Indicator function

This article is about the 0-1 indicator function. For the 0-infinity indicator function, see characteristic function (convex analysis).

In other contexts, such as computer science, this would more often be described as a boolean predicate function (to test set inclusion).

The Dirichlet function is an example of an indicator function and is the indicator of the rationals.

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 classic probability theory. For this reason, traditional 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[a] to describe the function that indicates membership in a set.

In fuzzy logic and modern many-valued logic, predicates are the characteristic functions of a probability distribution. That is, the strict true/false valuation of the predicate is replaced by a quantity interpreted as the degree of truth.

This mapping is surjective only when A is a non-empty proper subset of X. If A ≡ X, then 1A = 1. By a similar argument, if A ≡ ∅ then 1A = 0.

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.)

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":[1]

Kleene (1952)[2] 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 φ1*φ2* ... *φn = 0 whenever any one of the functions equals 0, it plays the role of logical OR: IF φ1 = 0 OR φ2 = 0 OR ... OR φn = 0 THEN their product is 0. What appears to the modern reader as the representing function's logical inversion, 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. 279 ff) mu operators (Kleene (1952)) and the CASE function (p. 229).

In classical mathematics, characteristic functions of sets only take values 1 (members) or 0 (non-members). 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 real-world predicates like "tall", "warm", etc.

A particular indicator function is the Heaviside step function. The Heaviside step function H(x) is the indicator function of the one-dimensional positive half-line, i.e. the domain [0, ∞). The distributional derivative of the Heaviside step function is equal to the Dirac delta function, i.e.

The derivative of the Heaviside step function can be seen as the inward normal derivative at the boundary of the domain given by the positive half-line. In higher dimensions, the derivative naturally generalises to the inward normal derivative, while the Heaviside step function naturally generalises to the indicator function of some domain D. The surface of D will be denoted by S. Proceeding, it can be derived that the inward normal derivative of the indicator gives rise to a 'surface delta function', which can be indicated by δS(x):

where n is the outward normal of the surface S. This 'surface delta function' has the following property:[3]

By setting the function f equal to one, it follows that the inward normal derivative of the indicator integrates to the numerical value of the surface area S.