 Indicator function

In mathematics, an indicator function or a characteristic function is a function defined on a set X that indicates membership of an element in a subset A of X, having the value 1 for all elements of A and the value 0 for all elements of X not in A.
Contents
Definition
The indicator function of a subset A of a set X is a function
defined as
The Iverson bracket allows the equivalent notation, , to be used instead of
The function is sometimes denoted or or even just . (The Greek letter χ appears because it is the initial letter of the Greek word characteristic.)
Remark on notation and terminology
 The notation may signify the identity function.
 The notation may signify the characteristic function in convex analysis.
A related concept in statistics is that of a dummy variable (this must not be confused with "dummy variables" as that term is usually used in mathematics, also called a bound variable).
The term "characteristic function" has an unrelated meaning in probability theory. For this reason, probabilists use the term indicator function for the function defined here almost exclusively, while mathematicians in other fields are more likely to use the term characteristic function to describe the function which indicates membership in a set.
Basic properties
The indicator or characteristic function of a subset A of some set X, maps elements of X to the range {0,1}.
This mapping is surjective only when A is a proper subset of X. If , then . By a similar argument, if then .
In the following, the dot represents multiplication, 1·1 = 1, 1·0 = 0 etc. "+" and "−" represent addition and subtraction. "" and "" is intersection and union, respectively.
If A and B are two subsets of X, then
and the indicator function of the complement of A i.e. A^{C} is:
More generally, suppose is a collection of subsets of X. For any ,
is clearly a product of 0s and 1s. This product has the value 1 at precisely those which belong to none of the sets A_{k} and is 0 otherwise. That is
Expanding the product on the left hand side,
where  F  is the cardinality of F. This is one form of the principle of inclusionexclusion.
As suggested by the previous example, the indicator function is a useful notational device in combinatorics. The notation is used in other places as well, for instance in probability theory: if X is a probability space with probability measure and A is a measurable set, then becomes a random variable whose expected value is equal to the probability of A:
This identity is used in a simple proof of Markov's inequality.
In many cases, such as order theory, the inverse of the indicator function may be defined. This is commonly called the generalized Möbius function, as a generalization of the inverse of the indicator function in elementary number theory, the Möbius function. (See paragraph below about the use of the inverse in classical recursion theory.)
Mean, variance and covariance
Given a probability space with , the indicator random variable is defined by if otherwise
 Mean:
Characteristic function in recursion theory, Gödel's and Kleene's representing function
Kurt Gödel described the representing function in his 1934 paper "On Undecidable Propositions of Formal Mathematical Systems". (The paper appears on pp. 4174 in Martin Davis ed. The Undecidable):
 "There shall correspond to each class or relation R a representing function φ(x_{1}, . . ., x_{n}) = 0 if R(x_{1}, . . ., x_{n}) and φ(x_{1}, . . ., x_{n})=1 if ~R(x_{1}, . . ., x_{n})." (p. 42; the "~" indicates logical inversion i.e. "NOT")
Stephen Kleene (1952) (p. 227) offers up the same definition in the context of the primitive recursive functions as a function φ of a predicate P, takes on values 0 if the predicate is true and 1 if the predicate is false.
For example, because the product of characteristic functions φ_{1}*φ_{2}* . . . *φ_{n} = 0 whenever any one of the functions equals 0, it plays the role of logical OR: IF φ_{1}=0 OR φ_{2}=0 OR . . . OR φ_{n}=0 THEN their product is 0. What appears to the modern reader as the representing function's logicalinversion, i.e. the representing function is 0 when the function R is "true" or satisfied", plays a useful role in Kleene's definition of the logical functions OR, AND, and IMPLY (p. 228), the bounded (p. 228) and unbounded (p. 279ff) mu operators (Kleene (1952)) and the CASE function (p. 229).
Characteristic function in fuzzy set theory
In classical mathematics, characteristic functions of sets only take values 1 (members) or 0 (nonmembers). In fuzzy set theory, characteristic functions are generalized to take value in the real unit interval [0, 1], or more generally, in some algebra or structure (usually required to be at least a poset or lattice). Such generalized characteristic functions are more usually called membership functions, and the corresponding "sets" are called fuzzy sets. Fuzzy sets model the gradual change in the membership degree seen in many realworld predicates like "tall", "warm", etc.
See also
 Dirac delta
 Extension (predicate logic)
 Free variables and bound variables
 Heaviside step function
 Iverson bracket
 Kronecker delta, a function that can be viewed as an indicator for the identity relation
 Multiset
 Membership function
 Simple function
References
 Folland, G.B. (1999). Real Analysis: Modern Techniques and Their Applications (Second ed.). John Wiley & Sons, Inc..
 Cormen, Thomas H.; Leiserson, Charles E.; Rivest, Ronald L.; Stein, Clifford (2001). "Section 5.2: Indicator random variables". Introduction to Algorithms (Second Edition ed.). MIT Press and McGrawHill. pp. 94–99. ISBN 0262032937.
 Davis, Martin, ed (1965). The Undecidable. New York: Raven Press Books, Ltd..
 Kleene, Stephen (1971) [1952]. Introduction to Metamathematics (Sixth Reprint with corrections). Netherlands: WoltersNoordhoff Publishing and North Holland Publishing Company.
 Boolos, George; Burgess, John P.; Jeffrey, Richard C. (2002). Computability and Logic. Cambridge UK: Cambridge University Press. ISBN 0521007585.
 Zadeh, Lotfi A. (June 1965). "Fuzzy sets" (PDF). Information and Control 8 (3): 338–353. doi:10.1016/S00199958(65)90241X. http://wwwbisc.cs.berkeley.edu/zadeh/papers/Fuzzy%20Sets1965.pdf.
 Goguen, Joseph (1967). "Lfuzzy sets". Journal of Mathematical Analysis and Applications 18 (1): 145–174. doi:10.1016/0022247X(67)901898.
Categories: Measure theory
 Integral calculus
 Real analysis
 Mathematical logic
 Basic concepts in set theory
 Probability theory
 Types of functions
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