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Chapter 3 Set Theory

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Title: Chapter 3 Set Theory


1
Chapter 3Set Theory
  • Yen-Liang Chen
  • Dept of Information Management
  • National Central University

2
3.1 Sets and subsets
  • Definitions
  • Element and set , Ex 3.1
  • Finite set and infinite set, cardinality ? A ?,
    Ex 3.2
  • C?D a subset, C?D a proper subset
  • CD, two sets are equal
  • Neither order nor repetition is relevant for a
    general set
  • null set, , ?

3
Subset relations
  • A?B
  • ? ?x x?A?x?B
  • A B
  • ? ??x x?A?x?B
  • ? ?x ? x?A?x?B
  • ? ?x ? (?x?A)?(x?B)
  • ? ?x x?A??(x?B)
  • ? ?x x?A?x?B

4
Subset relations
  • A?B
  • ??(A?B?B?A)
  • ??(A?B)??(B?A)
  • ?(A B)? (B A)
  • A?B
  • ?A?B ?A?B

5
Ex 3.5
6
Theorems 3.1. and 3.2
  • Theorem 3.1
  • If A?B and B?C, then A?C,
  • If A?B and B?C, then A?C,
  • If A?B and B?C, then A?C,
  • If A?B and B?C, then A?C,
  • Theorem 3.2
  • ?? A. If A is not empty, then ??A.

7
Power set
  • For any finite set A with ? A ?n, the total
    number of subsets of A is 2n.
  • Definition 3.4. the power set of A, denoted as
    ?(A) is the collection of all subsets of A.
  • What is the power set of 1, 2,3 4?

8
Ex 3.10
  • Count the number of paths in the xy-plane from
    (2,1) to (7,4)
  • The number of paths sought here equals the number
    of subsets A of 1,2,,8, where ? A ?3.

9
Ex 3.11
  • Count the number of compositions of an integer,
    say 7
  • 71111111, there are six plus signs.
  • Subset 1,4,6 ? (11)1(11)(11)?2122
  • Subset 1,2,5,6? (111)1(111)?313
  • Subset 3,4,5,6? 11(11111)?115
  • Consequently, there are 2m-1 compositions for the
    value m.

10
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11
An important identity
  • C(n1, r) C(n, r)C(n, r-1)
  • Pascala triangle in Ex 3.14

12
3.2 Set operations and the laws of set theory
  • Definition 3.5.
  • A?Bx?x?A ? x?B
  • A?Bx?x?A ? x?B
  • A?Bx?x?A?B ? x?A?B
  • Ex 3.15
  • Definitions 3.6, 3.7, 3.8
  • S, T are disjoint, written S?T?
  • The complement of A, denoted as
  • The relative complement of A in B, denoted B-A

13
Ex 3.18
14
Theorem 3.4
  • The following statements are equivalent
  • A?B
  • A?BB
  • A?BA

15
A?B ? A?BB ?A?BA
  • B?(A?B) for any sets
  • x?(A?B) ? (x?A)?(x?B)
  • since A?B, ? (x?B)
  • this means (A?B)?B
  • we conclude A?BB
  • A?A?B for any sets
  • y?A? y?A?B (1)
  • Since A?BB, (1)?y?B?y?(A?B)
  • This means A?A?B
  • we conclude AA?B

16
A?BA ? ? A?B
17
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18
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19
A?(B?C)(A?B)?(A?C)
20
The Duality
  • Definition 3.9 Let s be a statement dealing with
    the equality of two set expressions. The dual of
    s, denoted sd, is obtained from s by replacing
    (1) each occurrence of ? and U by U and ?,
    respectively and (2) each occurrence of ? and ?
    by ? and ?, respectively.
  • Theorem 3.5. s is a theorem if and only if sd is
    also a theorem.        

21
Three approaches to proof
  • The first approach to prove a theorem is by
    element argument.
  • The second is by Venn diagram, and
  • the third is by membership table.

22
Venn diagram to show
23
Venn diagram to show
24
membership table
25
Membership table for A?(B?C)(A?B)?(A?C)
26
Ex 3.20
27
Ex 3.22
28
4
1
2
3
A1,2, B2,3, A?B1,3, A3,4,
B1,4 A?B2, 4 B?A (A?B)
29
Generalized DeMorgans Law
30
3.3. Counting and Venn diagrams
  • Finite sets A and B are disjoint if and only if
    ? A ? B ? ? A ? ? B ? , Figures 3.9 and 3.10
  • Ex 3.25, If A and B are finite sets, then ?A?B?
    ?A? ?B?-?A?B? , Figure 3.11
  • When U is finite, we have

31
Ex 3.26
  • How many gates have at least one of the defects
    D1, D2, D3? How many are perfect?
  • Figure 3.12 and Figure 3.13. If A, B and C are
    finite sets, then ?A?B?C? ?A?
    ?B??C?-?A?B?-?A?C?-?B?C? ?A?B?C?
  • When U is finite, we have

32
3.4. A first world on probability
  • Let ? be the sample space for an experiment. Each
    subset A of ?, including the empty subset, is
    called an event. Each element of ? determines an
    outcome. If ???n, then Pr(a)1/n and Pr(A)? A
    ?/n
  • Ex 3.29, Ex 3.30, Ex 3.31
  • Definition 3.11. For sets A and B, the Cartesian
    product of A and B is denoted by A?B and equals
    (a, b)?a? A , b ? B. We call the elements of
    A?B ordered pairs.

33
Ex 3.33
  • Suppose we roll two fair dice.
  • Consider the following event
  • A rolls a 6
  • B The sum of dice is at least 7
  • C Rolls an even sum
  • D The sum of the dice is 6 or less
  • What are P(A), P(B), P(C), P(D), P(A?B), P(C?D)?

34
Examples
  • Ex 3.35. If we toss a fair coin four times, what
    is the prob that we get two heads and two tails?
  • Ex 3.36. Among the letters WYSIWYG, what is the
    prob that the arrangement has both consecutive
    Ws and Ys? and the prob that the arrangement
    starts and ends with W?

35
3.5. The axioms of probability
  • Ex 3.39. The outcomes of a sample space may have
    different likelihoods
  • A warehouse has 10 motors, three of which are
    defective. We select two motors.
  • A exactly one is defective
  • B at least one motor is defective
  • C both motors are defective
  • D Both motors are in good condition.

36
The axioms of probability
  • Let ? be the sample space for an experiment. If A
    and B are any events, then
  • Pr(A)?0
  • Pr(?)1
  • If A and B are disjoint, Pr(A ? B )Pr(A) Pr(B)
  • Theorem 3.7.

37
Ex 3.40
  • The letters PROBABILITY are arranged in a random
    manner. Determine the prob of the following
    event The first and last letters are different.
  • Neither B nor I appears at the start or finish.
  • (7)(9!/2!2!)(6)
  • Only B appears at the start or finish.
  • (2)(7)(9!/2!)
  • One of B is used at the start and I as the other.
  • (2)(9!)

38
Ex 3.41
  • The prob that our team can win any tournament is
    0.7. Suppose we need to play eight tournaments.
    Consider the following cases
  • Win all eight games. (0.3)8
  • Win exactly five of the eight. C(8,
    5)(0.7)5(0.3)3
  • Win at least one. 1-(0.3)8
  • If there are n trials and each trial has
    probability p of success and 1-p of failure, the
    probability that there are k successes among
    these n trials is

39
Theorem 3.8
  • Pr(A?B)
  • Pr(A?Bc) Pr(B)
  • Pr(A) Pr(B)- Pr(A?B )
  • Ex 3.42
  • What is the prob that the card drawn is a club
    and the value is between 3 and 7.
  • Ex 3.43

40
Theorem 3.9.
  • Pr(A?B?C) Pr(A) Pr(B)Pr(C)-Pr(A?B)-Pr(A?C)-Pr(B
    ?C) Pr(A?B?C)
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