Absolute continuity of an integral as a property of the (Lebesgue) integral. Let a function be -integrable on a set . The integral of over -measurable subsets is an absolutely continuous set function (see Subsection 3 below) with respect to the measure if for any there exists a such that the integral for any set with . In the general case the integral with respect to a finitely-additive set function with scalar or vectorial or is an absolutely continuous function.

Absolute continuity of a measure as a concept in the theory of measures. A measure is absolutely continuous with respect to a measure if is an absolutely continuous set function with respect to . Thus, let be a finite measure, given together with on some fixed -algebra ; will then be absolutely continuous with respect to if it follows from , , that . A generalized finite measure (cf. Charge) is absolutely continuous with respect to a generalized measure if , provided that , where is the total variation of .

Absolute continuity of a function is a stronger notion than continuity. A function defined on a segment is said to be absolutely continuous if for any there exists a such that for any finite system of pairwise non-intersecting intervals , , for which the inequality holds. Any absolutely continuous function on a segment is continuous on this segment. The opposite implication is not true: e.g. the function if and is continuous on the segment , but is not absolutely continuous on it. If, in the definition of an absolutely continuous function, the requirement that the pairwise intersections of intervals are empty be discarded, then the function will satisfy an even stronger condition: A Lipschitz condition with some constant.

If two functions and are absolutely continuous, then their sum, difference and product are also absolutely continuous and, if does not vanish, so is their quotient . The superposition of two absolutely continuous functions need not be absolutely continuous. However, if the function is absolutely continuous on a segment and if , , while the function satisfies a Lipschitz condition on the segment , then the composite function is absolutely continuous on . If a function , which is absolutely continuous on , is monotone increasing, while is absolutely continuous on , then the function is also absolutely continuous on .

An absolutely continuous function maps a set of measure zero into a set of measure zero, and a measurable set into a measurable set. Any continuous function of finite variation which maps each set of measure zero into a set of measure zero is absolutely continuous. Any absolutely continuous function can be represented as the difference of two absolutely continuous non-decreasing functions.

A function that is absolutely continuous on the segment has a finite variation on this segment and has a finite derivative at almost every point. The derivative is summable over this segment, and If the derivative of an absolutely continuous function is almost everywhere equal to zero, then the function itself is constant. On the other hand, for any function that is summable on the function is absolutely continuous on this segment. Accordingly, the class of functions that are absolutely continuous on a given segment coincides with the class of functions that can be represented as an indefinite Lebesgue integral, i.e. as a Lebesgue integral with a variable upper limit of a certain summable function plus a constant.

If is absolutely continuous on , then its total variation is

The concept of absolute continuity can be generalized to include both functions of several variables and set functions (see Subsection 4 below).

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Absolute continuity of a set function is a concept usually applied to countably-additive functions defined on a -ring of subsets of a set . Thus, if and are two countably-additive functions defined on having values in the extended real number line , then is absolutely continuous with respect to (in symbols this is written as ) if entails . Here is the total variation of : and are measures, known as the positive and negative variations of ; according to the Jordan–Hahn theorem, . It turns out that the relations 1) ; 2) , ; 3) are equivalent. If the measure is finite, if and only if for any there exists a such that entails . According to the Radon–Nikodým theorem, if are (completely) -finite, (i.e. and there exists a sequence , such that and if , then there exists on a finite measurable function such that

Conversely, if is (completely) -finite and the integral makes sense, then as a function of the set is absolutely continuous with respect to . If and are (completely) -finite measures on , there exist uniquely defined (completely) -finite measures and such that , and is singular with respect to (i.e. there exists a set such that , ) (Lebesgue's theorem). A measure, defined on the Borel sets of a finite-dimensional Euclidean space (or, more generally, of a locally compact group), is called absolutely continuous if it is absolutely continuous with respect to the Lebesgue (Haar) measure. A non-negative measure on the Borel sets of the real line is absolutely continuous if and only if the corresponding distribution function is absolutely continuous (as a function of a real variable). The concept of absolute continuity of a set function can also be defined for finitely-additive functions and for functions with vector values.

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