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While the Riemann integral considers the area under a curve as made out of vertical rectangles, the Lebesgue definition considers horizontal slabs that are not necessarily just rectangles, and so it is more flexible. For this reason, the Lebesgue definition makes it possible to calculate integrals for a broader class of functions.
The Lebesgue–Stieltjes integral ()is defined when : [,] is Borel-measurable and bounded and : [,] is of bounded variation in [a, b] and right-continuous, or when f is non-negative and g is monotone and right-continuous.
Measure and integration (as the English translation of the title reads) is a definitive monograph on integration and measure theory: the treatment of the limiting behavior of the integral of various kind of sequences of measure-related structures (measurable functions, measurable sets, measures and their combinations) is somewhat conclusive.
The definition of the Lebesgue integral thus begins with a measure, μ. In the simplest case, the Lebesgue measure μ ( A ) of an interval A = [ a , b ] is its width, b − a , so that the Lebesgue integral agrees with the (proper) Riemann integral when both exist. [ 26 ]
In Lebesgue integration, this is exactly the requirement for any measurable function f to be considered integrable, with the integral then equaling + (), so that in fact "absolutely integrable" means the same thing as "Lebesgue integrable" for measurable functions.
Lebesgue's dominated convergence theorem is a special case of the Fatou–Lebesgue theorem. Below, however, is a direct proof that uses Fatou’s lemma as the essential tool. Since f is the pointwise limit of the sequence ( f n ) of measurable functions that are dominated by g , it is also measurable and dominated by g , hence it is integrable.
The Lebesgue–Stieltjes integral is the ordinary Lebesgue integral with respect to a measure known as the Lebesgue–Stieltjes measure, which may be associated to any function of bounded variation on the real line. The Lebesgue–Stieltjes measure is a regular Borel measure, and conversely every regular Borel measure on the real line is of ...
Henri Léon Lebesgue ForMemRS [1] (French: [ɑ̃ʁi leɔ̃ ləbɛɡ]; June 28, 1875 – July 26, 1941) was a French mathematician known for his theory of integration, which was a generalization of the 17th-century concept of integration—summing the area between an axis and the curve of a function defined for that axis.