Fuzzy set


In mathematics, fuzzy 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 more general kind of structure called an "L-relation", which he studied in an abstract algebraic context;
fuzzy relations are special cases of L-relations when L is the unit interval .
They are now used throughout fuzzy mathematics, having applications in areas such as linguistics, decision-making, and clustering.
In classical set theory, 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 valued in the real unit interval . Fuzzy sets generalize classical sets, since the indicator functions of classical sets are special cases of the membership functions of fuzzy sets, if the latter only takes values 0 or 1. 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.

Definition

A fuzzy set is a pair where is a set and a membership function.
The reference set is called universe of discourse, and for each the value is called the grade of membership of in.
The function is called the membership function of the fuzzy set.
For a finite set the fuzzy set is often denoted by
Let. Then is called
  • not included in the fuzzy set if ,
  • fully included if ,
  • partially included if
The set of all fuzzy sets on a universe is denoted with .

Crisp sets related to a fuzzy set

For any fuzzy set and the following crisp sets are defined:
  • is called its α-cut
  • is called its strong α-cut
  • is called its support
  • is called its core.
Note that some authors understand "kernel" in a different way; see below.

Other definitions

  • A fuzzy set is empty iff
  • Two fuzzy sets and are equal iff
  • A fuzzy set is included in a fuzzy set iff
  • For any fuzzy set, any element that satisfies
  • Given a fuzzy set, any, for which is not empty, is called a level of A.
  • The level set of A is the set of all levels representing distinct cuts. It is the image of :
  • For a fuzzy set, its height is given by
  • A fuzzy set is said to be normalized iff
  • For fuzzy sets of real numbers with bounded support, the width is defined as
  • A real fuzzy set is said to be convex, iff

    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, its complement is defined by the following membership function:
  • Let t be a t-norm, and s the corresponding s-norm. Given a pair of fuzzy sets, their intersection is defined by:
By the definition of the t-norm, we see that the union and intersection are commutative, monotonic, associative, and have both a null and an identity element. 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 of fuzzy sets recursively. It is noteworthy that the generally accepted standard operators for the union and intersection of fuzzy sets are the max and min operators:
  • and.
  • If the standard negator is replaced by another strong negator, the fuzzy set difference may be generalized by
  • The triple of fuzzy intersection, union and complement form a De Morgan Triplet. That is, De Morgan's laws extend to this triple.
  • For any fuzzy set and the ν-th power of is defined by the membership function:
The case of exponent two is special enough to be given a name.
  • For any fuzzy set the concentration is defined
Taking, we have and
  • Given fuzzy sets, the fuzzy set difference, also denoted, may be defined straightforwardly via the membership function:
  • Proposals for symmetric fuzzy set differences have been made by Dubois and Prade, either by taking the absolute value, giving
  • 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 are disjoint iff
which is equivalent to
and also equivalent to
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 according to the standard definition for crisp sets.
For disjoint fuzzy sets any intersection will give ∅, and any union will give the same result, which is denoted as
with its membership function given by
Note that only one of both summands is greater than zero.
For disjoint fuzzy sets the following holds true:
This can be generalized to finite families of fuzzy sets as follows:
Given a family of fuzzy sets with index set I. This family is disjoint iff
A family of fuzzy sets is disjoint, iff the family of underlying supports is disjoint in the standard sense for families of crisp sets.
Independent of the t/s-norm pair, intersection of a disjoint family of fuzzy sets will give ∅ again, while the union has no ambiguity:
with its membership function given by
Again only one of the summands is greater than zero.
For disjoint families of fuzzy sets the following holds true:

Scalar cardinality

For a fuzzy set with finite support , its cardinality is given by
In the case that U itself is a finite set, the relative cardinality is given by
This can be generalized for the divisor to be a non-empty fuzzy set: For fuzzy sets with G ≠ ∅, we can define the relative cardinality by:
which looks very similar to the expression for conditional probability.
Note:
  • here.
  • The result may depend on the specific intersection chosen.
  • For the result is unambiguous and resembles the prior definition.

    Distance and similarity

For any fuzzy set the membership function can be regarded as a family. The latter is a metric space with several metrics known. A metric can be derived from a norm via
For instance, if is finite, i.e., such a metric may be defined by:
For infinite, the maximum can be replaced by a supremum.
Because fuzzy sets are unambiguously defined by their membership function, this metric can be used to measure distances between fuzzy sets on the same universe:
which becomes in the above sample:
Again for infinite the maximum must be replaced by a supremum. Other distances may diverge, if infinite fuzzy sets are too different, e.g., and.
Similarity measures may then be derived from the distance, e.g. after a proposal by Koczy:
or after Williams and Steele:
where is a steepness parameter and.

''L''-fuzzy sets

Sometimes, more general variants of the notion of fuzzy set are used, with membership functions taking values in a algebra or structure of a given kind; usually it is required that be at least a poset or lattice. These are usually called L-fuzzy sets, to distinguish them from those valued over the unit interval. The usual membership functions with values in are then called -valued membership functions. These kinds of generalizations were first considered in 1967 by Joseph Goguen, who was a student of Zadeh. A classical corollary may be indicating truth and membership values by instead of.
An extension of fuzzy sets has been provided by Atanassov. An intuitionistic fuzzy set is characterized by two functions:
with functions with.
This resembles a situation like some person denoted by voting
  • for a proposal :,
  • against it:,
  • or abstain from voting:.
After all, we have a percentage of approvals, a percentage of denials, and a percentage of abstentions.
For this situation, special "intuitive fuzzy" negators, t- and s-norms can be defined. With and by combining both functions to this situation resembles a special kind of L-fuzzy sets.
Once more, this has been expanded by defining picture fuzzy sets as follows: A PFS A is characterized by three functions mapping U to :, "degree of positive membership", "degree of neutral membership", and "degree of negative membership" respectively and additional condition
This expands the voting sample above by an additional possibility of "refusal of voting".
With and special "picture fuzzy" negators, t- and s-norms this resembles just another type of L-fuzzy sets.

Pythagorean fuzzy sets

One extension of IFS is what is known as Pythagorean fuzzy sets. Such sets satisfy the constraint, which is reminiscent of the Pythagorean theorem. Pythagorean fuzzy sets can be applicable to real life applications in which the previous condition of is not valid. However, the less restrictive condition of may be suitable in more domains.

Fuzzy logic

As an extension of the case of multi-valued logic, valuations of propositional variables into a set of membership degrees can be thought of as membership functions mapping predicates into fuzzy sets. With these valuations, many-valued logic can be extended to allow for fuzzy premises from which graded conclusions may be drawn.
This extension is sometimes called "fuzzy logic in the narrow sense" as opposed to "fuzzy logic in the wider sense," which originated in the engineering fields of automated control and knowledge engineering, and which encompasses many topics involving fuzzy sets and "approximated reasoning."
Industrial applications of fuzzy sets in the context of "fuzzy logic in the wider sense" can be found at fuzzy logic.