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In mathematics, a measure is a function that assigns "sizes", "volumes", or "probabilities" to subsets of a given set. The concept is important in mathematical analysis and probability theory. Formally, a measure μ is a function which assigns to every element S of a given sigma algebra X a value μ(S), a nonnegative real number or ∞. The following properties have to be satisfied:
If μ is a measure on the sigma algebra X, then the members of X are called the μmeasurable sets, or the measurable sets for short. A set Ω together with a sigma algebra X on Ω and a measure μ on X is called a measure space.
The following properties can be derived from the definition above:
A measurable set S is called a nullset if μ(S) = 0. The measure μ is called complete if every subset of a nullset is measurable (and then automatically itself a nullset).
Some important measures are listed here.
For certain purposes, it is useful to have a "measure" whose values are not restricted to the nonnegative reals or infinity. For instance, a countably additive set function with values in the (signed) real numbers is called a signed measure, while such a function with values in the complex numbers is called a complex measure. A measure that takes values in a Banach space is called a spectral measure; these are used mainly in functional analysis for the spectral theorem.
Another generalization is the finitely additive measure. This is the same as a measure except that instead of requiring countable additivity we require only finite additivity. Historically, this definition was used first, but proved to be not so useful.
The remarkable result in integral geometry[?] known as Hadwiger's theorem states that the space of translationinvariant, finitely additive, notnecessarilynonnegative set functions defined on finite unions of compact convex sets in R^{n} consists (up to scalar multiples) of one "measure" that is "homogeneous of degree k" for each k=0,1,2,...,n, and linear combinations of those "measures". "Homogeneous of degree k" means that rescaling any set by any factor c>0 multiplies the set's "measure" by c^{k}. The one that is homogeneous of degree n is the ordinary ndimensional volume. The one that is homogeneous of degree n1 is the "surface volume". The one that is homogeneous of degree 1 is a mysterious function called the "mean width", a misnomer. The one that is homogenous of degree 0 is the Euler characteristic.
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