In mathematics, a set function is a function whose input is a set. The output is usually a number. Often the input is a set of real numbers, a set of points in Euclidean space, or a set of points in some measure space.
Examples of set functions include:
The function that assigns to each set its cardinality, i.e. the number of members of the set, is a set function.
The function
assigning densities to sufficiently well-behaved subsets
A ⊆ {1, 2, 3, ...}, is a set function.
The Lebesgue measure is a set function that assigns a non-negative real number to each set of real numbers. (Kolmogorov and Fomin 1975)
A probability measure assigns a probability to each set in a σ-algebra. Specifically, the probability of the empty set is zero and the probability of the sample space is 1, with other sets given probabilities between 0 and 1.
A possibility measure assigns a number between zero and one to each set in the powerset of some given set. See possibility theory.
A Random set is a set-valued random variable. See Random compact set.
