Fuzzy Sets
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In mathematics, fuzzy sets (a.k.a. uncertain sets) are sets whose
elements Element or elements may refer to: Science * Chemical element, a pure substance of one type of atom * Heating element, a device that generates heat by electrical resistance * Orbital elements, parameters required to identify a specific orbit of ...
have degrees of membership. Fuzzy sets were introduced independently by
Lotfi A. Zadeh Lotfi Aliasker Zadeh (; az, Lütfi Rəhim oğlu Ələsgərzadə; fa, لطفی علی‌عسکرزاده; 4 February 1921 – 6 September 2017) was a mathematician, computer scientist, electrical engineer, artificial intelligence researcher, an ...
in 1965 as an extension of the classical notion of set. At the same time, defined a more general kind of structure called an ''L''-relation, which he studied in an
abstract algebra In mathematics, more specifically algebra, abstract algebra or modern algebra is the study of algebraic structures. Algebraic structures include groups, rings, fields, modules, vector spaces, lattices, and algebras over a field. The te ...
ic context. Fuzzy relations, which are now used throughout
fuzzy mathematics Fuzzy mathematics is the branch of mathematics including fuzzy set theory and fuzzy logic that deals with partial inclusion of elements in a set on a spectrum, as opposed to simple binary "yes" or "no" (0 or 1) inclusion. It started in 1965 ...
and have applications in areas such as
linguistics Linguistics is the scientific study of human language. It is called a scientific study because it entails a comprehensive, systematic, objective, and precise analysis of all aspects of language, particularly its nature and structure. Lingu ...
,
decision-making In psychology, decision-making (also spelled decision making and decisionmaking) is regarded as the cognitive process resulting in the selection of a belief or a course of action among several possible alternative options. It could be either r ...
, and clustering , are special cases of ''L''-relations when ''L'' is the
unit interval In mathematics, the unit interval is the closed interval , that is, the set of all real numbers that are greater than or equal to 0 and less than or equal to 1. It is often denoted ' (capital letter ). In addition to its role in real analys ...
, 1 In classical
set theory Set theory is the branch of mathematical logic that studies sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory, as a branch of mathematics, is mostly concer ...
, the membership of elements in a set is assessed in binary terms according to a bivalent condition—an element either belongs or does not belong to the set. By contrast, fuzzy set theory permits the gradual assessment of the membership of elements in a set; this is described with the aid of a
membership function In mathematics, an indicator function or a characteristic function of a subset of a set is a function that maps elements of the subset to one, and all other elements to zero. That is, if is a subset of some set , one has \mathbf_(x)=1 if x\ ...
valued in the
real Real may refer to: Currencies * Brazilian real (R$) * Central American Republic real * Mexican real * Portuguese real * Spanish real * Spanish colonial real Music Albums * ''Real'' (L'Arc-en-Ciel album) (2000) * ''Real'' (Bright album) (201 ...
unit interval , 1 Fuzzy sets generalize classical sets, since the
indicator function In mathematics, an indicator function or a characteristic function of a subset of a set is a function that maps elements of the subset to one, and all other elements to zero. That is, if is a subset of some set , one has \mathbf_(x)=1 if x ...
s (aka characteristic functions) of classical sets are special cases of the membership functions of fuzzy sets, if the latter only takes values 0 or 1.D. Dubois and H. Prade (1988) Fuzzy Sets and Systems. Academic Press, New York. In fuzzy set theory, classical bivalent sets are usually called ''crisp sets''. The fuzzy set theory can be used in a wide range of domains in which information is incomplete or imprecise, such as
bioinformatics Bioinformatics () is an interdisciplinary field that develops methods and software tools for understanding biological data, in particular when the data sets are large and complex. As an interdisciplinary field of science, bioinformatics combin ...
.


Definition

A fuzzy set is a pair (U, m) where U is a set (often required to be
non-empty In mathematics, the empty set is the unique set having no elements; its size or cardinality (count of elements in a set) is zero. Some axiomatic set theories ensure that the empty set exists by including an axiom of empty set, while in other ...
) and m\colon U \rightarrow ,1/math> a membership function. The reference set U (sometimes denoted by \Omega or X) is called universe of discourse, and for each x\in U, the value m(x) is called the grade of membership of x in (U,m). The function m = \mu_A is called the membership function of the fuzzy set A = (U, m). For a finite set U=\, the fuzzy set (U, m) is often denoted by \. Let x \in U. Then x is called * not included in the fuzzy set (U,m) if (no member), * fully included if (full member), * partially included if The (crisp) set of all fuzzy sets on a universe U is denoted with SF(U) (or sometimes just F(U)).


Crisp sets related to a fuzzy set

For any fuzzy set A = (U,m) and \alpha \in ,1/math> the following crisp sets are defined: * A^ = A_\alpha = \ is called its α-cut (aka α-level set) * A^ = A'_\alpha = \ is called its strong α-cut (aka strong α-level set) * S(A) = \operatorname(A) = A^ = \ is called its support * C(A) = \operatorname(A) = A^ = \ is called its core (or sometimes kernel \operatorname(A)). Note that some authors understand "kernel" in a different way; see below.


Other definitions

* A fuzzy set A = (U,m) is empty (A = \varnothing)
iff In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false. The connective is bicondi ...
(if and only if) :: \forall x \in U: \mu_A(x) = m(x) = 0 * Two fuzzy sets A and B are equal (A = B) iff ::\forall x \in U: \mu_A(x) = \mu_B(x) * A fuzzy set A is included in a fuzzy set B (A \subseteq B) iff ::\forall x \in U: \mu_A(x) \le \mu_B(x) * For any fuzzy set A, any element x \in U that satisfies ::\mu_A(x) = 0.5 :is called a crossover point. * Given a fuzzy set A, any \alpha \in ,1/math>, for which A^ = \ is not empty, is called a level of A. * The level set of A is the set of all levels \alpha\in ,1/math> representing distinct cuts. It is the
image An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensio ...
of \mu_A: ::\Lambda_A = \ = \ = \mu_A(U) * For a fuzzy set A, its height is given by ::\operatorname(A) = \sup \ = \sup(\mu_A(U)) :where \sup denotes the
supremum In mathematics, the infimum (abbreviated inf; plural infima) of a subset S of a partially ordered set P is a greatest element in P that is less than or equal to each element of S, if such an element exists. Consequently, the term ''greatest l ...
, which exists because \mu_A(U) is non-empty and bounded above by 1. If ''U'' is finite, we can simply replace the supremum by the maximum. * A fuzzy set A is said to be normalized iff ::\operatorname(A) = 1 :In the finite case, where the supremum is a maximum, this means that at least one element of the fuzzy set has full membership. A non-empty fuzzy set A may be normalized with result \tilde by dividing the membership function of the fuzzy set by its height: ::\forall x \in U: \mu_(x) = \mu_A(x)/\operatorname(A) :Besides similarities this differs from the usual
normalization Normalization or normalisation refers to a process that makes something more normal or regular. Most commonly it refers to: * Normalization (sociology) or social normalization, the process through which ideas and behaviors that may fall outside of ...
in that the normalizing constant is not a sum. * For fuzzy sets A of real numbers (''U'' ⊆ ℝ) with bounded support, the width is defined as ::\operatorname(A) = \sup(\operatorname(A)) - \inf(\operatorname(A)) :In the case when \operatorname(A) is a finite set, or more generally a
closed set In geometry, topology, and related branches of mathematics, a closed set is a set whose complement is an open set. In a topological space, a closed set can be defined as a set which contains all its limit points. In a complete metric spac ...
, the width is just ::\operatorname(A) = \max(\operatorname(A)) - \min(\operatorname(A)) :In the ''n''-dimensional case (''U'' ⊆ ℝ''n'') the above can be replaced by the ''n''-dimensional volume of \operatorname(A). :In general, this can be defined given any measure on ''U'', for instance by integration (e.g.
Lebesgue integration In mathematics, the integral of a non-negative function of a single variable can be regarded, in the simplest case, as the area between the graph of that function and the -axis. The Lebesgue integral, named after French mathematician Henri Le ...
) of \operatorname(A). * A real fuzzy set A (''U'' ⊆ ℝ) is said to be convex (in the fuzzy sense, not to be confused with a crisp
convex set In geometry, a subset of a Euclidean space, or more generally an affine space over the reals, is convex if, given any two points in the subset, the subset contains the whole line segment that joins them. Equivalently, a convex set or a convex ...
), iff ::\forall x,y \in U, \forall\lambda\in ,1 \mu_A(\lambda + (1-\lambda)y) \ge \min(\mu_A(x),\mu_A(y)). : Without loss of generality, we may take ''x'' ≤ ''y'', which gives the equivalent formulation ::\forall z \in ,y \mu_A(z) \ge \min(\mu_A(x),\mu_A(y)). : This definition can be extended to one for a general
topological space In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called po ...
''U'': we say the fuzzy set A is convex when, for any subset ''Z'' of ''U'', the condition ::\forall z \in Z: \mu_A(z) \ge \inf(\mu_A(\partial Z)) : holds, where \partial Z denotes the
boundary Boundary or Boundaries may refer to: * Border, in political geography Entertainment * ''Boundaries'' (2016 film), a 2016 Canadian film * ''Boundaries'' (2018 film), a 2018 American-Canadian road trip film *Boundary (cricket), the edge of the pla ...
of ''Z'' and f(X) = \ denotes the
image An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensio ...
of a set ''X'' (here \partial Z) under a function ''f'' (here \mu_A).


Fuzzy set operations

Although the complement of a fuzzy set has a single most common definition, the other main operations, union and intersection, do have some ambiguity. * For a given fuzzy set A, its complement \neg (sometimes denoted as A^c or cA) is defined by the following membership function: ::\forall x \in U: \mu_(x) = 1 - \mu_A(x). * Let t be a
t-norm In mathematics, a t-norm (also T-norm or, unabbreviated, triangular norm) is a kind of binary operation used in the framework of probabilistic metric spaces and in multi-valued logic, specifically in fuzzy logic. A t-norm generalizes intersection ...
, and s the corresponding s-norm (aka t-conorm). Given a pair of fuzzy sets A, B, their intersection A\cap is defined by: ::\forall x \in U: \mu_(x) = t(\mu_A(x),\mu_B(x)), :and their union A\cup is defined by: ::\forall x \in U: \mu_(x) = s(\mu_A(x),\mu_B(x)). By the definition of the t-norm, we see that the union and intersection are
commutative In mathematics, a binary operation is commutative if changing the order of the operands does not change the result. It is a fundamental property of many binary operations, and many mathematical proofs depend on it. Most familiar as the name o ...
,
monotonic In mathematics, a monotonic function (or monotone function) is a function between ordered sets that preserves or reverses the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of ord ...
,
associative In mathematics, the associative property is a property of some binary operations, which means that rearranging the parentheses in an expression will not change the result. In propositional logic, associativity is a valid rule of replacement ...
, and have both a
null Null may refer to: Science, technology, and mathematics Computing *Null (SQL) (or NULL), a special marker and keyword in SQL indicating that something has no value *Null character, the zero-valued ASCII character, also designated by , often used ...
and an
identity element In mathematics, an identity element, or neutral element, of a binary operation operating on a set is an element of the set that leaves unchanged every element of the set when the operation is applied. This concept is used in algebraic structures s ...
. For the intersection, these are ∅ and ''U'', respectively, while for the union, these are reversed. However, the union of a fuzzy set and its complement may not result in the full universe ''U'', and the intersection of them may not give the empty set ∅. Since the intersection and union are associative, it is natural to define the intersection and union of a finite
family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
of fuzzy sets recursively. * If the standard negator n(\alpha) = 1 - \alpha, \alpha \in
, 1 The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline o ...
/math> is replaced by another strong negator, the fuzzy set difference may be generalized by ::\forall x \in U: \mu_(x) = n(\mu_A(x)). * The triple of fuzzy intersection, union and complement form a De Morgan Triplet. That is,
De Morgan's laws In propositional logic and Boolean algebra, De Morgan's laws, also known as De Morgan's theorem, are a pair of transformation rules that are both valid rules of inference. They are named after Augustus De Morgan, a 19th-century British mathem ...
extend to this triple. :Examples for fuzzy intersection/union pairs with standard negator can be derived from samples provided in the article about
t-norm In mathematics, a t-norm (also T-norm or, unabbreviated, triangular norm) is a kind of binary operation used in the framework of probabilistic metric spaces and in multi-valued logic, specifically in fuzzy logic. A t-norm generalizes intersection ...
s. :The fuzzy intersection is not
idempotent Idempotence (, ) is the property of certain operations in mathematics and computer science whereby they can be applied multiple times without changing the result beyond the initial application. The concept of idempotence arises in a number of pl ...
in general, because the standard t-norm is the only one which has this property. Indeed, if the arithmetic multiplication is used as the t-norm, the resulting fuzzy intersection operation is not idempotent. That is, iteratively taking the intersection of a fuzzy set with itself is not trivial. It instead defines the ''m''-th power of a fuzzy set, which can be canonically generalized for non-
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
exponents in the following way: * For any fuzzy set A and \nu \in \R^+ the ν-th power of A is defined by the membership function: ::\forall x \in U: \mu_(x) = \mu_(x)^. The case of exponent two is special enough to be given a name. * For any fuzzy set A the concentration CON(A) = A^2 is defined ::\forall x \in U: \mu_(x) = \mu_(x) = \mu_(x)^2. Taking 0^0 = 1, we have A^0 = U and A^1 = A. * Given fuzzy sets A, B, the fuzzy set difference A \setminus B, also denoted A - B, may be defined straightforwardly via the membership function: ::\forall x \in U: \mu_(x) = t(\mu_A(x),n(\mu_B(x))), :which means A \setminus B = A \cap \neg, e. g.: ::\forall x \in U: \mu_(x) = \min(\mu_A(x),1 - \mu_B(x)).N.R. Vemuri, A.S. Hareesh, M.S. Srinath
Set Difference and Symmetric Difference of Fuzzy Sets
in: Fuzzy Sets Theory and Applications 2014, Liptovský Ján, Slovak Republic
:Another proposal for a set difference could be: ::\forall x \in U: \mu_(x) = \mu_A(x) - t(\mu_A(x),\mu_B(x)). * Proposals for symmetric fuzzy set differences have been made by Dubois and Prade (1980), either by taking the absolute value, giving ::\forall x \in U: \mu_(x) = , \mu_A(x) - \mu_B(x), , :or by using a combination of just , , and standard negation, giving ::\forall x \in U: \mu_(x) = \max(\min(\mu_A(x), 1 - \mu_B(x)), \min(\mu_B(x), 1 - \mu_A(x))). :Axioms for definition of generalized symmetric differences analogous to those for t-norms, t-conorms, and negators have been proposed by Vemur et al. (2014) with predecessors by Alsina et al. (2005) and Bedregal et al. (2009). * In contrast to crisp sets, averaging operations can also be defined for fuzzy sets.


Disjoint fuzzy sets

In contrast to the general ambiguity of intersection and union operations, there is clearness for disjoint fuzzy sets: Two fuzzy sets A, B are disjoint iff :\forall x \in U: \mu_A(x) = 0 \lor \mu_B(x) = 0 which is equivalent to : \nexists x \in U: \mu_A(x) > 0 \land \mu_B(x) > 0 and also equivalent to :\forall x \in U: \min(\mu_A(x),\mu_B(x)) = 0 We keep in mind that / is a t/s-norm pair, and any other will work here as well. Fuzzy sets are disjoint if and only if their supports are
disjoint Disjoint may refer to: *Disjoint sets, sets with no common elements *Mutual exclusivity, the impossibility of a pair of propositions both being true See also *Disjoint union *Disjoint-set data structure {{disambig in the 21st century.Michael Winter "Representation theory of Goguen categories"
Fuzzy Sets and Systems ''Fuzzy Sets and Systems'' is a peer-reviewed international scientific journal published by Elsevier on behalf of the International Fuzzy Systems Association (IFSA) and was founded in 1978. The editors-in-chief (as of 2010) are Bernard De Baets of ...
Volume 138, Issue 1, 16 August 2003, Pages 85–126
In these categories, rather than using two valued set membership, more general intervals are used, and may be lattices as in ''L''-fuzzy sets.


Fuzzy relation equation

The
fuzzy relation equation In mathematics, fuzzy sets (a.k.a. uncertain sets) are sets whose elements have degrees of membership. Fuzzy sets were introduced independently by Lotfi A. Zadeh in 1965 as an extension of the classical notion of set. At the same time, defined a ...
is an equation of the form , where ''A'' and ''B'' are fuzzy sets, ''R'' is a fuzzy relation, and stands for the
composition Composition or Compositions may refer to: Arts and literature * Composition (dance), practice and teaching of choreography *Composition (language), in literature and rhetoric, producing a work in spoken tradition and written discourse, to include ...
of ''A'' with ''R'' .


Entropy

A measure ''d'' of fuzziness for fuzzy sets of universe U should fulfill the following conditions for all x \in U: #d(A) = 0 if A is a crisp set: \mu_A(x) \in \ #d(A) has a unique maximum iff \forall x \in U: \mu_A(x) = 0.5 #\mu_A \leq \mu_B \iff :::\mu_A \leq \mu_B \leq 0.5 :::\mu_A \geq \mu_B \geq 0.5 ::which means that ''B'' is "crisper" than ''A''. #d(\neg) = d(A) In this case d(A) is called the entropy of the fuzzy set ''A''. For finite U = \ the entropy of a fuzzy set A is given by :d(A) = H(A) + H(\neg), ::H(A) = -k \sum_^n \mu_A(x_i) \ln \mu_A(x_i) or just :d(A) = -k \sum_^n S(\mu_A(x_i)) where S(x) = H_e(x) is Shannon's function (natural entropy function) :S(\alpha) = -\alpha \ln \alpha - (1-\alpha) \ln (1-\alpha),\ \alpha \in ,1/math> and k is a constant depending on the measure unit and the logarithm base used (here we have used the natural base e). The physical interpretation of ''k'' is the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas consta ...
''k''''B''. Let A be a fuzzy set with a continuous membership function (fuzzy variable). Then :H(A) = -k \int_^\infty \operatorname \lbrace A \geq t \rbrace \ln \operatorname \lbrace A \geq t \rbrace \,dt and its entropy is :d(A) = -k \int_^\infty S(\operatorname \lbrace A \geq t \rbrace )\,dt.


Extensions

There are many mathematical constructions similar to or more general than fuzzy sets. Since fuzzy sets were introduced in 1965, many new mathematical constructions and theories treating imprecision, inexactness, ambiguity, and uncertainty have been developed. Some of these constructions and theories are extensions of fuzzy set theory, while others try to mathematically model imprecision and uncertainty in a different way (; ; Deschrijver and Kerre, 2003).


See also

*
Alternative set theory In a general sense, an alternative set theory is any of the alternative mathematical approaches to the concept of set and any alternative to the de facto standard set theory described in axiomatic set theory by the axioms of Zermelo–Fraenkel s ...
*
Defuzzification Defuzzification is the process of producing a quantifiable result in crisp logic, given fuzzy sets and corresponding membership degrees. It is the process that maps a fuzzy set to a crisp set. It is typically needed in fuzzy control systems. Th ...
*
Fuzzy concept A fuzzy concept is a kind of concept of which the boundaries of application can vary considerably according to context or conditions, instead of being fixed once and for all. This means the concept is vague in some way, lacking a fixed, precise me ...
*
Fuzzy mathematics Fuzzy mathematics is the branch of mathematics including fuzzy set theory and fuzzy logic that deals with partial inclusion of elements in a set on a spectrum, as opposed to simple binary "yes" or "no" (0 or 1) inclusion. It started in 1965 ...
*
Fuzzy set operations Fuzzy set operations are a generalization of crisp set operations for fuzzy sets. There is in fact more than one possible generalization. The most widely used operations are called ''standard fuzzy set operations''; they comprise: fuzzy comple ...
*
Fuzzy subalgebra Fuzzy subalgebras theory is a chapter of fuzzy set theory. It is obtained from an interpretation in a multi-valued logic of axioms usually expressing the notion of subalgebra of a given algebraic structure. Definition Consider a first order la ...
*
Interval finite element In numerical analysis, the interval finite element method (interval FEM) is a finite element method that uses interval parameters. Interval FEM can be applied in situations where it is not possible to get reliable probabilistic characteristics ...
*
Linear partial information Linear partial information (LPI) is a method of making decisions based on insufficient or fuzzy information. LPI was introduced in 1970 by Polish–Swiss mathematician Edward Kofler (1911–2007) to simplify decision processes. Compared to other ...
*
Multiset In mathematics, a multiset (or bag, or mset) is a modification of the concept of a set that, unlike a set, allows for multiple instances for each of its elements. The number of instances given for each element is called the multiplicity of that ...
*
Neuro-fuzzy In the field of artificial intelligence, neuro-fuzzy refers to combinations of artificial neural networks and fuzzy logic. Overview Neuro-fuzzy hybridization results in a hybrid intelligent system that these two techniques by combining the human ...
*
Rough fuzzy hybridization {{No footnotes, date=April 2009 Rough fuzzy hybridization is a method of hybrid intelligent system or soft computing, where Fuzzy set theory is used for linguistic representation of patterns, leading to a ''fuzzy granulation'' of the feature spac ...
*
Rough set In computer science, a rough set, first described by Polish computer scientist Zdzisław I. Pawlak, is a formal approximation of a crisp set (i.e., conventional set) in terms of a pair of sets which give the ''lower'' and the ''upper'' approxima ...
*
Sørensen similarity index Sørensen () is a Danish- Norwegian patronymic surname meaning "son of Søren" ( given name equivalent of Severin). , it is the eighth most common surname in Denmark. Immigrants to English-speaking countries often changed the spelling to ''Sorens ...
* Type-2 fuzzy sets and systems *
Uncertainty Uncertainty refers to Epistemology, epistemic situations involving imperfect or unknown information. It applies to predictions of future events, to physical measurements that are already made, or to the unknown. Uncertainty arises in partially ...


References


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