In measure theory (a branch of mathematical analysis), a property holds almost everywhere if the set of elements for which the property does not hold is a null set, that is, a set of measure zero (Halmos 1974). In cases where the measure is not complete, it is sufficient that the set is contained within a set of measure zero. When discussing sets of real numbers, the Lebesgue measure is assumed unless otherwise stated.
The term almost everywhere is abbreviated a.e.; in older literature p.p. is used, to stand for the equivalent French language phrase presque partout.
A set with full measure is one whose complement is of measure zero. In probability theory, the terms almost surely, almost certain and almost always refer to sets with probability 1, which are exactly the sets of full measure in a probability space.
Occasionally, instead of saying that a property holds almost everywhere, it is said that the property holds for almost all elements (though the term almost all also has other meanings).
Properties
 If f : [a, b] → R is a monotonic function, then f is differentiable almost everywhere.
 If f : R → R is Lebesgue measurable and
 If f(x,y) is Borel measurable on R^{2} then for almost every x, the function y→f(x,y) is Borel measurable.
Definition using ultrafilters
Outside of the context of real analysis, the notion of a property true almost everywhere is sometimes defined in terms of an ultrafilter. An ultrafilter on a set X is a maximal collection F of subsets of X such that:
A property of points in X holds almost everywhere, relative to an ultrafilter F, if the set of points for which X holds is in F.
For example, one construction of the hyperreal number system defines a hyperreal number as an equivalence class of sequences that are equal almost everywhere as defined by an ultrafilter.
The definition of almost everywhere in terms of ultrafilters is closely related to the definition in terms of measures, because each ultrafilter defines a finitelyadditive measure taking only the values 0 and 1, where a set has measure 1 if and only if it is included in the ultrafilter.
References
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