**Unformatted text preview: ** W.D. Wallis A Beginner’s
Guide to Discrete
Mathematics
Second Edition W.D. Wallis
Department of Mathematics
Southern Illinois University
Carbondale, IL 62901
USA
[email protected] ISBN 978-0-8176-8285-9
e-ISBN 978-0-8176-8286-6
DOI 10.1007/978-0-8176-8286-6
Springer New York Dordrecht Heidelberg London
Library of Congress Control Number: 2011940047
Mathematics Subject Classification (2010): 05-01, 05Axx, 05Cxx, 60-01, 68Rxx, 97N70
1st edition: © Birkhäuser Boston 2003
2nd edition: © Springer Science+Business Media, LLC 2012
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Springer is part of Springer Science+Business Media ( ) For Nathan Preface This text is a basic introduction to those areas of discrete mathematics of interest to
students of mathematics. Introductory courses on this material are now standard at
many colleges and universities. Usually these courses are of one semester’s duration,
and usually they are offered at the sophomore level.
Very often this will be the first course where the students see several real proofs.
The preparation of the students is very mixed, and one cannot assume a strong background. In particular, the instructor should not assume that the students have seen a
linear algebra course, or any introduction to number systems that goes beyond college algebra.
In view of this, I have tried to avoid too much sophistication, while still retaining rigor. I hope I have included enough problems so that the student can reinforce
the concepts. Most of the problems are quite easy, with just a few difficult exercises
scattered through the text. If the class is weak, a small number of sections will be too
hard, while the instructor who has a strong class will need to include some supplementary material. I think this is preferable to a book at a higher mathematical level,
one that scares away the weaker students.
Readership
While the book is primarily directed at mathematics majors and minors, this material
is also studied by computer scientists. The face of computer science is changing due
to the influence of the internet, and many universities will also require a second
course, with more specialized material, but those students also need the basics.
Another developing area is the course on mathematical applications in the modern world, aimed at liberal arts majors and others. Much of the material in those
courses is discrete. I do not think this book should even be considered as a text for
such a course, but it could be a useful reference, and those who end up teaching
such a course will also find this text useful. Discrete mathematics is also an elective viii Preface topic for mathematically gifted students in high schools, and I consulted the Indiana
suggested syllabus for such courses.
Outline of Topics
The first two chapters include a brief survey of number systems and elementary set
theory. Included are discussions of scientific notation and the representation of numbers in computers; topics that were included at the suggestion of computer science
instructors. Mathematical induction is treated at this point although the instructor
could defer this until later. (There are a few references to induction later in the text,
but the student can omit these in a first reading.)
I introduce logic along with set theory. This leads naturally into an introduction
to Boolean algebra, which brings out the commonality of logic and set theory. The
latter part of Chapter 3 explains the application of Boolean algebra to circuit theory.
I follow this with a short chapter on relations and functions. The study of relations is an offshoot of set theory, and also lays the foundation for the study of graph
theory later. Functions are mentioned only briefly. The student will see them treated
extensively in calculus courses, but in discrete mathematics we mostly need basic
definitions.
Enumeration, or theoretical counting, is central to discrete mathematics. In Chapter 5 I present the main results on selections and arrangements, and also cover the
binomial theorem and derangements. Some of the harder problems here are rather
challenging, but I have omitted most of the more sophisticated results.
Counting leads naturally to probability theory. I have included the main ideas
of discrete probability, up to Bayes’ theorem. There was a conscious decision not
to include any real discussion of measures of central tendency (means, medians) or
spread (variance, quartiles) because most students will encounter them elsewhere,
e.g., in statistics courses.
Graph theory is studied, including Euler and Hamilton cycles and trees. This is
a vehicle for some (easy) proofs, as well as being an important example of a data
structure.
Matrices and vectors are defined and discussed briefly. This is not the place for
algebraic studies, but matrices are useful for studying other discrete objects, and this
is illustrated by a section on adjacency matrices of relations and graphs. A number of
students will never study linear algebra, and this chapter will provide some foundation for the use of matrices in programming, mathematical modeling, and statistics.
Those who have already seen vectors and matrices can skip most of this chapter, but
should read the section on adjacency matrices.
Chapter 9 is an introduction to cryptography, including the RSA cryptosystem,
together with the necessary elementary number theory (such as modular arithmetic
and the Euclidean algorithm). Cryptography is an important application area and is
a good place to show students that discrete mathematics has real-world applications. Preface ix Moreover, most computer science majors will later be presented with electives in this
area. The level of mathematical sophistication is higher in parts of this chapter than
in most of the book.
The final chapter is about voting systems. This topic has not been included in very
many discrete mathematics texts. However, voting methods are covered in many of
the elementary applied mathematics courses for liberal arts majors, and they make a
nice optional topic for mathematics and computer science majors.
Perhaps I should explain the omissions rather than the inclusions. I thought the
study of predicates and quantifiers belonged in a course on logic rather than here.
I also thought lattice theory was too deep, although it would fit nicely after the section
on Boolean forms.
There is no section on recursion and recurrence relations. Again, this is a deep
area. I have actually given some problems on recurrences in the induction section, but
I thought that a serious study belongs in a combinatorics course. Similarly, the deeper
enumeration results, such as counting partitions, belong in higher-level courses.
Another area is linear programming. This was once an important part of discrete mathematics courses. But, in recent years, syllabi have changed. Nowadays,
somewhat weaker students are using linear programming, and there are user-friendly
computer packages available. I do not think that it will be on the syllabus of many of
the courses at which this book is aimed.
Problems and Exercises
The book contains a large selection of exercises, collected at the end of sections.
There should be enough for students to practice the concepts involved. Most are
straightforward; in some sections there are one or two more sophisticated questions
at the end.
A number of worked examples, called Sample Problems, are included in the
body of each section. Most of these are accompanied by a Practice Exercise, designed primarily to test the reader’s comprehension of the ideas being discussed. It
is recommended that students work all the Practice Exercises. Complete solutions
are provided for all of them, as well as brief answers to the odd-numbered problems
from the sectional exercise sets.
Gender
In many places a mathematical discussion involves a protagonist—a person who flips
a coin or deals a card or traverses a road network. These people used to be exclusively
male in older textbooks. In recent years this has rightly been seen to be inappropriate.
Unfortunately this has led to frequent repetitions of nouns—“the player’s card” rather
than “his card”—and the use of the ugly “he or she.” x Preface I decided to avoid such problems by a method that was highly appropriate to this
text: I flipped a coin to decide whether a character was male or female. If the reader
detects an imbalance, please blame the coin.
There were two exceptions to this rule. Cryptographers traditionally write about
messages sent from Alice (A) to Bob (B), so I followed this rule in discussing RSA
cryptography. And in the discussion of the Monty Hall problem, the game show host
is male, in honor of Monty, and the player is female for balance.
Acknowledgments
My treatment of discrete mathematics owes a great deal to many colleagues and
mathematicians in other institutions with whom I have taught or discussed this material. Among my influences are Roger Eggleton, Ralph Grimaldi, Dawit Haile, Fred
Hoffman, Bob McGlynn, Nick Phillips, Bill Sticka, and Anne Street, although some
of them may not remember why their names are here.
I am grateful for the constant support and encouragement of the staff at
Birkhäuser.
Carbondale, USA W.D. Wallis Contents 1 2 3 4 Properties of Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Sums . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.3 Bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.4 Scientific Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 1.5 Arithmetic in Computers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Sets and Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.1 Propositions and Logic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.2 Elements of Set Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.3 Proof Methods in Set Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 2.4 Some Further Set Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 2.5 Mathematical Induction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Boolean Algebras and Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 3.1 Boolean Algebra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 3.2 Boolean Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.3 Finding Minimal Disjunctive Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 3.4 Digital Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 Relations and Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 4.1 Relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 xii Contents 4.2 Some Special Kinds of Relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.3 Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
5 The Theory of Counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.1 Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.2 Unions of Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
5.3 One-to-One Correspondences and Infinite Sets . . . . . . . . . . . . . . . . . . 129
5.4 Arrangement Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
5.5 Selections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 5.6 The Binomial Theorem and Its Applications . . . . . . . . . . . . . . . . . . . . 151
5.7 Some Further Counting Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
6 Probability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
6.1 Probability Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
6.2 Repeated Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
6.3 Counting and Probability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
6.4 Conditional Probabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
6.5 Bayes’ Formula and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 7 Graph Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
7.1 Introduction to Graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
7.2 The Königsberg Bridges; Traversability . . . . . . . . . . . . . . . . . . . . . . . . 222
7.3 Walks, Paths, and Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
7.4 Distances and Shortest Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
7.5 Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243
7.6 Hamiltonian Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
7.7 The Traveling Salesman Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 8 Matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
8.1 Vectors and Matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
8.2 Properties of the Matrix Product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271
8.3 Systems of Linear Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 Contents xiii 8.4 More About Linear Systems and Inverses . . . . . . . . . . . . . . . . . . . . . . 284
8.5 Adjacency Matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
9 Number Theory and Cryptography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
9.1 Some Elementary Number Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
9.2 Modular Arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
9.3 An Introduction to Cryptography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
9.4 Substitution Ciphers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317
9.5 Modern Cryptography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323
9.6 Other Cryptographic Ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329
9.7 Attacks on the RSA System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 10 The Theory of Voting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339
10.1 Simple Elections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339
10.2 Multiple Elections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350
10.3 Fair Elections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359
10.4 Properties of Electoral Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
Solutions to Practice Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379
Answers to Selected Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 1
Properties of Numbers When we study numbers, many of the problems involve continuous properties. Much
of the earliest serious study of mathematics was in geometry, and one essential property of the real world is that between any two points there is a line segment that is
continuous and infinitely divisible. All of calculus depends on the continuous nature
of the number line. Some of the most famous difficulties of Greek mathematics involved the existence of irrational numbers and the fact that between any two real
numbers one can always find another number.
But the discrete properties of numbers are also very important. The decimal notation in which we usually write numbers depends on properties of the number 10, and
we can also study the (essentially discrete) features of representations where other
positive whole numbers take the role of 10.
When we write the exact value of a number, or an approximation to its value, we
use only the ten integers 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, together with the decimal point.
When numbers are represented in a computer, only integers are used. So it is important to understand the discrete properties of numbers to talk about their continuous
properties. 1.1 Numbers
Sets and Number Systems
All of discrete mathematics—and, in fact, all of mathematics—rests on the foundations of set theory and numbers. In this first section we remind you of some basic
definitions and notations. Further properties of numbers will be explored in the rest
of this chapter; sets will be discussed further in Chapter 2.
We use the word set in everyday language: a set of tires, a set of saucepans. In
mathematics you have already encountered various sets of numbers. We shall use
W.D. Wallis, A Beginner’s Guide to Discrete Mathematics,
DOI 10.1007/978-0-8176-8286-6_1, © Springer Science+Business Media, LLC 2012 2 1 Properties of Numbers set to mean any collection of objects, provided only that there is a well-defined rule,
called the membership law, for determining whether a given object belongs to the
set. The individual objects in the set are called its elements or members and are said
to belong to the set. If S is a set and s is one of its elements, we denote this fact by
writing
s ∈ S,
which is read as “s belongs to S” or “s is an element of S.”
The notation S ⊆ T means that every member of S is a member of T :
x∈S ⇒ x ∈ T. Then S is called a subset of T . This definition allows for the possibility that S = T ,
and, in fact, a set is considered to be a subset of itself. If S ⊆ T , but S = T we say
S is a proper subset of T and write S ⊂ T .
One way of defining a set is to list all the elements, usually between braces;
thus the set of the first three members of the English alphabet is {a, b, c}. If S is the
set consisting of the numbers 0, 1, and 3, we could write S = {0, 1, 3}. We write
{1, 2, . . . , 16} to mean the set of all whole numbers from 1 to 16. This use of a string
of dots is not precise, but is usually easy to understand. Another method is the use
of the membe...

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