1. Taibah University College of Computer Science & Engineering Course Title: Discrete Mathematics Code: CS 103
Chapter 2
Sets
Slides are adopted from “Discrete Mathematics and It's Applications” Kenneth H. Rosen; 7th edition, 2012.

2. Chapter Sets

3. 2.1 Sets
Definition : A set is an unordered collection of objects.
Definition : The objects in a set are called the elements, or members, of
the set.
Capital letters (A, B, S…) used for sets
Italic lower-case letters (a, x, y…) used to denote elements of sets
There are several ways to describe a set.
Listing all members of a set, when this is possible
Some examples:
{1, 2, 3} is the set containing “1” and “2” and “3.”
{1, 1, 2, 3, 3} = {1, 2, 3} since repetition is irrelevant.
{1, 2, 3} = {3, 2, 1} since sets are unordered.
{1, 2, 3, …} is a way we denote an infinite set.
 = { } is the empty set, or the set containing no elements.
Note that   {}

4. 2.1 Sets 2. Set Builder notation
Characterize all those elements in the set by stating the property or properties they must have to be members.
D = {x | x is prime and x > 2}
E = {x | x is odd and x > 2}
The vertical bar means “such that”
N = {0, 1, 2, 3, …} is the set of natural numbers
Z = {…, -2, -1, 0, 1, 2, …} is the set of integers
Z+ = {1, 2, 3, …} is the set of positive integers
Z- = {-1, -2, -3, …} is the set of negative integers
Q = {p/q : p, q  Z, q ≠ 0} is the set of rational numbers
Any number that can be expressed as a fraction of two integers (where the bottom one is not zero)
R is the set of real numbers
R+, the set of positive real numbers
C, the set of complex numbers.

5. 2.1 Sets x  S means “x is an element of set S.”
x  S means “x is not an element of set S.”
Example: 4  {1, 2, 3, 4}
7  {1, 2, 3, 4}
Universal set “U” : the set of all of elements (or the
“universe”) from which given any set is drawn.
For the set {-2, 0.4, 2}, U would be the real
numbers
For the set {0, 1, 2}, U could be the N, Z, Q, R
depending on the context
For the set of the vowels of the alphabet, U would be
all the letters of the alphabet

6. 2.1 Sets
Venn diagrams
Sets can be represented graphically using Venn diagrams
universal set U, which contains all the objects under
consideration, is represented by a rectangle.
Circles or other geometrical figures Inside this
rectangle are used to represent sets.
Points represent the particular elements of the set. S U

7. 2.1 Sets Sets can contain other sets
V = { {{1}, {{2}}}, {{{3}}}, { {1}, {{2}}, {{{3}}} } }
V has only 3 elements!
Note that 1 ≠ {1} ≠ {{1}} ≠ {{{1}}}
They are all different
 ≠ {  }
The first is a set of zero elements
The second is a set of 1 element
Replace  by { }, and you get: { } ≠ {{ }}
It’s easier to see that they are not equal that way

8. 2.1 Sets
Definition:
Two sets are equal if and only if they have the same elements. Therefore, if A and B are sets, then A and B are equal if and only if
∀ x(x ∈ A ↔ x ∈ B).
We write A = B if A and B are equal sets.
The set A is a subset of B if and only if every element of A is also an element of B.We use the notation A ⊆ B to indicate that A is a subset of the set B.
Examples:
{1, 2, 3, 4, 5} = {5, 4, 3, 2, 1} but {1, 2, 3, 4, 5} ≠ {1, 2, 3, 4}
If A = {2, 4, 6}, B = {1, 2, 3, 4, 5, 6, 7}, A is a subset of B
This is specified by A  B meaning that  x (x  A  x  B)

9. 2.1 Sets A  B means “A is a subset of B.” or, “B contains A.”
or, “every element of A is also in B.”
or, x ((x  A)  (x  B)).
For any set S, S  S (S S  S)
For any set S,   S (S   S)
Every non-empty set S, it has at least two subsets:
1)   S ) S  S A B
Venn Diagram U

10. 2.1 Sets A  B means “A is a subset of B.”
A  B means “A is a superset of B.”
A = B if and only if A and B have exactly the same elements.
iff, A  B and B  A
iff, A  B and A  B
iff, x ((x  A)  (x  B)).

11. 2.1 Sets Proper Subsets U A  B means “A is a proper subset of B.”
A  B, and A  B.
x ((x  A)  (x  B))  x ((x  B)  (x  A)) A B U

12. 2.1 Sets
The difference between “subset” and “proper subset” is like the difference between “less than or equal to” and “less than” for numbers.
{1,2,3}  {1,2,3,4,5}
{1,2,3}  {1,2,3,4,5}
Examples:
Let B = {0, 1, 2, 3, 4, 5}
If A = {1, 2, 3}, A is not equal to B, and A is a subset of B
A proper subset is written as A  B
Sets may contain other sets as members:
A = {0, {a}, {b}, {a, b}} and B = {x I x is a subset of the set {a, b}}.
Note that A = B, {a}  A , but a  A

13. 2.1 Sets Quick examples: Is   {1,2,3}?
Yes! x (x  )  (x  {1,2,3}) holds, because (x  ) is false.
Is   {1,2,3}? NO
Yes
Is   {,1,2,3}?
Is   {,1,2,3}?
Is {x}  {x}? No

14. 2.1 Sets Cardinality of Set
If S is finite, then the cardinality of S is the number of distinct elements in S.
The cardinality of S is denoted by |S|.
Examples:
If S = {1, 2, 3, 4, 5}, Then |S| = 5
If S = {3,3,3,3,3}, Then |S| = 1
If S = , Then |S| = 0
If S = {, {a}, {b}, {a, b}}, Then |S| = 4
If S be the set of odd positive integers less than 10 Then
|S| = 5.
If S be the set of letters in the English alphabet Then |S| = 26

15. 2.1 Sets
Power Sets
The power set of S is the set of the subsets of S.
(written as P(S))
Given S = {0, 1, 2} . All the possible subsets of S?
P(S) = {, {0}, {l}, {2}, {0, 1}, {0, 2}, {l, 2}, {0, 1, 2}}
Note that |S| = 3 and |P(S)| = 8
If S has n elements then the power set of S has 2n elements
If S is a set, then the power set of S is
2S = { x : x  S }.
If S = {a}, then P(S) =2S = {, {a}}.
If S = {,{}}, then P(S) = 2S = {, {}, {{}}, {,{}}}.
Fact: if S is finite, |P(S)| =|2S| = 2|S|. (if |S| = n, |2S| = 2n)

16. 2.1 Sets Cartesian Product
The Cartesian Product of two sets A and B is:
A x B = { < a, b > : a  A  b  B}
Example: Given A = { a, b } and B = { 0, 1 }, what is
their Cartesian product?
C = A x B = { (a,0), (a,1), (b,0), (b,1) }
Example: What is the Cartesian product of:
A = { I , 2} and B = {a, b, c}
Solution: The Cartesian product A x B is:
A  B = {(1 , a), (1, b), (1 , c) , (2 , a), (2 , b), (2 , c) }.
Note that: A  B ≠ B  A
Since, B  A = {(a, I), (a, 2), (b, I), (b, 2), (c , 1), (c , 2)}
A1 x A2 x … x An = {<a1, a2,…, an>: a1  A1, a2  A2, …, an  An}

17. 2.1 Sets
Example: What is the Cartesian product A  B  C, where A = {0, l}, B = { I, 2}, and C = {0, 1 , 2}?
Solution:
A  B  C = {(0, 1 , 0) , (0 , I, I), (0 , 1, 2), (0 , 2 , 0), (0 , 2 , I), (0 , 2 , 2) , (1 , 1 , 0) , (1 , 1 , 1), (1 , 1 , 2), (1, 2 , 0), (1 , 2 , 1), (1 , 2 , 2)}.

18. 2.1 Sets Exercises 1. List the members of these sets.
a) {x I x is a real number such that x2 = I}
b) {x I x is a positive integer less than 12}
c) {x I x is the square of an integer and x < 100}
d) {x I x is an integer such that x2 = 2}
2. Use set builder notation to give a description of each of these sets.
a) {0, 3, 6, 9, I2}
b) {-3, -2, -I,0, I , 2, 3}
c) {m, n, o, p}
3. Determine whether each of these pairs of sets are equal.
a) {I, 3, 3, 3, 5, 5, 5, 5, 5}, {5, 3, I}
b) {{I}}, {I, {I}}
c) , {}
4. Suppose that A = {2, 4, 6}, B = {2, 6}, C = {4, 6}, and D = {4, 6, 8}. Determine which of these sets are subsets of which other of these sets.

19. 2.1 Sets
Exercises

20. 2.1 Sets
Exercises

21. 2.1 Sets
Exercises

22. Chapter 2.2 Set Operations

23. 2.2 Set Operations Definition: Union
Two sets can be combined in many different ways:
Definition: Union
The union of the sets A and B, denoted by A U B, is the set that
contains those elements that are either in A or in B, or in both.
A  B = { x : x  A v x  B}
Examples:
{1, 2, 3} U {3, 4, 5} = {1, 2, 3, 4, 5}
{a, b} U {3, 4} = {a, b, 3, 4}
{1, 2} U  = {1, 2} A B

24. 2.2 Set Operations Properties of the union operation
A U  = A Identity law
A U U = U Domination law
A U A = A Idempotent law
A U B = B U A Commutative law
A U (B U C) = (A U B) U C Associative law

25. 2.2 Set Operations Intersection
The intersection of the sets A and B, denoted by A  B, is
the set containing those elements in both A and B
A  B = { x : x  A  x  B}
Examples:
{1, 2, 3} ∩ {3, 4, 5} = {3}
{a, b} ∩ {3, 4} = 
{1, 2} ∩  =  A B

26. 2.2 Set Operations Properties of the intersection operation
A ∩ U = A Identity law
A ∩  =  Domination law
A ∩ A = A Idempotent law
A ∩ B = B ∩ A Commutative law
A ∩ (B ∩ C) = (A ∩ B) ∩ C Associative law

27. 2.2 Set Operations Disjoint
Formal definition for disjoint sets: two sets are disjoint if
their intersection is empty set
A  B = 
Examples:
{1, 2, 3} and {3, 4, 5} are not disjoint
{a, b} and {3, 4} are disjoint
{1, 2} and  are disjoint
Their intersection is empty set
 and  are disjoint!

28. 2.2 Set Operations Complement
Let U be the universal set. The complement of the set A, denoted by A , is the complement of A with respect to U.
The complement of the set A is U - A.
The complement of a set A is:
A = { x : x  A} = Ac
Note that:
 = U and U = 
Properties of complement sets
(Ac)c = A Complementation law
A U Ac = U Complement law
A ∩ Ac =  Complement law
Example: Let U is the set of all positive integers
If A = {x| x > 10}, Then Ac = {l,2,3,4,5,6,7,8,9,l0}. A A

29. 2.2 Set Operations Difference
The difference of A and B, denoted by A - B, is the set containing
those elements that are in A but not in B.
The difference of two sets A and B is:
A - B = { x : x  A  x  B }
A - B = A  B U

30. 2.2 Set Operations Examples: {1, 2, 3} - {3, 4, 5} = {1, 2}
{a, b} - {3, 4} = {a, b}
{1, 2} -  = {1, 2}
The difference of any set S with the empty set will be
the set S
S -  = S , where S is any set

31. 2.2 Set Operations symmetric difference
The symmetric difference of two sets A and B is:
A  B = { x : (x  A  x  B) v (x  B  x  A)}
= (A - B) U (B - A)
= (A  B) – (A  B)

32. 2.2 Set Operations
Set identities

33. 2.2 Set Operations Four methods: How to prove a set identity?
For example Prove that : (A U B) = A  B
Four methods:
Use the basic set identities
Use membership tables
Prove each set is a subset of each other
Use set builder notation and logical equivalences

34. 1- By using the basic set identities (A U B) = A U B = A  B
2.2 Set Operations
1- By using the basic set identities
(A U B) = A U B
= A  B

35. 2- Bu Using membership tables
2.2 Set Operations
2- Bu Using membership tables

36. 2.2 Set Operations 3- By using each set is a subset of each other
() (x  A U B)  (x  A U B)  (x  A and x  B)
 (x  A  B)
() (x  A  B)  (x  A and x  B)  (x  A U B)
 (x  A U B)

37. 2.2 Set Operations
4- By using set builder notation and logical equivalences
(A U B) = {x : (x  A v x  B)}
= {x : (x  A)  (x  B)}
= {x : (x  A)  (x  B)}
= A  B

38. 2.2 Set Operations Generalized Unions and Intersections
Let A, B, and C be sets then:
A  B  C contains those elements that are in at least one of the sets
A, B, and C
A  B  C contains those elements that are in all of A, B, and C.

39. 2.2 Set Operations
Example: Let A = {0, 2, 4, 6, 8}, B = {0, 1,2,3, 4}, and C = {0, 3, 6, 9}, determine the following combinations:
1. A  B  C
2. A  B  C
Solution:
1. A  B  C = {0, 1,2, 3,4, 6, 8, 9}.
2. A  B  C = {0}.

40. 2.2 Set Operations Computer Representation of Sets
Let U = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}.
The bit string (of length |U| = 10) that represents the set
A = {1, 3, 5, 6, 9} has a one in the first, third, fifth, sixth, and ninth position, and zero elsewhere.
It is
What bit strings represent the subset of:
All odd integers in U,
A= {I, 3, 5, 7, 9}, and the bit string is
2. Integers not exceeding 5 in U
A= {l, 2, 3, 4, 5}, and the bit string is

41. 2.2 Set Operations
Example: The bit strings for the sets {I, 2, 3, 4, 5} and {I, 3, 5, 7, 9} are and , respectively.
Use bit strings to find the union and intersection of these sets.
Solution:
The bit string for the union of these sets is:
 = ,
Corresponds to the set {l, 2, 3,4, 5, 7, 9}.
The bit string for the intersection of these sets is:
 = ,
Corresponds to the set {I, 3, 5}.

42. 2.2 Set Operations Exercises
1. Let A = { l , 2, 3,4, 5} and B = {0, 3, 6}. Find
A  B b) A  B c) A – B d) B - A
2. Let A = {a, b, c, d, e} and B = {a, b, c, d, e, f, g, h}. Find
A  B b) A  B c) A – B d) B – A
3. Find the sets A and B if A - B = {I, 5, 7, 8}, B - A = {2, l0}, and A  B = {3, 6,9}
4. Show that if A and B are sets, then
a) A - B = A  Bc b) (A  B)  (A  Bc)= A
5. Find the symmetric difference of:
a) { I , 3 , 5} and { I , 2, 3}.
b) the set of computer science majors at a school and the set of mathematics majors at this school

43. 2.2 Set Operations
6. Let A = {0, 2, 4, 6, 8, 10}, B = {0, 1 , 2, 3 ,4,5, 6}, and C = {4, 5 , 6, 7, 8 , 9, 10}. Find
a) A  B  C b) A  B  C c) (A  B)  C
d) (A  B)  C
7. What can you say about the sets A and B if we know that:
a) A  B=A b) A  B=A c) A – B = A
d) A  B= B  A e) A – B = B - A
8. Can you conclude that A = B if A, B, and C are sets such that:
a) A  C = B  C b) A  C = B  C
c) A  B = B  C and A  C = B  C

44. 2.2 Set Operations