# .

# Set function

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

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

\( d(A) = \lim_{n\to\infty} \frac{|A \cap \{1,\dots,n\}|}{n}, \)

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.

References

A.N. Kolmogorov and S.V. Fomin (1975), Introductory Real Analysis, Dover. ISBN 0-486-61226-0

Further reading

Sobolev, V.I. (2001), "Set function", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4

*Regular set function at Encyclopedia of Mathematics

Undergraduate Texts in Mathematics

Graduate Studies in Mathematics

Retrieved from "http://en.wikipedia.org/"

All text is available under the terms of the GNU Free Documentation License