a = a b Thus we have a b = b a . a,b G Hence < G , >is an abelian Group
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IV. Show that if every element in a group is its own inverse , then the group must be abelian
Solution : Let a , b G a b G (by closure property)Now, a-1 = a and b-1 = b ( a b)-1 = a bNow, ( a b)-1 = a b b-1 a-1 = a b b a = a b Thus we have a b = b a , a,b G Hence < G , >is an abelian Group
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V. Write down Composition table for <Z7, +5> and <Z7
*, 7> where Z7
* = Z7 - {0}
+7
0 1 2 3 4 5 6
0 0 1 2 3 4 5 61 1 2 3 4 5 6 02 2 3 4 5 6 0 13 3 4 5 6 0 1 24 4 5 6 0 1 2 35 5 6 0 1 2 3 46 6 0 1 2 3 4 5
7
1 2 3 4 5 6
1 1 2 3 4 5 62 2 4 6 1 3 53 3 6 2 5 1 44 4 1 5 2 6 35 5 3 1 6 4 26 6 5 4 3 2 1
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VII. Show that < {1} , > and < {1 , -1} , > are the only finite groups of nonzero real numbers under the operation of multiplication
Solution:
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• Definition of Sub Group : Let < G , > be a Group and S G, such
that it satisfies the following condition:1) e G , Where e is the identity of < G , > 2) For any a S , a-1 S3) For a , b S , a b SThen < S , > is called Sub Group of <G , >
For any group <G , > , <{e} , > and <G , > are Trivial Sub Groups of <G , >.Let <Z-{0}, X> is a Group then <{1}, X> & <Z,X> are Trivial Sub group of <Z, X>All other subgroups of <G , > are called Proper Subgroup Let < {,-1,0,1} , X > is Proper subgroup of <Z , X >
Sub Group and Homomorphism
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Theorem : A subset S of G is a subgroup of < G , > iff for any pair of elements a , b S , a b-1 S
Proof : Assume that S is a subgroup if a , b S then b-1 S and a b-1 S To prove the converse , let us assume that
a , b S and a b-1 S for any pair a , b. taking b = a , a a-1 = e SFrom e , a, b S e a-1 = a-1 SSimilarly , b-1 S.Finally , because a and b-1 are in S , we have
a b S.Hence , < S , > is a sub group of < G , >
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Definition of Group Homomorphism
Let < G , > and < H , > be two Group. A mapping g : G H is called a group homomorphism from < G , > to < H , > if for any a , b G
g (a b) = g(a) g(b) g(eG) = eH
g(a-1) = [g(a)]-1
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Definition of Group Isomorphism
Let f : < G , > < H , >.if f is one to one and onto. Then Group is called isomorphism
A homomorphism f : < G , > < H , > is called an endomorphism
A Isomorphism f : < G , > < H , > is called an automorphism
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Definition Kernal of Homomorphism
Let < G , > and < H , > be two Groups and let f is homomorphism of G into H. The set of elements of G which are mapped into eH , the identity of H is called the kernal of the homomorphism and is denoted by Kf or Ker(f)
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Theorem : The Kernal of homomorphism f : <G , > < H , > is sub group of < G , >
Proof : Here f : < G , > < H , > is homomorphism Ker (f) = {x G | f(x) = eH identity element of H} k (f) because eG K(f) (f(eG)=eH) let a , b Kf f (a) = eH & f(b) = eG Now, f(ab-1) = f(a) . f(b-1) = f(a) . [f(b)]-1
= eH . eH-1
= eH . eH = eH ab-1 Kf Kf is a sub group of < G , >
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• Show that every interval of lattice is a sub lattice of a lattice
Proof: Let < L , > be a lattice and a , b L a , b a a b a [ a , b ] also [a , b] = {x L | a x b} L Let x , y [ a , b ] a x b , a y b a a x y b b a a x y b b a x y b a x y b x y [ a , b ] and x y [ a , b ] [ a , b ] is sub lattice of the lattice < L , >
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Draw Hasse Diagram of the poset {2,3,5,6,9,15,24,45},D . Find
(i) Maximal and Minimal elements(ii) Greatest and Least members, if exist.(iii) Upper bound of {9,15} and l.u.b. of{9,15} , if exist.(iv) Lower bound of {15,24} and g.l.b. of{15,24} , if exist.
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Definition Of Right Cosets Let G be a Group and H is any sub
Group of G. Let a be any element of G . Then set Ha = {ha : hH} is called a right coset of H in G generated by a.• Definition of Left Cosets Let G be a Group and H is any sub
Group of G. Let a be any element of G . Then set aH = {aH : hH} is called a right coset of H in G generated by a.
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Lagrange’s Theorem : The order of each sub group of a
finite group G is a divisor of the order of G
Index in G : The number of left cosets of H in G
is called index of H in G. Definition of Normal
subgroup: A sub group < H , > is sub
group of < G , > is called a normal sub group if for any a G , aH = Ha
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Fins the Sub group of < Z12 , +12 > show that <{1,4,13,16} , 17> is
subgroup of < Z17* , 17 >
Show that every sub group of abelian group is normal
Let x G and h HXhx-1 = xx-1h = eh = h x G and h H xhx-1 H i.e. xH = Hx H is normal subgroup of G
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Definition of Irreflexive A relation R on a set A is
irreflexive if aRa for a A, if (a,a) R
For example A = {1,2,3,4}R =
{<1,1>,<1,2>,<2,3>,<1,3>,<4,4>}
R = {<1,3>,<2,1>}