Lipschutz s , lipson m schaums outline of discrete maths

Lipschutz−Lipson:
Schaum’s Outline of Theory
and Problems of Discrete
Math, 2/e

Front Matter

© The McGraw−Hill
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Preface

Preface
Discrete mathematics, the study of ®nite systems, has become increasingly
important as the computer age has advanced. The digital computer is basically a
®nite structure, and many of its properties can be understood and interpreted within
the framework of ®nite mathematical systems. This book, in presenting the more
essential material, may be used as a textbook for a formal course in discrete mathematics or as a supplement to all current texts.
The ®rst three chapters cover the standard material on sets, relations, and functions and algorithms. Next come chapters on logic, vectors and matrices, counting,
and probability. We than have three chapters on graph theory: graphs, directed
graphs, and binary trees. Finally there are individual chapters on properties of the
integers, algebraic systems, languages and machines, ordered sets and lattices, and
Boolean algebra. The chapter on functions and algorithms includes a discussion of
cardinality and countable sets, and complexity. The chapters on graph theory
include discussions on planarity, traversability, minimal paths, and Warshall's and
Hu€man's algorithms. The chapter on languages and machines includes regular
expressions, automata, and Turing machines and computable functions. We emphasize that the chapters have been written so that the order can be changed without
diculty and without loss of continuity.
This second edition of Discrete Mathmatics covers much more material and in
greater depth than the ®rst edition. The topics of probability, regular expressions and
regular sets, binary trees, cardinality, complexity, and Turing machines and computable functions did not appear in the ®rst edition or were only mentioned. This new

material re¯ects the fact that discrete mathematics now is mainly a one-year course
rather than a one-semester course.
Each chapter begins with a clear statement of pertinent de®nition, principles,
and theorems with illustrative and other descriptive material. This is followed by sets
of solved and supplementary problems. The solved problems serve to illustrate and
amplify the material, and also include proofs of theorems. The supplementary problems furnish a complete review of the material in the chapter. More material has
been included than can be covered in most ®rst courses. This has been done to make
the book more ¯exible, to provide a more useful book of reference, and to stimulate
further interest in the topics.
Finally, we wish to thank the sta€ of the McGraw-Hill Schaum's Outline Series,
especially Arthur Biderman and Maureen Walker, for their unfailing cooperation.
SEYMOUR LIPSCHUTZ
MARC LARS LIPSON

v


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1. Set Theory

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Text

Chapter 1

Set Theory
1.1

INTRODUCTION

The concept of a set appears in all mathematics. This chapter introduces the notation and terminology of set theory which is basic and used throughout the text.
Though logic is formally treated in Chapter 4, we introduce Venn diagram representation of sets
here, and we show how it can be applied to logical arguments. The relation between set theory and logic
will be further explored when we discuss Boolean algebra in Chapter 15.
This chapter closes with the formal de®nition of mathematical induction, with examples.
1.2

SETS AND ELEMENTS

A set may be viewed as a collection of objects, the elements or members of the set. We ordinarily use
capital letters, A, B, X, Y, . . ., to denote sets, and lowercase letters, a, b, x, y, . . ., to denote elements of
sets. The statement ``p is an element of A'', or, equivalently, ``p belongs to A'', is written
pPA
The statement that p is not an element of A, that is, the negation of p P A, is written
pP
= A
The fact that a set is completely determined when its members are speci®ed is formally stated as the
principle of extension.
Principle of Extension:

Two sets A and B are equal if and only if they have the same members.

As usual, we write A ˆ B if the sets A and B are equal, and we write A Tˆ B if the sets are not equal.
Specifying Sets
There are essentially two ways to specify a particular set. One way, if possible, is to list its members.

For example,
A ˆ fa; e; i; o; ug
denotes the set A whose elements are the letters a, e, i, o, u. Note that the elements are separated by
commas and enclosed in braces { }. The second way is to state those properties which characterized the
elements in the set. For example,
B ˆ fx: x is an even integer, x > 0g
which reads ``B is the set of x such that x is an even integer and x is greater than 0'', denotes the set B
whose elements are the positive integers. A letter, usually x, is used to denote a typical member of the set;
the colon is read as ``such that'' and the comma as ``and''.
EXAMPLE 1.1
(a)

The set A above can also be written as
A ˆ fx: x is a letter in the English alphabet, x is a vowelg
Observe that b P
= A, e P A, and p P
= A.

(b)

We could not list all the elements of the above set B although frequently we specify the set by writing
B ˆ f2; 4; 6; . . .g

1


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1. Set Theory

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SET THEORY

[CHAP. 1

where we assume that everyone knows what we mean. Observe that 8 P B but À7 P
= B.
(c)

Let E ˆ fx: x2 À 3x ‡ 2 ˆ 0g. In other words, E consists of those numbers which are solutions of the equation
x2 À 3x ‡ 2 ˆ 0, sometimes called the solution set of the given equation. Since the solutions of the equation are
1 and 2, we could also write E ˆ f1; 2g.

(d ) Let E ˆ fx: x2 À 3x ‡ 2 ˆ 0g, F ˆ f2; 1g and G ˆ f1; 2; 2; 1; 63g. Then E ˆ F ˆ G. Observe that a set does
not depend on the way in which its elements are displayed. A set remains the same if its elements are repeated or
rearranged.

Some sets will occur very often in the text and so we use special symbols for them. Unless otherwise
speci®ed, we will let
N
Z

Q
R
C

ˆ
ˆ
ˆ
ˆ
ˆ

the
the
the
the
the

set
set
set
set
set

of
of
of
of
of

positive integers: 1, 2, 3, . . .
integers: . . ., À2, À1, 0, 1, 2, . . .

rational numbers
real numbers
complex numbers

Even if we can list the elements of a set, it may not be practical to do so. For example, we would not
list the members of the set of people born in the world during the year 1976 although theoretically it is
possible to compile such a list. That is, we describe a set by listing its elements only if the set contains a
few elements; otherwise we describe a set by the property which characterizes its elements.
The fact that we can describe a set in terms of a property is formally stated as the principle of
abstraction.
Principle of Abstraction: Given any set U and any property P, there is a set A such that the elements of
A are exactly those members of U which have the property P.

1.3

UNIVERSAL SET AND EMPTY SET

In any application of the theory of sets, the members of all sets under investigation usually belong to
some ®xed large set called the universal set. For example, in plane geometry, the universal set consists of
all the points in the plane, and in human population studies the universal set consists of all the people in
the world. We will let the symbol
U
denote the universal set unless otherwise stated or implied.
For a given set U and a property P, there may not be any elements of U which have property P. For
example, the set
S ˆ fx: x is a positive integer, x2 ˆ 3g
has no elements since no positive integer has the required property.
The set with no elements is called the empty set or null set and is denoted by
D
There is only one empty set. That is, if S and T are both empty, then S ˆ T since they have exactly the

same elements, namely, none.
1.4

SUBSETS

If every element in a set A is also an element of a set B, then A is called a subset of B. We also say
that A is contained in B or that B contains A. This relationship is written
AB

or

BA


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SET THEORY

If A is not a subset of B, i.e., if at least one element of A does not belong to B, we write A \ B or B ] A.
EXAMPLE 1.2
(a)

Consider the sets
A ˆ f1; 3; 4; 5; 8; 9g

B ˆ f1; 2; 3; 5; 7g

C ˆ f1; 5g

Then C  A and C  B since 1 and 5, the elements of C, are also members of A and B. But B \ A since some
of its elements, e.g., 2 and 7, do not belong to A. Furthermore, since the elements of A, B, and C must also
belong to the universal set U, we have that U must at least contain the set f1; 2; 3; 4; 5; 6; 7; 8; 9g.
(b)

Let N, Z, Q, and R be de®ned as in Section 1.2. Then
NZQR

(c)

The set E ˆ f2; 4; 6g is a subset of the set F ˆ f6; 2; 4g, since each number 2, 4, and 6 belonging to E also
belongs to F. In fact, E ˆ F. In a similar manner it can be shown that every set is a subset of itself.

The following properties of sets should be noted:
(i) Every set A is a subset of the universal set U since, by de®nition, all the elements of A belong to U.
Also the empty set D is a subset of A.
(ii) Every set A is a subset of itself since, trivially, the elements of A belong to A.

(iii) If every element of A belongs to a set B, and every element of B belongs to a set C, then clearly
every element of A belongs to C. In other words, if A  B and B  C, then A  C.
(iv) If A  B and B  A, then A and B have the same elements, i.e., A ˆ B. Conversely, if A ˆ B then
A  B and B  A since every set is a subset of itself.
We state these results formally.
Theorem 1.1:

(i) For any set A, we have D  A  U.
(ii) For any set A, we have A  A.
(iii) If A  B and B  C, then A  C.
(iv) A ˆ B if and only if A  B and B  A.

If A  B, then it is still possible that A ˆ B. When A  B but A Tˆ B, we say A is a proper subset of B.
We will write A & B when A is a proper subset of B. For example, suppose
A ˆ f1; 3g

B ˆ f1; 2; 3g;

C ˆ f1; 3; 2g

Then A and B are both subsets of C; but A is a proper subset of C, whereas B is not a proper subset of C
since B ˆ C.

1.5

VENN DIAGRAMS

A Venn diagram is a pictoral representation of sets in which sets are represented by enclosed areas in
the plane.
The universal set U is represented by the interior of a rectangle, and the other sets are represented by

disks lying within the rectangle. If A  B, then the disk representing A will be entirely within the disk
representing B as in Fig. 1-1(a). If A and B are disjoint, i.e., if they have no elements in common, then the
disk representing A will be separated from the disk representing B as in Fig. 1-1(b).
However, if A and B are two arbitrary sets, it is possible that some objects are in A but not in B,
some are in B but not in A, some are in both A and B, and some are in neither A nor B; hence in general
we represent A and B as in Fig. 1-1(c).


Lipschutz−Lipson:
Schaum’s Outline of Theory
and Problems of Discrete
Math, 2/e

1. Set Theory

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Text

SET THEORY

[CHAP. 1

Fig. 1-1

Arguments and Venn Diagrams
Many verbal statements are essentially statements about sets and can therefore be described by Venn

diagrams.
Hence Venn diagrams can sometimes be used to determine whether or not an argument is valid.
Consider the following example.
EXAMPLE 1.3 Show that the following argument (adapted from a book on logic by Lewis Carroll, the author of
Alice in Wonderland) is valid:
S1 :
S2 :
S3 :

My saucepans are the only things I have that are made of tin.
I ®nd all your presents very useful.
None of my saucepans is of the slightest use.

S:

Your presents to me are not made of tin.

(The statements S1 , S2 , and S3 above the horizontal line denote the assumptions, and the statement S below the line
denotes the conclusion. The argument is valid if the conclusion S follows logically from the assumptions S1 , S2 , and
S3 .)
By S1 the tin objects are contained in the set of saucepans and by S3 the set of saucepans and the set of useful
things are disjoint: hence draw the Venn diagram of Fig. 1-2.

Fig. 1-2


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and Problems of Discrete
Math, 2/e

1. Set Theory

CHAP. 1]

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SET THEORY

By S2 the set of ``your presents'' is a subset of the set of useful things; hence draw Fig. 1-3.

Fig. 1-3
The conclusion is clearly valid by the above Venn diagram because the set of ``your presents'' is disjoint from
the set of tin objects.

1.6

SET OPERATIONS
This section introduces a number of important operations on sets.

Union and Intersection
The union of two sets A and B, denoted by A ‘ B, is the set of all elements which belong to A or to B;
that is,
A ‘ B ˆ fx: x P A or x P Bg
Here ``or'' is used in the sense of and/or. Figure 1-4(a) is a Venn diagram in which A ‘ B is shaded.

The intersection of two sets A and B, denoted by A ’ B, is the set of elements which belong to both A
and B; that is,
A ’ B ˆ fx: x P A and x P Bg
Figure 1-4(b) is a Venn diagram in which A ’ B is shaded.
If A ’ B ˆ D, that is, if A and B do not have any elements in common, then A and B are said to be
disjoint or nonintersecting.

Fig. 1-4

EXAMPLE 1.4
(a)

Let A ˆ f1; 2; 3; 4g, B ˆ f3; 4; 5; 6; 7g, C ˆ f2; 3; 5; 7g. Then
A ‘ B ˆ f1; 2; 3; 4; 5; 6; 7g
A ‘ C ˆ f1; 2; 3; 4; 5; 7g

A ’ B ˆ f3; 4g
A ’ C ˆ f2; 3g


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(b)

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SET THEORY

[CHAP. 1

Let M denote the set of male students in a university C, and let F denote the set of female students in C. Then
M‘F ˆC
since each student in C belongs to either M or F. On the other hand,
M’F ˆD
since no student belongs to both M and F.

The operation of set inclusion is closely related to the operations of union and intersection, as shown
by the following theorem.
Theorem 1.2:

The following are equivalent: A  B,

A ’ B ˆ A, and

A ‘ B ˆ B.

Note: This theorem is proved in Problem 1.27. Other conditions equivalent to A  B are given in
Problem 1.37.
Complements
Recall that all sets under consideration at a particular time are subsets of a ®xed universal set U. The

absolute complement or, simply, complement of a set A, denoted by Ac , is the set of elements which belong
to U but which do not belong to A; that is,
= Ag
Ac ˆ fx: x P U, x P
 Figure 1-5(a) is a Venn diagram in which Ac is
Some texts denote the complement of A by A H or A.
shaded.
The relative complement of a set B with respect to a set A or, simply, the di€erence of A and B,
denoted by AnB, is the set of elements which belong to A but which do not belong to B; that is
AnB ˆ fx: x P A; x P
= Bg
The set AnB is read ``A minus B''. Many texts denote AnB by A À B or A $ B. Figure 1-5(b) is a Venn
diagram in which AnB is shaded.

Fig. 1-5

EXAMPLE 1.5

Suppose U ˆ N ˆ f1; 2; 3; . . .g, the positive integers, is the universal set. Let
A ˆ f1; 2; 3; 4; g;

B ˆ f3; 4; 5; 6; 7g;

C ˆ f6; 7; 8; 9g

and let E ˆ f2; 4; 6; 8; . . .g, the even integers. Then
Ac ˆ f5; 6; 7; 8; . . .g;

B c ˆ f1; 2; 8; 9; 10; . . .g;

C c ˆ f1; 2; 3; 4; 5; 10; 11; . . .g

and
AnB ˆ f1; 2g;
c

BnC ˆ f3; 4; 5g;

Also, E ˆ f1; 3; 5; . . .g, the odd integers.

BnA ˆ f5; 6; 7g;

CnE ˆ f7; 9g


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CHAP. 1]

7

SET THEORY

Fundamental Products
Consider n distinct sets A1 ; A2 ; . . . ; An . A fundamental product of the sets is a set of the form
AÃ1 ’ AÃ2 ’ Á Á Á ’ AÃn
where AÃi is either Ai or Aci . We note that (1) there are 2n such fundamental products, (2) any two such
fundamental products are disjoint, and (3) the universal set U is the union of all the fundamental
products (Problem 1.64). There is a geometrical description of these sets which is illustrated below.
EXAMPLE 1.6
sets:

Consider three sets A, B, and C. The following lists the eight fundamental products of the three

P1 ˆ A ’ B ’ C;
P2 ˆ A ’ B ’ C c

P3 ˆ A ’ B c ’ C;
P4 ˆ A ’ B c ’ C c ;

P5 ˆ Ac ’ B ’ C;
P6 ˆ Ac ’ B ’ C c

P7 ˆ Ac ’ B c ’ C
P8 ˆ Ac ’ B c ’ C c

These eight products correspond precisely to the eight disjoint regions in the Venn diagram of sets A, B, C in Fig. 1-6
as indicated by the labeling of the regions.

Fig. 1-6

Fig. 1-7

Symmetric Di€erence
The symmetric di€erence of sets A and B, denoted by A È B, consists of those elements which belong
to A or B but not to both; that is,
A È B ˆ …A ‘ B†n…A ’ B†
One can also show (Problem 1.18) that
A È B ˆ …AnB† ‘ …BnA†
For example, suppose A ˆ f1; 2; 3; 4; 5; 6g and B ˆ f4; 5; 6; 7; 8; 9g. Then
AnB ˆ f1; 2; 3g;

BnA ˆ f7; 8; 9g

and so

A È B ˆ f1; 2; 3; 7; 8; 9g

Figure 1-7 is a Venn diagram in which A È B is shaded.

1.7

ALGEBRA OF SETS AND DUALITY

Sets under the operations of union, intersection, and complement satisfy various laws or identities
which are listed in Table 1-1. In fact, we formally state this:
Theorem 1.3:

Sets satisfy the laws in Table 1-1.

There are two methods of proving equations involving set operations. One way is to use what it
means for an object x to be an element of each side, and the other way is to use Venn diagrams. For
example, consider the ®rst of DeMorgan's laws.
…A ‘ B†c ˆ Ac ’ B c


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Table 1-1

[CHAP. 1

Laws of the algebra of sets
Idempotent laws

(1a)

A‘AˆA

(1b)

A’AˆA

Associative laws
(2a)

…A ‘ B† ‘ C ˆ A ‘ …B ‘ C†

(3a)

A‘BˆB‘A

(2b)

…A ’ B† ’ C ˆ A ’ …B ’ C†

Commutative laws
(3b)

A’BˆB’A

Distributive laws
(4a)

A ‘ …B ’ C† ˆ …A ‘ B† ’ …A ‘ C†

(4b)

A ’ …B ‘ C† ˆ …A ’ B† ‘ …A ’ C†

Identity laws
(5a)

A‘DˆA

(6a)

A‘U ˆU

(5b)

A’U ˆA

(6b)

A’DˆD

Involution laws
(7) …Ac †c ˆ A
Complement laws
(8a)

A ‘ Ac ˆ U

(8b)

A ’ Ac ˆ D

(9a)

Uc ˆD

(9b)

Dc ˆ U

DeMorgan's laws
(10a)

…A ‘ B†c ˆ Ac ’ B c

(10b)

…A ’ B†c ˆ Ac ‘ B c

Method 1:

We ®rst show that …A ‘ B†c  Ac ’ B c . If x P …A ‘ B†c , then x P
= A ‘ B.
Thus x P
= A and x P
= B, and so x P Ac and x P B c . Hence x P Ac ’ B c .
Next we show that Ac ’ B c  …A ‘ B†C . Let x P Ac ’ B c . Then x P Ac
and x P B c , so x P
= A and x P
= B. Hence x P

= A ‘ B, so x P …A ‘ B†c .
We have proven that every element of …A ‘ B†c belongs to Ac ’ Bc
and that every element of Ac ’ Bc belongs to …A ‘ B†c . Together, these
inclusions prove that the sets have the same elements, i.e., that
…A ‘ B†c ˆ Ac ’ Bc .

Method 2:

From the Venn diagram for A ‘ B in Fig. 1-4, we see that …A ‘ B†c is
represented by the shaded area in Fig. 1-8(a). To ®nd Ac ’ B c , the area in
both Ac and B c , we shaded Ac with strokes in one direction and B c with
strokes in another direction as in Fig. 1-8(b). Then Ac ’ B c is represented
by the crosshatched area, which is shaded in Fig. 1-8(c). Since …A ‘ B†c
and Ac ’ B c are represented by the same area, they are equal.

Fig. 1-8


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SET THEORY

Duality
Note that the identities in Table 1-1 are arranged in pairs, as, for example, (2a) and (2b). We now
consider the principle behind this arrangement. Suppose E is an equation of set algebra. The dual E Ã of
E is the equation obtained by replacing each occurrence of ‘, ’, U, and D in E by ’, ‘, D and U,
respectively. For example, the dual of
…U ’ A† ‘ …B ’ A† ˆ A

is

…D ‘ A† ’ …B ‘ A† ˆ A

Observe that the pairs of laws in Table 1-1 are duals of each other. It is a fact of set algebra, called the
principle of duality, that, if any equation E is an identity, then its dual E Ã is also an identity.

1.8

FINITE SETS, COUNTING PRINCIPLE

A set is said to be ®nite if it contains exactly m distinct elements where m denotes some nonnegative
integer. Otherwise, a set is said to be in®nite. For example, the empty set D and the set of letters of the
English alphabet are ®nite sets, whereas the set of even positive integers, f2; 4; 6; . . .g, is in®nite.
The notation n…A† will denote the number of elements in a ®nite set A. Some texts use #…A†; jAj or
card…A† instead of n…A†.
Lemma 1.4:

If A and B are disjoint ®nite sets, then A ‘ B is ®nite and
n…A ‘ B† ˆ n…A† ‡ n…B†

Proof. In counting the elements of A ‘ B, ®rst count those that are in A. There are n…A† of these. The
only other elements of A ‘ B are those that are in B but not in A. But since A and B are disjoint, no
element of B is in A, so there are n…B† elements that are in B but not in A. Therefore,
n…A ‘ B† ˆ n…A† ‡ n…B†.
We also have a formula for n…A ‘ B† even when they are not disjoint. This is proved in Problem 1.28.
Theorem 1.5:

If A and B are ®nite sets, then A ‘ B and A ’ B are ®nite and
n…A ‘ B† ˆ n…A† ‡ n…B† À n…A ’ B†

We can apply this result to obtain a similar formula for three sets:
Corollary 1.6:

If A, B, and C are ®nite sets, then so is A ‘ B ‘ C, and

n…A ‘ B ‘ C† ˆ n…A† ‡ n…B† ‡ n…C† À n…A ’ B† À n…A ’ C† À n…B ’ C† ‡ n…A ’ B ’ C†
Mathematical induction (Section 1.10) may be used to further generalize this result to any ®nite
number of sets.
EXAMPLE 1.7 Consider the following data for 120 mathematics students at a college concerning the languages
French, German, and Russian:
65 study French
45 study German
42 study Russian
20 study French and German
25 study French and Russian
15 study German and Russian

8 study all three languages.
Let F, G, and R denote the sets of students studying French, German and
Russian, respectively. We wish to ®nd the number of students who study at
least one of the three languages, and to ®ll in the correct number of students
in each of the eight regions of the Venn diagram shown in Fig. 1-9.

Fig. 1-9


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SET THEORY

[CHAP. 1

By Corollary 1.6,
…F ‘ G ‘ R† ˆ n…F† ‡ n…G† ‡ n…R† À n…F ’ G† À n…F ’ R† À n…G ’ R† ‡ n…F ’ G ’ R†
ˆ 65 ‡ 45 ‡ 42 À 20 À 25 À 15 ‡ 8 ˆ 100

That is, n…F ‘ G ‘ R† ˆ 100 students study at least one of the three
languages.
We now use this result to ®ll in the Venn diagram. We have:
8 study all three languages,
20 À 8 ˆ 12 study French and German but not Russian
25 À 8 ˆ 17 study French and Russian but not German
15 À 8 ˆ 7 study German and Russian but not French
65 À 12 À 8 À 17 ˆ 28 study only French
45 À 12 À 8 À 7 ˆ 18 study only German

Fig. 1-10

42 À 17 À 8 À 7 ˆ 10 study only Russian
120 À 100 ˆ 20 do not study any of the languages

Accordingly, the completed diagram appears in Fig. 1-10. Observe that 28 ‡ 18 ‡ 10 ˆ 56 students study only one of
the languages.

1.9

CLASSES OF SETS, POWER SETS, PARTITIONS

Given a set S, we might wish to talk about some of its subsets. Thus we would be considering a set of
sets. Whenever such a situation occurs, to avoid confusion we will speak of a class of sets or collection of
sets rather than a set of sets. If we wish to consider some of the sets in a given class of sets, then we speak
of a subclass or subcollection.
EXAMPLE 1.8
S. Then

Suppose S ˆ f1; 2; 3; 4g. Let A be the class of subsets of S which contain exactly three elements of

A ˆ ‰f1; 2; 3g; f1; 2; 4g; f1; 3; 4g; f2; 3; 4gŠ

The elements of A are the sets f1; 2; 3g, f1; 2; 4g, f1; 3; 4g, and f2; 3; 4g.
Let B be the class of subsets of S which contain 2 and two other elements of S. Then
B ˆ ‰f1; 2; 3g; f1; 2; 4g; f2; 3; 4gŠ
The elements of B are the sets f1; 2; 3g, f1; 2; 4g, and f2; 3; 4g. Thus B is a subclass of A, since every element of B is
also an element of A. (To avoid confusion, we will sometimes enclose the sets of a class in brackets instead of braces.)

Power Sets
For a given set S, we may speak of the class of all subsets of S. This class is called the power set of S,
and will be denoted by Power(S). If S is ®nite, then so is Power(S). In fact, the number of elements in
Power(S) is 2 raised to the power of S; that is,
n…Power…S†† ˆ 2n…S†
(For this reason, the power set of S is sometimes denoted by 2S .)
EXAMPLE 1.9 Suppose S ˆ f1; 2; 3g. Then
Power…S† ˆ ‰D; f1g; f2g; f3g; f1; 2g; f1; 3g; f2; 3g; SŠ
Note that the empty set D belongs to Power(S) since D is a subset of S. Similarly, S belongs to Power(S). As
expected from the above remark, Power(S) has 23 ˆ 8 elements.


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CHAP. 1]

11

SET THEORY

Partitions
Let S be a nonempty set. A partition of S is a subdivision
of S into nonoverlapping, nonempty subsets. Precisely, a partition of S is a collection fAi g of nonempty subsets of S such
that:
(i) Each a in S belongs to one of the Ai .
(ii) The sets of fAi g are mutually disjoint; that is, if
Ai Tˆ Aj
then
Ai ’ Aj ˆ D

Fig. 1-11

The subsets in a partition are called cells. Figure 1-11 is a Venn diagram of a partition of the rectangular
set S of points into ®ve cells, A1 , A2 , A3 , A4 , and A5 .
EXAMPLE 1.10

Consider the following collections of subsets of S ˆ f1; 2; . . . ; 8; 9g:
(i) ‰f1; 3; 5g; f2; 6g; f4; 8; 9gŠ
(ii)

‰f1; 3; 5g; f2; 4; 6; 8g; f5; 7; 9gŠ

(iii)

‰f1; 3; 5g; f2; 4; 6; 8g; f7; 9gŠ

Then (i) is not a partition of S since 7 in S does not belong to any of the subsets. Furthermore, (ii) is not a partition
of S since f1; 3; 5g and f5; 7; 9g are not disjoint. On the other hand, (iii) is a partition of S.

Generalized Set Operations
The set operations of union and intersection were de®ned above for two sets. These operations can
be extended to any number of sets, ®nite or in®nite, as follows.
Consider ®rst a ®nite number of sets, say, A1 , A2 , . . ., Am . The union and intersection of these sets
are denoted and de®ned, respectively, by
A1 ‘ A2 ‘ Á Á Á ‘ Am ˆ ‘m
iˆ1 Ai ˆ fx: x P Ai for some Ai g

A1 ’ A2 ’ Á Á Á ’ Am ˆ ’m
iˆ1 Ai ˆ fx: x P Ai for every Ai g
That is, the union consists of those elements which belong to at least one of the sets, and the intersection
consists of those elements which belong to all the sets.
Now let A be any collection of sets. The union and the intersection of the sets in the collection A is
denoted and de®ned, respectively, by
‘…A: A P A) ˆ fx: x P A for some A P Ag
’…A: A P A) ˆ fx: x P A for every A P Ag
That is, the union consists of those elements which belong to at least one of the sets in the collection A,
and the intersection consists of those elements which belong to every set in the collection A.
EXAMPLE 1.11

Consider the sets

A1 ˆ f1; 2; 3; . . .g ˆ N;

A2 ˆ f2; 3; 4; . . .g;

A3 ˆ f3; 4; 5; . . .g;

An ˆ fn; n ‡ 1; n ‡ 2; . . .g

Then the union and intersection of the sets are as follows:
‘…An : n P N† ˆ N

and

’ …An : n P N† ˆ D

DeMorgan's laws also hold for the above generalized operations. That is:
Theorem 1.7:

Let A be a collection of sets. Then
(i) …‘…A: A P A††c ˆ ’…Ac : A P A†
(ii) …’…A: A P A††c ˆ ‘…Ac : A P A†


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SET THEORY

[CHAP. 1

MATHEMATICAL INDUCTION
An essential property of the set
N ˆ f1; 2; 3; . . .g

which is used in many proofs, follows:
Principle of Mathematical Induction I: Let P be a proposition de®ned on the positive integers N, i.e.,
P…n† is either true or false for each n in N. Suppose P has the following two properties:
(i) P…1† is true.
(ii) P…n ‡ 1† is true whenever P…n† is true.
Then P is true for every positive integer.
We shall not prove this principle. In fact, this principle is usually given as one of the axioms when N
is developed axiomatically.
EXAMPLE 1.12

Let P be the proposition that the sum of the ®rst n odd numbers is n2 ; that is,
P…n†: 1 ‡ 3 ‡ 5 ‡ . . . ‡ …2n À 1† ˆ n2

(The nth odd number is 2n À 1, and the next odd number is 2n ‡ 1). Observe that P…n† is true for n ˆ 1, that is,

P…1†: 1 ˆ 12
Assuming P…n† is true, we add 2n ‡ 1 to both sides of P…n†, obtaining
1 ‡ 3 ‡ 5 ‡ . . . ‡ …2n À 1† ‡ …2n ‡ 1† ˆ n2 ‡ …2n ‡ 1† ˆ …n ‡ 1†2
which is P…n ‡ 1†. That is, P…n ‡ 1† is true whenever P…n† is true. By the principle of mathematical induction, P is
true for all n.

There is a form of the principle of mathematical induction which is sometimes more convenient to
use. Although it appears di€erent, it is really equivalent to the principle of induction.
Principle of Mathematical Induction II:
that:

Let P be a proposition de®ned on the positive integers N such

(i) P…1† is true.
(ii) P…n† is true whenever P…k† is true for all 1

k < n.

Then P is true for every positive integer.
Remark:

Sometimes one wants to prove that a proposition P is true for the set of integers
fa; a ‡ 1; a ‡ 2; . . .g

where a is any integer, possibly zero. This can be done by simply replacing 1 by a in either of the above
Principles of Mathematical Induction.

Solved Problems
SETS AND SUBSETS
1.1.

Which of these sets are equal: fr; t; sg, fs; t; r; sg, ft; s; t; rg, fs; r; s; tg?
They are all equal. Order and repetition do not change a set.


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SET THEORY

List the elements of the following sets; here N ˆ f1; 2; 3; . . .g.
(a) A ˆ fx: x P N; 3 < x < 12g
(b) B ˆ fx: x P N, x is even, x < 15g
(c) C ˆ fx: x P N, 4 ‡ x ˆ 3g.

1.3.

(a)

A consists of the positive integers between 3 and 12; hence
A ˆ f4; 5; 6; 7; 8; 9; 10; 11g

(b)

B consists of the even positive integers less than 15; hence
B ˆ f2; 4; 6; 8; 10; 12; 14g

(c)

There are no positive integers which satisfy the condition 4 ‡ x ˆ 3; hence C contains no elements. In
other words, C ˆ D, the empty set.

Consider the following sets:
D;

A ˆ f1g;

B ˆ f1; 3g;
E ˆ f1; 3; 5; 7; 9g;

C ˆ f1; 5; 9g;

D ˆ f1; 2; 3; 4; 5g;

U ˆ f1; 2; . . . ; 8; 9g

Insert the correct symbol  or \ or between each pair of sets:
(a) D, A
(b) A, B
(a)

(c)
(d )

B, C
B, E

(e) C, D
( f ) C, E

( g)
(h)

D, E
D, U

D  A because D is a subset of every set.

(b)

A  B because 1 is the only element of A and it belongs to B.

(c)

B \ C because 3 P B but 3 P

= C.

(d ) B \ E because the elements of B also belong to E.
(e)

C \ D because 9 P C but 9 P
= D.

( f ) C  E because the elements of C also belong to E.
( g) D \ E because 2 P D but 2 P
= E.
(h)

1.4.

D  U because the elements of D also belong to U.

Show that A ˆ f2; 3; 4; 5g is not a subset of B ˆ fx: x P N, x is even}.
It is necessary to show that at least one element in A does not belong to B. Now 3 P A and, since B
consists of even numbers, 3 P
= B; hence A is not a subset of B.

1.5.

Show that A ˆ f2; 3; 4; 5g is a proper subset of C ˆ f1; 2; 3; . . . ; 8; 9g.
Each element of A belongs to C so A  C. On the other hand, 1 P C but 1 P
= A. Hence A Tˆ C.
Therefore A is a proper subset of C.

SET OPERATIONS

Problems 1.6 to 1.8 refer to the universal set U ˆ f1; 2; . . . ; 9g and the sets
A ˆ f1; 2; 3; 4; 5g;

C ˆ f5; 6; 7; 8; 9g;

E ˆ f2; 4; 6; 8g

B ˆ f4; 5; 6; 7g;

D ˆ f1; 3; 5; 7; 9g;

F ˆ f1; 5; 9g


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[CHAP. 1

Find:
(a) A ‘ B and A ’ B

(c) A ‘ C and A ’ C

(e)

E ‘ E and E ’ E

(b) B ‘ D and B ’ D

(d ) D ‘ E and D ’ E

( f ) D ‘ F and D ’ F

Recall that the union X ‘ Y consists of those elements in either X or Y (or both), and that the
intersection X ’ Y consists of those elements in both X and Y.
(a)

A ‘ B ˆ f1; 2; 3; 4; 5; 6; 7g

A ’ B ˆ f4; 5g

(b)

B ‘ D ˆ f1; 3; 4; 5; 6; 7; 9g

B ’ D ˆ f5; 7g

(c)

A ‘ C ˆ f1; 2; 3; 4; 5; 6; 7; 8; 9g ˆ U

A ’ C ˆ f5g

(d ) D ’ E ˆ f1; 2; 3; 4; 5; 6; 7; 8; 9g ˆ U

D’E ˆD

(e)

E ’ E ˆ f2; 4; 6; 8g ˆ E

E ‘ E ˆ f2; 4; 6; 8g ˆ E

( f ) D ‘ F ˆ f1; 3; 5; 7; 9g ˆ D

D ’ F ˆ f1; 5; 9g ˆ F

Observe that F  D; so by Theorem 1.2 we must have D ‘ F ˆ D and D ’ F ˆ F.

1.7.

Find:

(a) Ac ; B c ; Dc ; E c ;

(b) AnB,

BnA, DnE,

FnD;

(c) A È B,

C È D,

E È F.

Recall that:
(1)

The complement X c consists of those elements in the universal set U which do not belong to X.

(2)

The di€erence XnY consist of the elements in X which do not belong to Y.

(3)

The symmetric di€erence X È Y consists of the elements in X or in Y but not in both X and Y.

Therefore:
(a)

1.8.

Ac ˆ f6; 7; 8; 9g; B c ˆ f1; 2; 3; 8; 9g; Dc ˆ f2; 4; 6; 8g ˆ E; E c ˆ f1; 3; 5; 7; 9g ˆ D.

(b)

AnB ˆ f1; 2; 3g; BnA ˆ f6; 7g; DnE ˆ f1; 3; 5; 7; 9g ˆ D; FnD ˆ D.

(c)

A È B ˆ f1; 2; 3; 6; 7g; C È D ˆ f1; 3; 8; 9g; E È F ˆ f2; 4; 6; 8; 1; 5; 9g ˆ E ‘ F.

Find:

(a) A ’ …B ‘ E†;
(c) …A ’ D†nB;

(b) …AnE†c ;
(d ) …B ’ F† ‘ …C ’ E†.

(a)

First compute B ‘ E ˆ f2; 4; 5; 6; 7; 8g. Then A ’ …B ‘ E† ˆ f2; 4; 5g.

(b)

AnE ˆ f1; 3; 5g. Then …AnE†c ˆ f2; 4; 6; 7; 8; 9g.

(c)

A ’ D ˆ f1; 3; 5g. Now …A ’ D†nB ˆ f1; 3g.

(d ) B ’ F ˆ f5g and C ’ E ˆ f6; 8g. So …B ’ F† ‘ …C ’ E† ˆ f5; 6; 8g.

1.9.

Show that we can have A ’ B ˆ A ’ C without B ˆ C.
Let A ˆ f1; 2g, B ˆ f2; 3g, and C ˆ f2; 4g. Then A ’ B ˆ f2g and A ’ C ˆ f2g. Thus A ’ B ˆ A ’ C
but B Tˆ C.

VENN DIAGRAMS
1.10. Consider the Venn diagram of two arbitrary sets A and B in Fig. 1-1(c). Shade the sets:
(a) A ’ B c ; (b) …BnA†c .
(a)

First shade the area represented by A with strokes in one direction (///), and then shade the area
represented by Bc (the area outside B), with strokes in another direction (\\\). This is shown in Fig.
1-12(a). The cross-hatched area is the intersection of these two sets and represents A ’ B c and this is
shown in Fig. 1-12(b). Observe that A ’ B c ˆ AnB. In fact, AnB is sometimes de®ned to be A ’ B c .


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Fig. 1-12
(b)

First shade the area represented by BnA (the area of B which does not lie in A) as in Fig. 1-13(a). Then
the area outside this shaded region, which is shown in Fig. 1-13(b), represents …BnA†c .

Fig. 1-13

1.11. Illustrate the distributive law A ’ …B ‘ C† ˆ …A ’ B† ‘ …A ’ C† with Venn diagrams.
Draw three intersecting circles labeled A, B, C, as in Fig. 1-14(a). Now, as in Fig. 1-14(b) shade A with
strokes in one direction and shade B ‘ C with strokes in another direction; the crosshatched area is
A ’ …B ‘ C†, as in Fig. 1-14(c). Next shade A ’ B and then A ’ C, as in Fig. 1-14(d); the total area shaded
is …A ’ B† ‘ …A ’ C†, as in Fig. 1-14(e).
As expected by the distributive law, A ’ …B ‘ C† and …A ’ B† ‘ …A ’ C† are both represented by the
same set of points.

Fig. 1-14


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[CHAP. 1

1.12. Determine the validity of the following argument:
S1 :

All my friends are musicians.

S2 :
S3 :

John is my friend.
None of my neighbors are musicians.

S:

John is not my neighbor.

The premises S1 and S3 lead to the Venn diagram in Fig. 1-15. By S2 , John belongs to the set of friends
which is disjoint from the set of neighbors. Thus S is a valid conclusion and so the argument is valid.

Fig. 1-15

FINITE SETS AND THE COUNTING PRINCIPLE
1.13. Determine which of the following sets are ®nite.
(a) A ˆ {seasons in the year}
(b) B ˆ {states in the Union}

(d )
(e)

D ˆ {odd integers}
E ˆ {positive integral divisors of 12}

(c) C ˆ {positive integers less than 1}

( f ) F ˆ {cats living in the United States}

(a)

A is ®nite since there are four seasons in the year, i.e., n…A† ˆ 4.

(b)

B is ®nite because there are 50 states in the Union, i.e. n…B† ˆ 50.

(c)

There are no positive integers less than 1; hence C is empty. Thus C is ®nite and n…C† ˆ 0.

(d ) D is in®nite.
(e)

The positive integer divisors of 12 are 1, 2, 3, 4, 6, and 12. Hence E is ®nite and n…E† ˆ 6:

( f ) Although it may be dicult to ®nd the number of cats living in the United States, there is still a ®nite
number of them at any point in time. Hence F is ®nite.

1.14. In a survey of 60 people, it was found that:
25 read Newsweek magazine
26 read Time
26 read Fortune
9 read both Newsweek and Fortune
11 read both Newsweek and Time
8 read both Time and Fortune
3 read all three magazines
(a) Find the number of people who read at least one of the three magazines.
(b) Fill in the correct number of people in each of the eight regions of the Venn diagram in Fig.
1-16(a) where N, T, and F denote the set of people who read Newsweek, Time, and Fortune,
respectively.


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CHAP. 1]

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SET THEORY

(c) Find the number of people who read exactly one magazine.
(a)

We want n…N ‘ T ‘ F†. By Corollary 1.6,
n…N ‘ T ‘ F† ˆ n…N† ‡ n…T† ‡ n…F† À n…N ’ T† À n…N ’ F† À n…T ’ F† ‡ n…N ’ T ’ F†
ˆ 25 ‡ 26 ‡ 26 À 11 À 9 À 8 ‡ 3 ˆ 52:

Fig. 1-16
(b)

The required Venn diagram in Fig. 1-16(b) is obtained as follows:
3 read all three magazines
11 À 3 ˆ 8 read Newsweek and Time but not all three magazines
9 À 3 ˆ 6 read Newsweek and Fortune but not all three magazines
8 À 3 ˆ 5 read Time and Fortune but not all three magazines
25 À 8 À 6 À 3 ˆ 8 read only Newsweek
26 À 8 À 5 À 3 ˆ 10 read only Time
26 À 6 À 5 À 3 ˆ 12 read only Fortune

60 À 52 ˆ 8 read no magazine at all

(c)

8 ‡ 10 ‡ 12 ˆ 30 read only one magazine.

ALGEBRA OF SETS AND DUALITY
1.15. Write the dual of each set equation:
…A ’ U† ’ …D ‘ Ac † ˆ D

(a) …U ’ A† ‘ …B ’ A† ˆ A

(c)

(b) …A ‘ B ‘ C†c ˆ …A ‘ C†c ’ …A ‘ B†c

(d ) …A ’ U†c ’ A ˆ D

Interchange ‘ and ’ and also U and D in each set equation:
(a)
(b)

…D ‘ A† ’ …B ‘ A† ˆ A
c

(c)
c

…A ’ B ’ C† ˆ …A ’ C† ‘ …A ’ B†

c

(d )

…A ‘ D† ‘ …U ’ Ac † ˆ U
…A ‘ D†c ‘ A ˆ U

1.16. Prove the Commutative laws: (a) A ‘ B ˆ B ‘ A,

and

(b)

(a)

A ‘ B ˆ fx: x P A or x P Bg ˆ fx: x P B or x P Ag ˆ B ‘ A:

(b)

A ’ B ˆ fx: x P A and x P Bg ˆ fx: x P B and x P Ag ˆ B ’ A:

A ’ B ˆ B ’ A.


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[CHAP. 1

1.17. Prove the following identity: …A ‘ B† ’ …A ‘ B c † ˆ A:
Statement
1.
2.
3.
4.
5.

Reason

…A ‘ B† ’ …A ‘ B c † ˆ A ‘ …B ’ B c †
B’ Bc ˆ D
…A ‘ B† ’ …A ‘ B c † ˆ A ‘ D
A‘DˆA
…A ‘ B† ’ …A ‘ B c † ˆ A

Distributive law
Complement law

Substitution
Identity law
Substitution

1.18. Prove …A ‘ B†n…A ’ B† ˆ …AnB† ‘ …BnA†. (Thus either one may be used to de®ne A È B:)
Using XnY ˆ X ’ Y c and the laws in Table 1-1, including DeMorgan's laws, we obtain:
…A ‘ B†n…A ’ B† ˆ …A ‘ B† ’ …A ’ B†c ˆ …A ‘ B† ’ …Ac ‘ B c †
ˆ …A ’ Ac † ‘ …A ’ B c † ‘ …B ’ Ac † ‘ …B ’ B c †
ˆ D ‘ …A ’ B c † ‘ …B ’ Ac † ‘ D
ˆ …A ’ B c † ‘ …B ’ Ac † ˆ …AnB† ‘ …BnA†:

CLASSES OF SETS
1.19. Find the elements of the set A ˆ ‰f1; 2; 3g, f4; 5g, f6; 7; 8gŠ:
A is a class of sets; its elements are the sets f1; 2; 3g, f4; 5g, and f6; 7; 8g:

1.20. Consider the class A of sets in Problem 1.19. Determine whether each of the following is true or
false:
(a) 1 P A
(c) f6; 7; 8g P A
(e) D P A
(b) f1; 2; 3g  A (d ) ff4; 5gg  A
(f) DA
(a)

False. 1 is not one of the elements of A.

(b)

False. f1; 2; 3g is not a subset of A; it is one of the elements of A.

(c)

True. f6; 7; 8g is one of the elements of A.

(d ) True. ff4; 5gg, the set consisting of the element f4; 5g, is a subset of A.
(e)

False. The empty set is not an element of A, i.e., it is not one of the three sets listed as elements of A.

( f ) True. The empty set is a subset of every set; even a class of sets.

1.21. Determine the power set Power…A† of A ˆ fa; b; c; dg.
The elements of Power…A† are the subsets of A. Hence
Power…A† ˆ ‰A; fa; b; cg; fa; b; dg; fa; c; dg; fb; c; dg; fa; bg; fa; cg;
fa; dg; fb; cg; fb; dg; fc; dg; fag; fbg; fcg; fdg; DŠ
As expected, Power…A† has 24 ˆ 16 elements.

1.22. Let S ˆ {red, blue, green, yellow}. Determine which of the following is a partition of S:
(a) P1 ˆ ‰fredg; fblue; greengŠ.
(b) P2 ˆ ‰fred; blue; green; yellowgŠ:

(c) P3 ˆ ‰D; fred; blueg; fgreen; yellowgŠ.
(d ) P4 ˆ ‰fbluegfred; yellow; greeng:


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SET THEORY

(a)

No, since yellow does not belong to any cell.

(b)

Yes, since P2 is a partition of S whose only element is S itself.

(c)

No, since the empty set D cannot belong to a partition.

(d ) Yes, since each element of S appears in exactly one cell.

1.23. Find all partitions of S ˆ f1; 2; 3g.
Note that each partition of S contains either 1, 2, or 3 cells. The partitions for each number of cells are
as follows:

(1) :

[S]

(2) :

[{1}, {2, 3}],

(3) :

[{1}, {2}, {3}]

[{2}, {1, 3}],

[{3}, {1, 2}]

Thus we see that there are ®ve di€erent partitions of S.

MISCELLANEOUS PROBLEMS
1.24. Prove the proposition P that the sum of the ®rst n positive integers is 12 n…n ‡ 1†; that is,
P…n†:

1
1 ‡ 2 ‡ 3 ‡ . . . ‡ n ˆ n…n ‡ 1†
2

The proposition holds for n ˆ 1 since
P…1† : 1 ˆ 12 …1†…1 ‡ 1†
Assuming P…n† is true, we add n ‡ 1 to both sides of P…n†, obtaining
1 ‡ 2 ‡ 3 ‡ . . . ‡ n ‡ …n ‡ 1† ˆ 12 n…n ‡ 1† ‡ …n ‡ 1†

ˆ 12 ‰n…n ‡ 1† ‡ 2…n ‡ 1†Š
ˆ 12 ‰…n ‡ 1†…n ‡ 2†Š
which is P…n ‡ 1†. That is, P…n ‡ 1† is true whenever P…n† is true. By the principle of induction, P is true for
all n.

1.25. Prove the following proposition (for n ! 0†:
P…n†: 1 ‡ 2 ‡ 22 ‡ 23 ‡ Á Á Á ‡ 2n ˆ 2n‡1 À 1
P…0† is true since 1 ˆ 21 À 1. Assuming P…n† is true, we add 2n‡1 to both sides of P…n†, obtaining
1 ‡ 21 ‡ 22 ‡ Á Á Á ‡ 2n ‡ 2n‡1 ˆ 2n‡1 À 1 ‡ 2n‡1
ˆ 2…2n‡1 † À 1
ˆ 2n‡2 À 1
which is P…n ‡ 1†. Thus P…n ‡ 1† is true whenever P…n† is true. By the principle of induction, P is true for all
n ! 0.

1.26. Prove:

…A ’ B†  A  …A ‘ B†

and

…A ’ B†  B  …A ‘ B†:

Since every element in A ’ B is in both A and B, it is certainly true that if x P …A ’ B† then x P A; hence
…A ’ B†  A. Furthermore, if x P A, then x P …A ‘ B† (by the de®nition of A ‘ B), so A  …A ‘ B†. Putting
these together gives …A ’ B†  A  …A ‘ B†. Similarly, …A ’ B†  B  …A ‘ B†.


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SET THEORY

1.27. Prove Theorem 1.2:

[CHAP. 1

The following are equivalent: A  B, A ’ B ˆ A, and A ‘ B ˆ B.

Suppose A  B and let x P A. Then x P B, hence x P A ’ B and A  A ’ B. By Problem 1.26,
…A ’ B†  A. Therefore A ’ B ˆ A. On the other hand, suppose A ’ B ˆ A and let x P A. Then
x P …A ’ B†, hence x P A and x P B. Therefore, A  B. Both results show that A  B is equivalent to
A ’ B ˆ A.
Suppose again that A  B. Let x P …A ‘ B†. Then x P A or x P B. If x P A, then x P B because A  B.
In either case, x P B. Therefore A ‘ B  B. By Problem 1.26, B  A ‘ B. Therefore A ‘ B ˆ B. Now suppose A ‘ B ˆ B and let x P A. Then x P A ‘ B by de®nition of union sets. Hence x P B ˆ A ‘ B. Therefore
A  B. Both results show that A  B is equivalent to A ‘ B ˆ B.
Thus A  B, A ’ B ˆ A and A ‘ B ˆ B are equivalent.

1.28. Prove Theorem 1.5:

If A and B are ®nite sets, then A ‘ B and A ’ B are ®nite and
n…A ‘ B† ˆ n…A† ‡ n…B† À n…A ’ B†

If A and B are ®nite, then clearly A ’ B and A ‘ B are ®nite.
Suppose we count the elements of A and then count the elements of B. Then every element in A ’ B
would be counted twice, once in A and once in B. Hence
n…A ‘ B† ˆ n…A† ‡ n…B† À n…A ’ B†
Alternatively, (Problem 1.36) A is the disjoint union of AnB and A ’ B, B is the disjoint union of BnA
and A ’ B, and A ‘ B is the disjoint union of AnB, A ’ B, and BnA. Therefore, by Lemma 1.4,
n…A ‘ B† ˆ n…AnB† ‡ n…A ’ B† ‡ n…BnA†
ˆ n…AnB† ‡ n…A ’ B† ‡ n…BnA† ‡ n…A ’ B† À n…A ’ B†
ˆ n…A† ‡ n…B† À n…A ’ B†

Supplementary Problems
SETS AND SUBSETS
1.29.

Which of the following sets are equal?
A ˆ fx: x2 À 4x ‡ 3 ˆ 0g,
C ˆ fx: x P N; x < 3g,
2

B ˆ fx: x À 3x ‡ 2 ˆ 0g,
1.30.

D ˆ fx: x P N, x is odd, x < 5g,

G ˆ f3; 1g

F ˆ f1; 2; 1g,

H ˆ f1; 1; 3g

List the elements of the following sets if the universal set is U ˆ fa; b; c; . . . ; y; zg. Furthermore, identify
which of the sets, if any, are equal.
A ˆ fx: x is a vowel}
C ˆ fx: x precedes f in the alphabet}
B ˆ fx: x is a letter in the word ``little''}

1.31.

E ˆ f1; 2g,

D ˆ fx: x is a letter in the word ``title''}

Let A ˆ f1; 2; . . . ; 8; 9g, B ˆ f2; 4; 6; 8g, C ˆ f1; 3; 5; 7; 9g, D ˆ f3; 4; 5g, E ˆ f3; 5g:
Which of the above sets can equal a set X under each of the following conditions?
(a)

X and B are disjoint.

(c)

(b)

X  D but X\B.

(d ) X  C but X\A.

X  A but X\C:

SET OPERATIONS
Problems 1.32 to 1.34 refer to the sets U ˆ f1; 2; 3; . . . ; 8; 9g and A ˆ f1; 2; 5; 6g, B ˆ f2; 5; 7g, C ˆ f1; 3; 5; 7; 9g:


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CHAP. 1]

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(b)

Find:

(a)

A ’ B and A ’ C;

1.33.

Find:

(a)

AnB and AnC;

1.34.

Find:

(a)

…A ‘ C†nB;

1.35.

Let A ˆ fa; b; c; d; eg, B ˆ fa; b; d; f ; gg, C ˆ fb; c; e; g; hg, D ˆ fd; e; f ; g; hg.

(b)

(b)

A ‘ B and B ‘ C;

(c)

Ac and C c .

1.32.

A È B and A È C.

…A ‘ B†c ;

(c)

…B È C†nA.

Find:

1.36.

1.37.

(a)

A‘B

(d )

A ’ …B ‘ D†

( g)

…A ‘ D†nC

…j† A È B

(b)

B’C

(e)

Bn…C ‘ D†

(h)

B’C’D

…k† A È C

(c)

CnD

(f)

…A ’ D† ‘ B

(i )

…CnA†nD

…l† …A È D†nB

Let A and B be any sets. Prove:

(a)

A is the disjoint union of AnB and A ’ B:

(b)

A ‘ B is the disjoint union of AnB, A ’ B, and BnA:

Prove the following:
(a)

A  B if and only if A ’ B c ˆ D.

(b)

A  B if and only if Ac ‘ B ˆ U.

(c)

A  B if and only if B c  Ac :

(d ) A  B if and only if AnB ˆ D:
(Compare results with Theorem 1.2.)
1.38.

Prove the Absorption laws: (a)

A ‘ …A ’ B† ˆ A;

(b) A ’ …A ‘ B† ˆ A:

1.39.

The formula AnB ˆ A ’ B c de®nes the di€erence operation in terms of the operations of intersection and
complement. Find a formula that de®nes the union A ‘ B in terms of the operations of intersection and
complement.

VENN DIAGRAMS
1.40.

The Venn diagram in Fig. 1-17 shows sets A, B, C. Shade the following sets: (a)
(b) Ac ’ …B ‘ C†;
(c) Ac ’ …CnB†.

Fig. 1-17

An…B ‘ C†;


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1.41.

SET THEORY

[CHAP. 1

Use the Venn diagram Fig. 1-6 and Example 1.6 to write each set as the (disjoint) union of fundamental
products:
(a)

1.42.

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A ’ …B ‘ C†,

(b)

Ac ’ …B ‘ C†,

(c)

A ‘ …BnC†.

Draw a Venn diagram of sets A, B, C where A  B, sets B and C are disjoint, but A and C have elements in
common.

ALGEBRA OF SETS AND DUALITY
1.43.

1.44.

Write the dual of each equation:
(a)

A ‘ B ˆ …B c ’ Ac †c

(b)

A ˆ …B c ’ A† ‘ …A ’ B†

(c)

A ‘ …A ’ B† ˆ A

(d )

…A ’ B† ‘ …Ac ’ B† ‘ …A ’ B c † ‘ …Ac ’ B c † ˆ U

Use the laws in Table 1-1 to prove each set identity:
(a)

…A ’ B† ‘ …A ’ B c † ˆ A.

(b) A ‘ …A ’ B† ˆ A:
(c)

A ‘ B ˆ …A ’ B c † ‘ …Ac ’ B† ‘ …A ’ B†:

FINITE SETS AND THE COUNTING PRINCIPLE
1.45.

Determine which of the following sets are ®nite:
(a)

The set of lines parallel to the x axis

(b)

The set of letters in the English alphabet

(c)

The set of numbers which are multiples of 5

(d ) The set of animals living on the earth
(e)

The set of numbers which are solutions of the equation:
x27 ‡ 26x18 À 17x11 ‡ 7x3 À 10 ˆ 0

( f ) The set of circles through the origin (0, 0)
1.46.

Use Theorem 1.5 to prove Corollary 1.6: If A, B, and C are ®nite sets, then so is A ‘ B ‘ C and
n…A ‘ B ‘ C† ˆ n…A† ‡ n…B† ‡ n…C† À n…A ’ B† À n…A ’ C† À n…B ’ C† ‡ n…A ’ B ’ C†

1.47.

A survey on a sample of 25 new cars being sold at a local auto dealer was conducted to see which of three
popular options, air-conditioning …A†, radio …R†, and power windows …W†, were already installed. The
survey found:
15 had air-conditioning.
12 had radio.
11 had power windows.
5 had air-conditioning and power windows.
9 had air-conditioning and radio.
4 had radio and power windows.
3 had all three options.
Find the number of cars that had: (a) only power windows; (b) only air-conditioning; (c) only radio; (d )
radio and power windows but not air-conditioning; (e) air-conditioning and radio, but not power windows;
and ( f ) only one of the options; …g† at least one option; …h† none of the options.


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CHAP. 1]

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CLASSES OF SETS
1.48.

Find the power set Power…A† of A ˆ f1; 2; 3; 4; 5g:

1.49.

Given A ˆ ‰fa; bg, fcg, fd; e; f gŠ.
(a)

State whether each of the following is true or false:
…i† a P A,

(b)

1.50.

(ii) fcg  A,

(iii) fd; e; f g P A;

(iv) ffa; bgg  A,

(v) D  A:

Find the power set of A.

Suppose A is a ®nite set and n…A† ˆ m. Prove that Power…A† has 2m elements.

PARTITIONS
1.51.

1.52.

1.53.

Let X ˆ f1; 2; . . . ; 8; 9g. Determine whether or not each of the following is a partition of X:
(a)

‰f1; 3; 6g; f2; 8g, f5; 7; 9g]

(c)

(b)

‰f1; 5; 7g, f2; 4; 8; 9g, f3; 5; 6g]

(d ) ‰f1; 2; 7g; f3; 5g, f4; 6; 8; 9g, f3; 5gŠ

Let S ˆ f1; 2; 3; 4; 5; 6g. Determine whether or not each of the following is a partition of S:
(a)

P1 ˆ ‰f1; 2; 3g, f1; 4; 5; 6gŠ

(c)

(b)

P2 ˆ ‰f1; 2g, f3; 5; 6g]

(d ) P4 ˆ ‰f1; 3; 5g, f2; 4; 6; 7gŠ

P3 ˆ ‰f1; 3; 5g, f2; 4g, f6gŠ

Determine whether or not each of the following is a partition of the set N of positive integers:
(a)

1.54.

‰f2; 4; 5; 8g, f1; 9g, f3; 6; 7gŠ

‰fn: n > 5g, fn: n < 5gŠ

(b)

‰fn: n > 5g, f0g, f1; 2; 3; 4; 5gŠ,

(c)

‰fn: n2 > 11g, fn: n2 < 11gŠ

Let ‰A1 ; A2 ; . . . ; Am Š and ‰B1 ; B2 ; . . . ; Bn Š be partitions of a set X. Show that the collection of sets
P ˆ ‰Ai ’ Bj : i ˆ 1; . . . ; m; j ˆ 1; . . . ; nŠnD
is also a partition (called the cross partition) of X. (Observe that we have deleted the empty set D.)

1.55.

Let X ˆ f1; 2; 3; . . . ; 8; 9g. Find the cross partition P of the following partitions of X:
P1 ˆ ‰f1; 3; 5; 7; 9g; f2; 4; 6; 8gŠ

and

P2 ˆ ‰f1; 2; 3; 4g; f5; 7g; f6; 8; 9g

ARGUMENTS AND VENN DIAGRAMS
1.56.

Use a Venn diagram to show that the following argument is valid:
S1 : Babies are illogical.
S2 : Nobody is despised who can manage a crocodile.
S3 : Illogical people are despised.
________________________________________________
S: Babies cannot manage crocodiles.
(This argument is adopted from Lewis Carroll, Symbolic Logic; he is also the author of Alice in Wonderland.)


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1.57.

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SET THEORY

[CHAP. 1

Consider the following assumptions:
S1 :
S2 :
S3 :

All dictionaries are useful.
Mary owns only romance novels.
No romance novel is useful.

Determine the validity of each of the following conclusions: (a) Romance novels are not dictionaries. (b)
Mary does not own a dictionary. (c) All useful books are dictionaries.
INDUCTION
1.58.

Prove: 2 ‡ 4 ‡ 6 ‡ Á Á Á ‡ 2n ˆ n…n ‡ 1†:

1.59.

Prove: 1 ‡ 4 ‡ 7 ‡ Á Á Á ‡ …3n À 2† ˆ 2n…3n À 1†:

1.60.

Prove:

1.61.

Prove: 12 ‡ 22 ‡ 32 ‡ . . . ‡ n2 ˆ

1
1
1
1
1
‡
‡
‡ ÁÁÁ ‡
ˆ
:
1Á3 3Á5 5Á7
…2n À 1†…2n ‡ 1† 2n ‡ 1
n…n ‡ 1†…2n ‡ 1†
:
6

MISCELLANEOUS PROBLEMS
1.62.

1.63.

Suppose N ˆ f1; 2; 3; . . .g is the universal set and
A ˆ fx: x

6g,

Find:

A È B; (b)

(a)

B ˆ fx: 4

x

B È C;

9g,
(c)

C ˆ f1; 3; 5; 7; 9g,
A ’ …B È D†;

(d )

D ˆ f2; 3; 5; 7; 8g

…A ’ B† È …A ’ D†:

Prove the following properties of the symmetric di€erence:
(i) A È …B È C† ˆ …A È B† È C (Associative law).
(ii)

A È B ˆ B È A (Commutative law).

(iii)

If A È B ˆ A È C, then B ˆ C (Cancellation law).

(iv) A ’ …B È C† ˆ …A ’ B† È …A ’ C† (Distribution law).
1.64.

Consider n distinct sets A1 ; A2 ; . . . ; An in a universal set U. Prove:
(a)

There are 2n fundamental products of the n sets.

(b)

Any two fundamental products are disjoint.

(c)

U is the union of all the fundamental products.

Answers to Supplementary Problems
1.29.

B ˆ C ˆ E ˆ F; A ˆ D ˆ G ˆ H:

1.30.

A ˆ fa; e; i; o; ug;

1.31.

(a)

1.32.

(a) A ’ B ˆ f2; 5g; A ’ C ˆ f1; 5g: (b)
(c) Ac ˆ f3; 4; 7; 8; 9g; C c ˆ f2; 4; 6; 8g:

C and E;

(b)

B ˆ D ˆ f1; i; t; eg;
D and E;

(c)

C ˆ fa; b; c; d; eg:

A; B; D;

(d )

None.

A ‘ B ˆ f1; 2; 5; 6; 7g;

B ‘ C ˆ f1; 2; 3; 5; 7; 9g: