Introduction to Fuzzy Set Theory

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Introduction to Fuzzy Set Theory

• ??? ???

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Content

• Fuzzy Sets
• Set-Theoretic Operations
• MF Formulation
• Extension Principle
• Fuzzy Relations
• Approximate Reasoning

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Introduction to Fuzzy Set Theory

• Fuzzy Sets

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Types of Uncertainty

• Stochastic uncertainty
• E.g., rolling a dice
• Linguistic uncertainty
• E.g., low price, tall people, young age
• Informational uncertainty
• E.g., credit worthiness, honesty

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Crisp or Fuzzy Logic

• Crisp Logic
• A proposition can be true or false only.
• Bob is a student (true)
• Smoking is healthy (false)
• The degree of truth is 0 or 1.
• Fuzzy Logic
• The degree of truth is between 0 and 1.
• William is young (0.3 truth)
• Ariel is smart (0.9 truth)

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Crisp Sets

• Classical sets are called crisp sets
• either an element belongs to a set or not, i.e.,
• Member Function of crisp set

or
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Crisp Sets
P the set of all people.
Y
Y the set of all young people.
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Fuzzy Sets
Crisp sets
Example
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Fuzzy Sets
Lotfi A. Zadeh, The founder of fuzzy logic.
L. A. Zadeh, Fuzzy sets, Information and
Control, vol. 8, pp. 338-353, 1965.
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DefinitionFuzzy Sets and Membership Functions
U universe of discourse.
If U is a collection of objects denoted
generically by x, then a fuzzy set A in U is
defined as a set of ordered pairs
membership function
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Example (Discrete Universe)
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Example (Discrete Universe)
Alternative Representation
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Example (Continuous Universe)
U the set of positive real numbers
about 50 years old
Alternative Representation
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Alternative Notation
U discrete universe
U continuous universe
Note that ? and integral signs stand for the
union of membership grades / stands for a
marker and does not imply division.
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Membership Functions (MFs)

• A fuzzy set is completely characterized by a
  membership function.
• a subjective measure.
• not a probability measure.

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Fuzzy Partition

• Fuzzy partitions formed by the linguistic values
  young, middle aged, and old

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MF Terminology
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More Terminologies

• Normality
• core non-empty
• Fuzzy singleton
• support one single point
• Fuzzy numbers
• fuzzy set on real line R that satisfies convexity
  and normality
• Symmetricity
• Open left or right, closed

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Convexity of Fuzzy Sets

• A fuzzy set A is convex if for any ? in 0, 1.

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Introduction to Fuzzy Set Theory

• Set-Theoretic Operations

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Set-Theoretic Operations

• Subset
• Complement
• Union
• Intersection

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Set-Theoretic Operations
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Properties
Involution
De Morgans laws
Commutativity
Associativity
Distributivity
Idempotence
Absorption
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Properties

• The following properties are invalid for fuzzy
  sets
• The laws of contradiction
• The laws of exclude middle

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Other Definitions for Set Operations

• Union
• Intersection

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Other Definitions for Set Operations

• Union
• Intersection

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Generalized Union/Intersection

• Generalized Union
• Generalized Intersection

t-norm
t-conorm
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T-Norm
Or called triangular norm.

1. Symmetry
2. Associativity
3. Monotonicity
4. Border Condition

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T-Conorm
Or called s-norm.

1. Symmetry
2. Associativity
3. Monotonicity
4. Border Condition

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Examples T-Norm T-Conorm

• Minimum/Maximum
• Lukasiewicz
• Probabilistic

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Introduction to Fuzzy Set Theory

• MF Formulation

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MF Formulation

• Triangular MF
• Trapezoidal MF
• Gaussian MF
• Generalized bell MF

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MF Formulation
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Manipulating Parameter of theGeneralized Bell
Function
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Sigmoid MF
Extensions
Abs. difference of two sig. MF
Product of two sig. MF
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L-R MF
Example
c65 ?60 ?10
c25 ?10 ?40
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Introduction to Fuzzy Set Theory

• Extension Principle

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Functions Applied to Crisp Sets
B
A
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Functions Applied to Fuzzy Sets
y
B
?B(y)
A
?A(x)
x
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Functions Applied to Fuzzy Sets
y
B
?B(y)
A
?A(x)
x
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The Extension Principle
Assume a fuzzy set A and a function f.
How does the fuzzy set f(A) look like?
y
B
?B(y)
A
?A(x)
x
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The Extension Principle
Assume a fuzzy set A and a function f.
How does the fuzzy set f(A) look like?
y
B
?B(y)
A
?A(x)
x
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The Extension Principle
The extension of f operating on A1, , An gives a
fuzzy set F with membership function
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Introduction to Fuzzy Set Theory

• Fuzzy Relations

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Binary Relation (R)
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Binary Relation (R)
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The Real-Life Relation

• x is close to y
• x and y are numbers
• x depends on y
• x and y are events
• x and y look alike
• x and y are persons or objects
• If x is large, then y is small
• x is an observed reading and y is a corresponding
  action

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Fuzzy Relations
A fuzzy relation R is a 2D MF
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Example (Approximate Equal)
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Max-Min Composition
R fuzzy relation defined on X and Y.
S fuzzy relation defined on Y and Z.
R?S the composition of R and S.
A fuzzy relation defined on X an Z.
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Example
max
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Max-Product Composition
Max-min composition is not mathematically
tractable, therefore other compositions such as
max-product composition have been suggested.
R fuzzy relation defined on X and Y.
S fuzzy relation defined on Y and Z.
R?S the composition of R and S.
A fuzzy relation defined on X an Z.
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Projection
Dimension Reduction
R
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Projection
Dimension Reduction
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Cylindrical Extension
Dimension Expansion
A a fuzzy set in X.
C(A) A?X?Y cylindrical extension of A.
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Introduction to Fuzzy Set Theory

• Approximate Reasoning

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Linguistic Variables

• Linguistic variable is a variable whose values
  are words or sentences in a natural or artificial
  language.
• Each linguistic variable may be assigned one or
  more linguistic values, which are in turn
  connected to a numeric value through the
  mechanism of membership functions.

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Motivation

• Conventional techniques for system analysis are
  intrinsically unsuited for dealing with systems
  based on human judgment, perception emotion.

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Example
if temperature is cold and oil is cheapthen
heating is high
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Example
Linguistic Variable
?cold
if temperature is cold and oil is cheapthen
heating is high
Linguistic Value
Linguistic Value
Linguistic Variable
?cheap
?high
Linguistic Variable
Linguistic Value
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Definition Zadeh 1973
A linguistic variable is characterized by a
quintuple
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Example
A linguistic variable is characterized by a
quintuple
Example semantic rule
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Example
Linguistic Variable temperature Linguistics
Terms (Fuzzy Sets) cold, warm, hot
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Fuzzy If-Than Rules
?
A ? B
If x is A then y is B.
antecedent or premise
consequence or conclusion
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Examples
A ? B
?
If x is A then y is B.

• If pressure is high, then volume is small.
• If the road is slippery, then driving is
  dangerous.
• If a tomato is red, then it is ripe.
• If the speed is high, then apply the brake a
  little.

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Fuzzy Rules as Relations
A ? B
?
R
If x is A then y is B.
Depends on how to interpret A ? B
A fuzzy rule can be defined as a binary relation
with MF
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Interpretations of A ? B