Fuzzy mathematics in economics and engineering

Fuzzy Mathematics
in Economics and Engineering


Studies in Fuzziness and Soft Computing
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Systems Research Institute
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James J. Buckley
Esfandiar Eslami
Thomas Feuring

Fuzzy Mathematics
in Economics
and Engineering
With 69 Figures
and 27 Tables

Springer-Verlag Berlin Heidelberg GmbH


Professor James J. Buckley
University of Alabama at Birmingham
Mathematics Department
Birmingham, AL 35294
USA

Professor Esfandiar Eslami 1
Shahid Bahonar University
Department of Mathematics
Kerman
Iran

eslami @math.uab.edu
Dr. Thomas Feuring
University of Siegen
Electrical Engineering and Computer Science
HolderlinstraBe 3
57068 Siegen
Germany
I Thanks to the University of Shahid Bahonar. Kerman, Iran, for financial support during my sabbatical
leave at UAB. Thanks to UAB for producing a good atmosphere to do research and teaching. Special
thanks to Prof. James J. BuckJey for his kind cooperation that made all possible.

ISSN 1434-9922
ISBN 978-3-7908-2505-3
ISBN 978-3-7908-1795-9 (eBook)
DOI 10.1007/978-3-7908-1795-9
Cataloging-in-Publication Data applied for
Die Deutsche Bibliothek - CIP-Einheitsaufnahme
Buckley, James J.: Fuzzy mathematics in economics and engineering: with 27 tables / James J. Buckley;
Esfandiar Eslami; Thomas Feuring. - Heidelberg; New York: Physica-VerI., 2002
(Studies in fuzziness and soft computing; Vol. 91)
This work is subject to copyright. All rights are reserved, whether the whole or part of the material is
concerned. specifically the rights of translation, reprinting, reuse of illustrations, recitation. broadcasting,
reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication
or parts thereof is permitted only under the provisions of the German Copyright Law of September 9,

1965, in its current version, and permission for use must always be obtained from Physica-Verlag. Violations are liable for prosecution under the German Copyright Law.

© Springer-Verlag Berlin Heidelberg 2002
Originally published by Physica-Verlag Heidelberg in 2002
Softcover reprint of the hardcover I st edition 2002
The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply.
even in the absence of a specific statement, that such names are exempt from the relevant protective laws
and regulations and therefore free for general use.
Hardcover Design: Erich Kirchner, Heidelberg


To Julianne, Birgit and Mehra

Helen, Lioba, Jason, Pooya, Peyman and Payam


Contents
1
2

3

Introduction
Bibliography

1

3

Fuzzy Sets

2.1 Fuzzy Sets . . . . . . .
2.1.1 Fuzzy Numbers
2.1.2 Alpha-Cuts ..
2.1.3 Inequalities ..
2.1.4 Discrete Fuzzy Sets.
2.2 Fuzzy Arithmetic . . . . . .
2.2.1 Extension Principle
2.2.2 Interval Arithmetic .
2.2.3 Fuzzy Arithmetic ..
2.3 Fuzzy Functions . . . . . .
2.3.1 Extension Principle
2.3.2 Alpha-Cuts and Interval Arithmetic
2.3.3 Differences
2.4 Possibility Theory
Bibliography . . . . . .

5
5
5
7
9
9
9
9
10

Solving Fuzzy Equations
3.1 AX +B = C . . . . . .
3.2 New Solutions . . . . . . . . . . . .
3.3 Systems of Fuzzy Linear Equations

3.4 Applications . . . . . . . . . . . . .
3.4.1 Fuzzy Linear Equation . . .
3.4.2 Fuzzy Quadratic Equation.
3.4.3 System of Linear Equations
3.5 Fuzzy Input-Output Analysis
3.5.1 The Open Model ..
3.5.2 Fuzzy Model . . . .
3.6 Summary and Conclusions.
Bibliography . . . . . . . . . . .

19
19

11
12
13
13
14
15
17

22
24
33
33
34
35

39
39

41
43
45


CONTENTS

viii
4

5

6

Fuzzy Mathematics in Finance
4.1 Future Value
4.2 Present Value . . . .
4.3 Annuities . . . . . .
4.3.1 Future Value
4.3.2 Present Value
4.4 Portfolio Analysis. .
4.4.1 NPV Method
4.4.2 IRR Method
4.5 Summary and Conclusions .
Bibliography . . . . . . . . . . .

47
48

Fuzzy Non-Linear Regression

5.1 Univariate Non-Linear Fuzzy Regression.
5.1.1 Testing the EA . . . . . . . . . . .
5.1.2 Application . . . . . . . . . . . . .
5.2 Multivariate Non-Linear Fuzzy Regression
5.2.1 Testing . . . . .
5.2.2 Application .. .
5.3 Conclusions and Results
Bibliography . . . . . .

69
70

Operations Research
6.1 Fuzzy Linear Programming
6.1.1 Maximize Z .....
6.1.2 Fuzzy Inequality ..
6.1.3 Evolutionary Algorithm
6.1.4 Applications . . . . . .
6.1.5 Summary and Conclusions.
6.2 Fuzzy PERT . . . . . . . . . . . .
6.2.1 Job Times Fuzzy Numbers
6.2.2 Job Times Discrete Fuzzy Sets
6.2.3 Summary . . . . . .
6.3 Fuzzy Inventory Control . .
6.3.1 Demand Not Fuzzy.
6.3.2 Fuzzy Demand . . .
6.3.3 Backorders . . . . .
6.3.4 Evolutionary Algorithm
6.4 Fuzzy Queuing Theory.
6.4.1 Service . . . . . . . . . .

6.4.2 Arrivals . . . . . . . . .
6.4.3 Finite or Infinite System Capacity
6.4.4 Machine Servicing Problem . . .
6.4.5 Fuzzy Queuing Decision Problem
6.4.6 Summary and Conclusions . . . .

81
81
82
85
88
89
97
98
102
104
104
105
109
111
117

50
53
53
54
55
55
59
62

65

71
72

74
75
75

76
77

118
118

120
121
122
124
126
129


CONTENTS

ix

6.5

Fuzzy Network Analysis . . . . . . . . . . . .

6.5.1 Fuzzy Shortest Route . . . . . . . . .
6.5.2 Fuzzy Min-Cost Capacitated Network
6.5.3 Evolutionary Algorithm ..
6.5.4 Summary and Conclusions.
6.6 Summary and Conclusions.
Bibliography . . . . . . . . . . .

129
130
132
136
137
137
139

7

Fuzzy Differential Equations
7.1 Fuzzy Initial Conditions
7.1.1 Electrical Circuit . .
7.1.2 Vibrating Mass . . .
7.1.3 Dynamic Supply and Demand.
7.2 Other Fuzzy Parameters ..
7.3 Summary and Conclusions.
Bibliography . . . . . . . . . .

145
146
150
153

155
158
161
163

8

Fuzzy Difference Equations
8.1 Difference Equations . . .
8.2 Fuzzy Initial Conditions .
8.2.1 Classical Solution.
8.2.2 Extension Principle Solution
8.2.3 Interval Arithmetic Solution.
8.2.4 Summary ..
8.3 Recursive Solutions .. .
8.4 Applications . . . . . . .
8.4.1 National Income
8.4.2 Transmission of Information.
8.4.3 Fuzzy Fibonacci Numbers
8.5 Summary and Conclusions.
Bibliography . . . . . . . . . . . . . .

165
166
167
167
169
172
174
175

176
176
178
179
180
183

9

Fuzzy Partial Differential Equations
9.1 Elementary Partial Differential Equations
9.2 Classical Solution . . . . . . .
9.3 Extension Principle Solution.
9.4 Summary and Conclusions.
Bibliography . . . . . . . . . . .

185
185
187
190
194
197

10 Fuzzy Eigenvalues
10.1 Fuzzy Eigenvalue Problem
10.1.1 Algorithm . . . . .
10.2 Fuzzy Input-Output Analysis
10.3 Fuzzy Hierarchical Analysis
10.3.1 The Amax-Method ..

199
199
203
206
209
210


CONTENTS

x

10.3.2 Fuzzy Amax-Method . . . . . . . . .
10.3.3 Fuzzy Geometric Row Mean Method
10.4 Summary and Conclusions.
Bibliography . . . . . . . . .

212
222
224
227

11 Fuzzy Integral Equations
11.1 Resolvent Kernel Method
11.1.1 Classical Solution.
11.1.2 Second Solution Method.
11.2 Symmetric Kernel Method . . . .
11.2.1 Classical Solution . . . . .
11.2.2 Second Solution Method.
11.3 Summary and Conclusions.

Bibliography . . . . . . . . . .

229
230
231
235
237
237
239
240
241

12 Summary and Conclusions
12.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . .
12.1.1 Chapter 3: Solving Fuzzy Equations . . . . . .
12.1.2 Chapter 4: The Fuzzy Mathematics in Finance
12.1.3 Chapter 5: Fuzzy Non-Linear Regression
12.1.4 Chapter 6: Operations Research . . . . . .
12.1.5 Chapter 7: Fuzzy Differential Equations . .
12.1.6 Chapter 8: The Fuzzy Difference Equations
12.1. 7 Chapter 9: Fuzzy Partial Differential Equations.
12.1.8 Chapter 10: Fuzzy Eigenvalues . . . .
12.1.9 Chapter 11: Fuzzy Integral Equations
12.2 Research Agenda , . . . . . . . , . . . . . . .
12.2.1 Chapter 3: Solving Fuzzy Equations .
12.2.2 Chapter 4: The Fuzzy Mathematics of Finance
12.2.3 Chapter 5: Fuzzy Non-Linear Regression
12.2.4 Chapter 6: Operations Research . . . ,
12.2.5 Chapter 7: Fuzzy Differential Equations .
12.2.6 Chapter 8: Fuzzy Difference Equations. .

12.2.7 Chapter 9: Fuzzy Partial Differential Equations.
12.2.8 Chapter 10: Fuzzy Eigenvalues . . . .
12.2,9 Chapter 11: Fuzzy Integral Equations
12.3 Conclusions . . . . , . , , . . . . . . . . . . .

243
243
243
244
244
245
247
248
248
249
249
250
250
250
250
250
251
252
252
252
252
252

13 Evolutionary Algorithms
13.1 Introduction . . . , . . .

13.2 General Purpose Algorithm
Bibliography . . . . , ' . . .

253
253
253
257

Index

259


CONTENTS

Xl

List of Figures

267

List of Tables

271


Chapter 1

Introduction
This book surveys certain applications of fuzzy sets to economics and engineering based on the authors' research over the last 16 years. We cover many

of the topics addressed in the literature, and if we omit an area of application, we may state this within the pertinent chapter or it will be found in the
references.
No previous knowledge of fuzzy sets is needed because in Chapter 2 we
survey the basic ideas needed for the rest of the book. The basic prerequisite
is elementary differential calculus because we use derivatives, and partial
derivatives, from time to time. The chapters on differential equations are all
elementary and should be understandable from basic calculus.
We sometimes define new concepts in the chapters. Usually, when we do
this the concept is useful only for that chapter and therefore was not included
in Chapter 2.
Our policy on "theorems" is always to state the theorem and present
the proof whenever the proof is short and elementary. Longer, and more
complicated proofs, are relegated to the references.
We do not give a complete list of references. We, of course, give all
our references since the book is based on these papers. For other references
we: (1) give the recent (last couple of years) references; and (2) for older
references we only give a few "key" citations from which the reader can find
other relevant papers.
An overview of the book can be seen from the table of contents. A more
detailed overview is in Chapter 12. So, if you want a quick reading about
what is in the book, please turn to the summary section of Chapter 12.
To achieve a uniform notation in a book with lots of mathematics is always
difficult. What we have done is introduce the basic notation, to be uniform
throughout the book, in Chapter 2. Other notation is chapter dependent. By
chapter dependent we mean some symbols may change their meaning from
chapter to chapter. For example, the letters "a" and "b" may be used as [a, b]
to represent a closed interval in one chapter but they can be parameters in a
J. J. Buckley et al., Fuzzy Mathematics in Economics and Engineering
© Springer-Verlag Berlin Heidelberg 2002

2

CHAPTER 1. INTRODUCTION

differential equation in another chapter.
Our recommendation reading for applications to economics is
I-t 2 -t {3,4,5,6,8, 10} -t 12,

where "1" stands for Chapter 1, etc. For engineering we suggest
I-t 2 -t {3, 7,8,9,10, 11} -t 12 .

This book is basically about solving fuzzy equations (linear, eigenvalue,
differential, etc.) and fuzzy optimization. What is new is that we introduce
three solution concepts for fuzzy equations: (1) the classical solution; (2)
the extension principle solution; and (3) the a-cut and interval arithmetic
solution. Therefore, it is most important that the reader understand the ideas
involved in these three methods of solving fuzzy equations before proceeding
to the rest of the book. The easiest introduction to the solution methods is
in Sections 3.1 and 3.2 of Chapter 3 where we look at solving a simple fuzzy
linear equation.
Also what is new is that we use an evolutionary algorithm to solve fuzzy
problems, especially the fuzzy optimization problems. Usually the classical
methods (calculus, etc.) do not apply to solving fuzzy optimization problems so we employ a direct search algorithm to generate good (approximate)
solutions to these fuzzy problems.
Our general purpose evolutionary algorithm (abbreviated EA ) is described in Chapter 13 at the end of this book. It has to be adopted to
the different fuzzy problems and this is usually discussed in the chapters
where it is used. We have not included our evolutionary algorithm in the
book since you can download this software (genetic, evolutionary) from the
internet. Use the terms "genetic", or "evolutionary" algorithm in your search

engine and then you can solve your own fuzzy optimization problems.
Some of the figures in the book are difficult to obtain so they were created
using different methods. First, many were made using the graphics package
in LaTeX2,. For example, Figures 3.6,3.11, many in Chapters 6 and 10, and
Figures 7.1, 7.3, were done this way. Some others, impossible to do within
the graphics package in LaTeX2" were first drawn in Maple [1] and then
exported to LaTeX2,. We did those in Chapter 7 ( not Figures 7.1 and
7.3) and some in Chapter 3 this way. There are some other figures that we
considered easier to do in Maple and then export to LaTeX2, ( all those in
Chapter 2 plus some in Chapter 3). These figures are defined as x = f(y),
for a ~ y ~ 1, which is backwards from the usual y a function of x, and the
"implicitplot" command in Maple made it easy to graph x a function of y ( y
axis vertical and the x axis horizontal). Finally, there were figures done using
the graphics package in LaTeX2" but the data for the graph was obtained
from our evolutionary algorithm or from Maple. The graphs in Chapter 4
plus some in Chapter 3, 6 and Figure 10.9 were done this way.


Bibliography
[1] Maple 6, Waterloo Maple Inc., Waterloo, Canada.


Chapter 2

Fuzzy Sets
In this chapter we have collected together the basic ideas from fuzzy sets and
fuzzy functions needed for the book. Any reader familiar with fuzzy sets,
fuzzy numbers, the extension principle, a-cuts, interval arithmetic, possibility
theory and fuzzy functions may go on to the rest of the book. A good general
reference for fuzzy sets and fuzzy logic is [1], [6].

Our notation specifying a fuzzy set is to place a "bar" over a letter. SO
A, B, ... , X, Y, . .. , Ct, /3, ... , will all denote fuzzy sets.

2.1

Fuzzy Sets

If 0 is some set, then a fuzzy subset A of 0 is defined by its membership
function, written A(x), which produces values in [0,1] for all x in O. So, A(x)
is a function mapping 0 into [0,1]. If A(xo) = 1, then we say Xo belongs to
A, if A(xt} = 0 we say Xl does not belong to A, and if A(X2) = 0.6 we say
the membership value of X2 in A is 0.6. When A(x) is always equal to one or
zero we obtain a crisp (non-fuzzy) subset of O. For all fuzzy sets B, C, ...
we use B(x), C(x), ... for the value of their membership function at x. Most
of the fuzzy sets we will be using will be fuzzy numbers.
The term "crisp" will mean not fuzzy. A crisp set is a regular set. A
crisp number is just a real number. A crisp matrix (vector) has real numbers
as its elements. A crisp function maps real numbers (or real vectors) into
real numbers. A crisp solution to a problem is a solution involving crisp sets,
crisp numbers, crisp functions, etc.

2.1.1

Fuzzy Numbers

A general definition of fuzzy number may be found in [1],[6], however our
fuzzy numbers will be almost always triangular (shaped), or trapezoidal
(shaped), fuzzy numbers. A triangular fuzzy number N is defined by three
J. J. Buckley et al., Fuzzy Mathematics in Economics and Engineering
© Springer-Verlag Berlin Heidelberg 2002

CHAPTER 2. FUZZY SETS

6

numbers a < b < c where the base of the triangle is the interval [a, c] and its
vertex is at x = b. Triangular fuzzy numbers will be written as N = (a/b/c).
A triangular fuzzy number N = (1.2/2/2.4) is shown in Figure 2.1. We see
that N(2) = 1, N(1.6) = 0.5, etc.

0.8

0.6
0.4
0.2

o

Figure 2.1: Triangular Fuzzy Number N
A trapezoidal fuzzy number M is defined by four numbers a < b < c < d
where the base of the trapezoid is the interval [a,d] and its top (where the
membership equal one) is over [b,c]. We write M = (a/b,c/d) for trapezoidal
fuzzy numbers. Figure 2.2 shows M = (1.2/2,2.4/2.7).
A triangular shaped fuzzy number P is given in Figure 2.3. P is only
partially specified by the three numbers 1.2, 2, 2.4 since the graph on [1.2,2],
and [2,2.4], is not a straight line segment. To be a triangular shaped fuzzy
number we require the graph to be continuous and: (1) monotonically increasing on [1.2,2]; and (2) monotonically decreasing on [2,2.4]. For triangular shaped fuzzy number P we use the notation P ~ (1.2/2/2.4) to
show that it is partially defined by the three numbers 1.2, 2, and 2.4. If
P ~ (1.2/2/2.4) we know its base is on the interval [1.2,2.4] with vertex

(membership value one) at x = 2. Similarly we define trapezoidal shaped
fuzzy number Q ~ (1.2/2,2.4/2.7) whose base is [1.2,2.7] and top is over
the interval [2,2.4]. The graph of Q is similar to M in Figure 2.2 but it has
continuous curves for its sides.
Although we will be using triangular (shaped) and trapezoidal (shaped)
fuzzy numbers throughout the book, many results can be extended to more


2.1. FUZZY SETS

7

0.8
0.6

0.4
0.2

o 0.5

1.5

2

x 2.5

3

3.5

4

Figure 2.2: Trapezoidal Fuzzy Number M
general fuzzy numbers, but we shall be content to work with only these special
fuzzy numbers.
We will be using fuzzy numbers in this book to describe uncertainty.
For example, in Chapter 4 if a future interest rate is uncertain but believed to be between 7 and 8%, we could model it using a trapezoidal fuzzy
number (0.065/0.07,0.08/0.085). Also, in Chapter 7, if the initial value of
the derivative is uncertain but known to be around 5, we may model it as
y'(O) = (4/5/6).

2.1.2

Alpha-Cuts

Alpha---cuts are slices through a fuzzy set producing regular (non-fuzzy) sets.
n, then an a-cut of A, written A[a] is
defined as
(2.1)
A[a] = {x E nIA(x) ~ a} ,

If A is a fuzzy subset of some set

for all a, 0 < a ~ 1. The a = 0 cut, or A[O], must be defined separately.
Let N be the fuzzy number in Figure 2.1. Then N[O] = [1.2,2.4]. Using
equation (2.1) to define N[O] would give N[O] = all the real numbers. Similarly, M[O] = [1.2,2.7] from Figure 2.2 and in Figure 2.3 prO] = [1.2,2.4].
For any fuzzy set A, A[O] is called the support, or base, of A. Many authors
call the support of a fuzzy number the open interval (a, b) like the support

CHAPTER 2. FUZZY SETS

8

0.8

0.6

0.4
0.2

o 0.5

1.5

x

2

2.5

3

Figure 2.3: Triangular Shaped Fuzzy Number P
of N in Figure 2.1 would then be (1.2,2.4). However in this book we use the
closed interval [a, b] for the support (base) of the fuzzy number.
The core of a fuzzy number is the set of values where the membership
value equals one. If N = (a/b/c), or N ~ (a/b/c), then the core of N is the
single point b. However, if M = (a/b, c/d), or M ~ (a/b, c/d), then the core
of M = [b,cJ.

For any fuzzy number Q we know that Q[a] is a closed, bounded, interval
for 0 ::; a ::; 1. We will write this as
(2.2)
where ql (a) (q2(a)) will be an increasing (decreasing) function of a with
ql(l) ::; q2(1). If Q is a triangular shaped or a trapezoidal shaped fuzzy
number then: (1) ql (a) will be a continuous, monotonically increasing function of a in [0,1]; (2) q2(a) will be a continuous, monotonically decreasing
function of a, 0 ::; a ::; 1; and (3) ql(l) = q2(1) (ql (1) < q2(1) for trapezoids). We sometimes check monotone increasing (decreasing) by showing
that dql (a)/da > 0 (dq2(a)/da < 0) holds.
For the N in Figure 2.1 we obtain N[a] = [nl(a),n2(a)J, nl(a) = 1.2 +
0.8a and n2(a) = 2.4 - O.4a, 0 S; a ::; 1. Similarly, M in Figure 2.2 has
M[a] = [ml(a), m2(a)], miCa) = 1.2 + 0.8a and m2(a) = 2.7 - 0.3a, 0 S;
a ::; 1. The equations for ni(a) and mi(a) are backwards. With the y-axis
vertical and the x-axis horizontal the equation nl (a) = 1.2 + 0.8a means


2.2. FUZZY ARITHMETIC

9

x = 1.2 + O.By, 0 :::; y :::; 1. That is, the straight line segment from (1.2,0) to
(2,1) in Figure 2.1 is given as x a function of y whereas it is usually stated as
y a function of x. This is how it will be done for all a-cuts of fuzzy numbers.

2.1.3

Inequalities

Let N = (a/b/c). We write N ~ d, d some real number, if a ~ d, N > d when
a> d, N :::; d for c :::; d and N < d if c < d. We use the same notation for
triangular shaped and trapezoidal (shaped) fuzzy numbers whose support is

the interval [a, c].
If A and B are two fuzzy subsets of a set 0, then A :::; B means A(x) :::;
B(x) for all x in 0, or A is a fuzzy subset of B. A < B holds when A(x) <
B(x), for all x. There is a potential problem with the symbol :::;. In some
places in the book (Chapters 4, 6 and 10) M :::; N, for fuzzy numbers M and
N, means that M is less than or equal to N. It should be clear on how we
use ":::;" as to which meaning is correct.

2.1.4

Discrete Fuzzy Sets

Let A be a fuzzy subset of O. If A(x) is not zero only at a finite number of
x values in 0, then A is called a discrete fuzzy set. Suppose A(x) is not zero
only at Xl, X2, X3 and X4 in O. Then we write the fuzzy set as

(2.3)
where the /li are the membership values. That is, A(Xi) = /li, 1 :::; i :::; 4,
and A(x) = 0 otherwise. We can have discrete fuzzy subsets of any space O.
Notice that a-cuts of discrete fuzzy sets of R, the set of real numbers, do not
produce closed, bounded, intervals.

2.2

Fuzzy Arithmetic

If A and B are two fuzzy numbers we will need to add, subtract, multiply and
divide them. There are two basic methods of computing A + B, A - B, etc.
which are: (1) extension principle; and (2) a-cuts and interval arithmetic.

2.2.1

Extension Principle

Let A and B be two fuzzy numbers. If A
function for C is defined as

+B

=

C, then the membership

C(z) = sup{min(A(x),B(y))lx + y = z} .
x,Y

(2.4)


10

CHAPTER 2. FUZZY SETS

If we set C = A - B, then

C(z) = sup{min(A(x),B(y))lx - y = z} .

(2.5)

X,Y

Similarly, C

= A· B, then
C(z) = sup{min(A(x),B(y))lx· y = z}

,

(2.6)

C(z) = sup{min(A(x),B(y»lx/y = z} .

(2.7)

X,Y

and if C = A/B,
X,Y

In all cases C is also a fuzzy number [6]. We assume that zero does not belong
to the support of B in C = A/B. If A and B are triangular (trapezoidal)
fuzzy numbers then so are A + B and A - B, but A . B and A/ B will be
triangular (trapezoidal) shaped fuzzy numbers.
We should mention something about the operator "sup" in equations (2.4)
- (2.7). If 0 is a set of real numbers bounded above (there is a M so that
x :::; M, for all x in 0), then sup(O) = the least upper bound for O. If 0
has a maximum member, then sup(O) = max(O). For example, if 0 = [0,1),
sup(O) = 1 but if 0 = [0,1], then sup(O) = max(O) = 1. The dual operator
to "sup" is "inf'. If 0 is bounded below (there is a M so that M :::; x for all
x EO), then infCO) = the greatest lower bound. For example, for 0 = (0,1]

inf(O) = 0 but if 0 = [0,1], then inf(O) = minCO) = O.
Obviously, given A and B, equations (2.4) - (2.7) appear quite complicated to compute A + B, A - B, etc. So, we now present an equivalent
procedure based on a-cuts and interval arithmetic. First, we present the
basics of interval arithmetic.

2.2.2

Interval Arithmetic

We only give a brief introduction to interval arithmetic. For more information the reader is referred to ([7],[8]). Let [al,b l ] and [a2,b 2] be two closed,
bounded, intervals of real numbers. If * denotes addition, subtraction, multiplication, or division, then [al, bd * [a2, b2] = [a,,8] where
(2.8)
If * is division, we must assume that zero does not belong to [a2, b2]. We may
simplify equation (2.8) as follows:

[al, bl] + [a2, b2]
[al, bl] - [a2, b2]
[al, bl] / [a2, b2]

[al + a2, bl + b2] ,
[al - b2,bl - a2] ,
[al,b 1 ]·

[b~' a~]

,

(2.9)
(2.10)
(2.11)

2.2. FUZZY ARITHMETIC

11

and
(2.12)
where

a

min{ al a2, al b2, b1 a2, b1 b2} ,

f3

max{ala2, a1b2, b1a2, b1b2} .

°

(2.13)
(2.14)

°

Multiplication and division may be further simplified if we know that
al > and b2 < 0, or b1 > 0 and b2 < 0, etc. For example, if al ~ and
a2 ~ 0, then
(2.15)
and if b1

< 0 but a2

~

0, we see that
(2.16)

Also, assuming b1

< 0 and b2 < 0 we get
(2.17)

but al

~

0, b2 < 0 produces
(2.18)

2.2.3

Fuzzy Arithmetic

Again we have two fuzzy numbers A and B. We know a-cuts are closed,
bounded, intervals so let A[a] = [al(a),a2(a)], B[a] = [b1(a),b 2(a)]. Then
if C = A + B we have
(2.19)
C[a] = A[a] + B[a] .
We add the intervals using equation (2.9). Setting C =

A - B we get

= A[a] - B[a] ,

(2.20)

C[a] = A[a] . B[a] ,

(2.21)

C[a] = A[a]/ B[a] ,

(2.22)

C[a]
for all a in [0,1]. Also
for C

= A· Band

when C = A/ B. This method is equivalent to the extension principle method
of fuzzy arithmetic [6]. Obviously, this procedure, of a-cuts plus interval
arithmetic, is more user (and computer) friendly.


12

CHAPTER 2. FUZZY SETS

0.8
0.6
0.4

0.2

/

o -18

-16

-14

-12 x -10

-8

-6

-4

Figure 2.4: The Fuzzy Number C = A· B

Example 2.2.3.1
Let A = (-3/ - 2/ -1) and B = (4/5/6). We determine A· B using a-cuts
and interval arithmetic. We compute A[a) = [-3 + a, -1- a) and B[a) =
[4+a,6-a). So, if C = A·B we obtain C[a) = [(a-3)(6-a), (-1-a)(4+a)),
o :::; a :::; 1. The graph of C is shown in Figure 2.4.

2.3

Fuzzy Functions

In this book a fuzzy function is a mapping from fuzzy numbers into fuzzy
numbers. We write H(X) = Z for a fuzzy function with one independent
variable X. Usually X will be a triangular (trapezoidal) fuzzy number and
then we usually obtain Z as a triangular (trapezoidal) shaped fuzzy number.
For two independent variables we have H(X,Y) = z.
Where do these fuzzy functions come from? They are usually extensions
of real-valued functions. Let h: [a,b) -t R. This notation means z = hex)
for x in [a, b) and z a real number. One extends h: [a, b) -t R to H(X) = Z
in two ways: (1) the extension principle; or (2) using a-cuts and interval
arithmetic.


13

2.3. FUZZY FUNCTIONS

2.3.1

Extension Principle

Any h: [a, b] -t R may be extended to H(X) =

Z(z) = sup { X(x)

'"

I hex) =

Z as follows

z, a ~ x ~

b} .

(2.23)

Equation (2.23) defines the membership function of Z for any triangular
(trapezoidal) fuzzy number X in [a, b].
If h is continuous, then we have a way to find a-cuts of Z. Let Z[a] =
[Zl (a), z2(a)]. Then [3]
mini hex) I x E X[a] } ,
maxi hex) I x E X[a] } ,

(2.24)
(2.25)

for 0 ~ a ~ 1.
If we have two independent variables, then let z = hex, y) for x in [aI, bl ],
y in [a2' b2]. We extend h to H(X, Y) = Z as

Z(z) = sup {min (X(x), Y(y))

"',v

I hex, y)

= z } ,

(2.26)

for X (Y) a triangular or trapezoidal fuzzy number in [aI, bl ] ([a2' b2]). For
a~cuts of Z, assuming h is continuous, we have

I x E X[a],
maxi h(x,y) I x E X[a],
mini h(x,y)

o ~ a ~ 1. We use equations (2.24)
this book.
2.3.2

~

y E Y[a] } ,
y E Y[a] } ,

(2.25) and (2.27)

~

(2.27)
(2.28)

(2.28) throughout

Alpha-Cuts and Interval Arithmetic

All the functions we usually use in engineering and science have a computer
algorithm which, using a finite number of additions, subtractions, multiplications and divisions, can evaluate the function to required accuracy ([2]).
Such functions can be extended, using a-cuts and interval arithmetic, to
fuzzy functions. Let h : [a, b] -t R be such a function. Then its extension H(X) = Z, X in [a, b] is done, via interval arithmetic, in computing
h(X[a]) = Z[a], a in [0,1]. We input the interval X[a], perform the arithmetic operations needed to evaluate h on this interval, and obtain the interval
Z[a]. Then put these a-cuts together to obtain the value Z. The extension
to more independent variables is straightforward.
For example, consider the fuzzy function

Z

= H(X) =

A X + 13
CX+D'

(2.29)


CHAPTER 2. FUZZY SETS

14

for triangular fuzzy numbers A, B, C, D and triangular fuzzy number X in
[0,10]. We assume that C ~ 0, D> 0 so that C X + D > O. This would be
the extension of
(2.30)
We would substitute the intervals A[a] for Xl, B[a] for X2, O[a] for X3, D[a]
for X4 and X[a] for X, do interval arithmetic, to obtain interval Z[a] for Z.

Alternatively, the fuzzy function

Z=

H(X) =

2X + 10

(2.31)

3X+4 '

would be the extension of
h(x)

2.3.3

=

2x+ 10 .
3x+4

(2.32)

Differences

Let h : [a, b] -t R. Just for this subsection let us write Z* = H(X) for the
extension principle method of extending h to H for X in [a, b]. We denote
Z = H(X) for the a-cut and interval arithmetic extension of h.
We know that Z can be different from Z*. But for basic fuzzy arithmetic

in Section 2.2 the two methods give the same results. In the example below
we show that for h(x) = x(1 - x), X in [0,1], we can get Z* i= Z for some
X in [0,1]. What is known ([3],[7]) is that for usual functions in science and
engineering Z* ~ Z. Otherwise, there is no known necessary and sufficient
conditions on h so that Z* = Z for all X in [a, b].
There is nothing wrong in using a-cuts and interval arithmetic to evaluate
fuzzy functions. Surely, it is user, and computer, friendly. However, we should
be aware that whenever we use a-cuts plus interval arithmetic to compute
Z = H(X) we may be getting something larger than that obtained from
the extension principle. The same results hold for functions of two or more
independent variables.

Example 2.3.3.1
The example is the simple fuzzy expression
Z = (1- X) X ,

for X a triangular fuzzy number in [0,1]. Let X[a] =
interval arithmetic we obtain
(l-x2(a))xl(a) ,
(1- Xl (a))x2(a) ,

(2.33)
[Xl (a),

x2(a)]. Using

(2.34)
(2.35)

15

2.4. POSSIBILITY THEORY
for Z[o:] = [Zl (0:), Z2(0:)], 0: in [0,1].
The extension principle extends the regular equation
x :::; 1, to fuzzy numbers as follows

z* (z) = sup {X(x)I(1 z

x)x

Z

= z, 0:::; x :::; I}

= (1 - x)x, 0 :::;

(2.36)

Let Z*[o:] = [z;(0:),z2(0:)]. Then
min{(l- x)xlx E X[o:]} ,
max{(1 - x)xlx E X[o:]} ,

(2.37)
(2.38)

for all 0:::; 0: :::; 1. Now let X = (0/0.25/0.5), then Xl (0:) = 0.250: and X2 (0:) =
0.50 - 0.250:. Equations (2.34) and (2.35) give Z[0.50] = [5/64,21/64] but
equations (2.37) and (2.38) produce Z* [0.50] = [7/64, 15/64]. Therefore,
Z* -I- Z. We do know that if each fuzzy number appears only once in the fuzzy

expression, the two methods produce the same results ([3],[7]). However,
if a fuzzy number is used more than once, as in equation (2.33), the two
procedures can give different results.

2.4

Possibility Theory

We will be using some of possibility theory only in Section 6.2 (Fuzzy PERT)
and Section 6.4 (Fuzzy Queuing Theory). For a general introduction to possibility theory we suggest ([4],[5]), and a brief review of the parts of possibility
theory needed in Chapter 6 is in this section.
Let X be a fuzzy variable whose values are restricted by a possibility
distribution A, where A is a fuzzy subset of R. Possibility distributions need
to be normalized which means that A(x) = 1 for some x in It. If E is any
subset of R, not a fuzzy set, we compute the possibility that X takes its
values in E as follows
Poss[X E E] = sup {A(x)

I x E E} .

(2.39)

This is analogous to probability theory where we use in fuzzy set theory sup
(or max) in place of summing (addition) in probability and min in place of
multiplication. If X is a random variable with probability density f(x), then
the probability that X takes its values in E is
Prob[X E E] =

f

f(x)dx .

(2.40)

E

In place of integration (summing) we use "sup" in fuzzy set theory.
Now consider Xl, ... , Xn fuzzy variables with associated possibility distributions AI' ... ' An, respectively. Let X = (Xl' ... ' Xn) and x =


16

CHAPTER 2. FUZZY SETS

(Xl, ... , Xn) ERn. Assuming that the fuzzy variables are non-interactive
(analogous to independent in probability theory) we form their joint
sibility distribution as

7r

pos-

(2.41)

all

X

ERn. For any crisp (non-fuzzy) E C

m,n we compute

Poss[X E E] = sup { 7r(x)
x

IX

E

E} .

(2.42)

Finally, given two EI and~, subsets ofRn , we need to find the possibility
that X takes its values in EI U E 2 • It is

Poss[X E EI U~] = max {Poss[X E EIJ,Poss[X E E 2 ]}

•

(2.43)

The possibility of a union is the maximum of the individual possibilities.