Lebesgue integration
In mathematics, the integral of a nonnegative function of a single variable can be regarded, in the simplest case, as the area between the graph of that function and the xaxis. The Lebesgue integral extends the integral to a larger class of functions. It also extends the domains on which these functions can be defined.
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Long before the 20th century, mathematicians already understood that for nonnegative functions with a smooth enough graph—such as continuous functions on closed bounded intervals—the area under the curve could be defined as the integral, and computed using approximation techniques on the region by polygons. However, as the need to consider more irregular functions arose—e.g., as a result of the limiting processes of mathematical analysis and the mathematical theory of probability—it became clear that more careful approximation techniques were needed to define a suitable integral. Also, one might wish to integrate on spaces more general than the real line. The Lebesgue integral provides the necessary abstractions for this.
The Lebesgue integral plays an important role in probability theory, real analysis, and many other fields in mathematics. It is named after Henri Lebesgue (1875–1941), who introduced the integral (Lebesgue 1904). It is also a pivotal part of the axiomatic theory of probability.
The term Lebesgue integration can mean either the general theory of integration of a function with respect to a general measure, as introduced by Lebesgue, or the specific case of integration of a function defined on a subdomain of the real line with respect to the Lebesgue measure.
Introduction
The integral of a positive function f between limits a and b can be interpreted as the area under the graph of f. This is straightforward for functions such as polynomials, but what does it mean for more exotic functions? In general, for which class of functions does "area under the curve" make sense? The answer to this question has great theoretical and practical importance.
As part of a general movement toward rigor in mathematics in the nineteenth century, mathematicians attempted to put integral calculus on a firm foundation. The Riemann integral—proposed by Bernhard Riemann (1826–1866)—is a broadly successful attempt to provide such a foundation. Riemann's definition starts with the construction of a sequence of easily calculated areas that converge to the integral of a given function. This definition is successful in the sense that it gives the expected answer for many alreadysolved problems, and gives useful results for many other problems.
However, Riemann integration does not interact well with taking limits of sequences of functions, making such limiting processes difficult to analyze. This is important, for instance, in the study of Fourier series, Fourier transforms, and other topics. The Lebesgue integral is better able to describe how and when it is possible to take limits under the integral sign (via the monotone convergence theorem and dominated convergence theorem).
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. For example, the Dirichlet function, which is 0 where its argument is irrational and 1 otherwise, has a Lebesgue integral, but does not have a Riemann integral. Furthermore, the Lebesgue integral of this function is zero, which agrees with the intuition that when picking a real number uniformly at random from the unit interval, the probability of picking a rational number should be zero.
Lebesgue summarized his approach to integration in a letter to Paul Montel:
I have to pay a certain sum, which I have collected in my pocket. I take the bills and coins out of my pocket and give them to the creditor in the order I find them until I have reached the total sum. This is the Riemann integral. But I can proceed differently. After I have taken all the money out of my pocket I order the bills and coins according to identical values and then I pay the several heaps one after the other to the creditor. This is my integral.
— Source: (SiegmundSchultze 2008)
The insight is that one should be able to rearrange the values of a function freely, while preserving the value of the integral. This process of rearrangement can convert a very pathological function into one that is "nice" from the point of view of integration, and thus let such pathological functions be integrated.
Intuitive interpretation
To get some intuition about the different approaches to integration, let us imagine that we want to find a mountain's volume (above sea level).
 The Riemann–Darboux approach
 Divide the base of the mountain into a grid of 1 meter squares. Measure the altitude of the mountain at the center of each square. The volume on a single grid square is approximately 1 m^{2} × (that square's altitude), so the total volume is 1 m^{2} times the sum of the altitudes.
 The Lebesgue approach
 Draw a contour map of the mountain, where adjacent contours are 1 meter of altitude apart. The volume of earth a single contour contains is approximately 1 m × (that contour's area), so the total volume is the sum of these areas times 1 m.
Folland summarizes the difference between the Riemann and Lebesgue approaches thus: "to compute the Riemann integral of f, one partitions the domain [a, b] into subintervals", while in the Lebesgue integral, "one is in effect partitioning the range of f ."[1]
Towards a formal definition
To define the Lebesgue integral requires the formal notion of a measure that, roughly, associates to each set A of real numbers a nonnegative number μ(A) representing the "size" of A. This notion of "size" should agree with the usual length of an interval or disjoint union of intervals. Suppose that f : ℝ → ℝ^{+} is a nonnegative realvalued function. Using the "partitioning the range of f " philosophy, the integral of f should be the sum over t of the elementary area contained in the thin horizontal strip between y = t and y = t − dt. This elementary area is just
Let
The Lebesgue integral of f is then defined by[2]
where the integral on the right is an ordinary improper Riemann integral. Note that f^{∗} is a nonnegative decreasing function, and therefore has a welldefined improper Riemann integral with value in the interval [0,∞). For a suitable class of functions (the measurable functions), this defines the Lebesgue integral.
A general (not necessarily positive) measurable function f is called Lebesgue integrable if the area between the graph of f and the xaxis is finite:
In that case, as in the Riemannian case, the integral is the difference between the area above the xaxis and the area below the xaxis:
where is the decomposition of f into the difference of two nonnegative functions given by
Construction
The theory of the Lebesgue integral requires a theory of measurable sets and measures on these sets, as well as a theory of measurable functions and integrals on these functions.
Measure theory
Measure theory was initially created to provide a useful abstraction of the notion of length of subsets of the real line—and, more generally, area and volume of subsets of Euclidean spaces. In particular, it provided a systematic answer to the question of which subsets of ℝ have a length. As later set theory developments showed (see nonmeasurable set), it is actually impossible to assign a length to all subsets of ℝ in a way that preserves some natural additivity and translation invariance properties. This suggests that picking out a suitable class of measurable subsets is an essential prerequisite.
The Riemann integral uses the notion of length explicitly. Indeed, the element of calculation for the Riemann integral is the rectangle [a, b] × [c, d], whose area is calculated to be (b − a)(d − c). The quantity b − a is the length of the base of the rectangle and d − c is the height of the rectangle. Riemann could only use planar rectangles to approximate the area under the curve, because there was no adequate theory for measuring more general sets.
In the development of the theory in most modern textbooks (after 1950), the approach to measure and integration is axiomatic. This means that a measure is any function μ defined on a certain class X of subsets of a set E, which satisfies a certain list of properties. These properties can be shown to hold in many different cases.
Measurable functions
We start with a measure space (E, X, μ) where E is a set, X is a σalgebra of subsets of E, and μ is a (nonnegative) measure on E defined on the sets of X.
For example, E can be Euclidean nspace ℝ^{n} or some Lebesgue measurable subset of it, X is the σalgebra of all Lebesgue measurable subsets of E, and μ is the Lebesgue measure. In the mathematical theory of probability, we confine our study to a probability measure μ, which satisfies μ(E) = 1.
Lebesgue's theory defines integrals for a class of functions called measurable functions. A realvalued function f on E is measurable if the preimage of every interval of the form (t, ∞) (in fact, any Borel set) is in X:
We can show that this is equivalent to requiring that the preimage of any Borel subset of ℝ be in X. The set of measurable functions is closed under algebraic operations, but more importantly it is closed under various kinds of pointwise sequential limits:
are measurable if the original sequence (f_{k})_{k}, where k ∈ ℕ, consists of measurable functions.
There are several approaches for defining an integral:
for measurable realvalued functions f defined on E.
Constructing the integral
One approach to constructing the Lebesgue integral is to make use of socalled simple functions: finite reallinear combinations of indicator functions. For a measure theory novice, this construction of the Lebesgue integral makes more intuitive sense when it is compared to the way Riemann sum is used with the definition/construction of the Riemann integral. Simple functions can be used to approximate a measurable function, by partitioning the range into layers. The integral of a simple function is equal to the measure of a given layer, times the height of that layer. The integral of a nonnegative general measurable function is then defined as an appropriate supremum of approximations by simple functions, and the integral of a (not necessarily positive) measurable function is the difference of two integrals of nonnegative measurable functions, as mentioned earlier.
Indicator functions
To assign a value to the integral of the indicator function 1_{S} of a measurable set S consistent with the given measure μ, the only reasonable choice is to set:
Notice that the result may be equal to +∞, unless μ is a finite measure.
Simple functions
A finite linear combination of indicator functions
where the coefficients a_{k} are real numbers and S_{k} are disjoint measurable sets, is called a measurable simple function. We extend the integral by linearity to nonnegative measurable simple functions. When the coefficients a_{k} are nonnegative, we set
The convention 0 × ∞ = 0 must be used, and the result may be infinite. Even if a simple function can be written in many ways as a linear combination of indicator functions, the integral is always the same. This can be shown using the additivity property of measures.
Some care is needed when defining the integral of a realvalued simple function, to avoid the undefined expression ∞ − ∞: one assumes that the representation
is such that μ(S_{k}) < ∞ whenever a_{k} ≠ 0. Then the above formula for the integral of f makes sense, and the result does not depend upon the particular representation of f satisfying the assumptions.
If B is a measurable subset of E and s is a measurable simple function one defines
Nonnegative functions
Let f be a nonnegative measurable function on E, which we allow to attain the value +∞, in other words, f takes nonnegative values in the extended real number line. We define
We need to show this integral coincides with the preceding one, defined on the set of simple functions, when E is a segment [a, b]. There is also the question of whether this corresponds in any way to a Riemann notion of integration. It is possible to prove that the answer to both questions is yes.
We have defined the integral of f for any nonnegative extended realvalued measurable function on E. For some functions, this integral ∫_{E} f dμ is infinite.
It is often useful to have a particular sequence of simple functions that approximates the Lebesgue integral well (analogously to a Riemann sum). For a nonnegative measurable function f, let be the simple function whose value is whenever , for k a nonnegative integer less than (say) . Then it can be proven directly that
and that the limit on the right hand side exists as an extended real number. This bridges the connection between the approach to the Lebesgue integral using simple functions, and the motivation for the Lebesgue integral using a partition of the range.
Signed functions
To handle signed functions, we need a few more definitions. If f is a measurable function of the set E to the reals (including ±∞), then we can write
where
Note that both f^{+} and f^{−} are nonnegative measurable functions. Also note that
We say that the Lebesgue integral of the measurable function f exists, or is defined if at least one of and is finite:
In this case we define
If
we say that f is Lebesgue integrable.
It turns out that this definition gives the desirable properties of the integral.
Complexvalued functions
Complexvalued functions can be similarly integrated, by considering the real part and the imaginary part separately.
If h=f+ig for realvalued integrable functions f, g, then the integral of h is defined by
The function is Lebesgue integrable if and only if its absolute value is Lebesgue integrable (see Absolutely integrable function).
Example
Consider the indicator function of the rational numbers, 1_{Q}, also known as the Dirichlet function. This function is nowhere continuous.
 is not Riemannintegrable on [0, 1]: No matter how the set [0, 1] is partitioned into subintervals, each partition contains at least one rational and at least one irrational number, because rationals and irrationals are both dense in the reals. Thus the upper Darboux sums are all one, and the lower Darboux sums are all zero.
 is Lebesgueintegrable on [0, 1] using the Lebesgue measure: Indeed, it is the indicator function of the rationals so by definition
 because Q is countable.
Domain of integration
A technical issue in Lebesgue integration is that the domain of integration is defined as a set (a subset of a measure space), with no notion of orientation. In elementary calculus, one defines integration with respect to an orientation:
Generalizing this to higher dimensions yields integration of differential forms. By contrast, Lebesgue integration provides an alternative generalization, integrating over subsets with respect to a measure; this can be notated as
to indicate integration over a subset A. For details on the relation between these generalizations, see Differential form § Relation with measures.
Limitations of the Riemann integral
With the advent of Fourier series, many analytical problems involving integrals came up whose satisfactory solution required interchanging limit processes and integral signs. However, the conditions under which the integrals
are equal proved quite elusive in the Riemann framework. There are some other technical difficulties with the Riemann integral. These are linked with the limittaking difficulty discussed above.
Failure of monotone convergence. As shown above, the indicator function 1_{Q} on the rationals is not Riemann integrable. In particular, the Monotone convergence theorem fails. To see why, let {a_{k}} be an enumeration of all the rational numbers in [0, 1] (they are countable so this can be done.) Then let
The function g_{k} is zero everywhere, except on a finite set of points. Hence its Riemann integral is zero. Each g_{k} is nonnegative, and this sequence of functions is monotonically increasing, but its limit as k → ∞ is 1_{Q}, which is not Riemann integrable.
Unsuitability for unbounded intervals. The Riemann integral can only integrate functions on a bounded interval. It can however be extended to unbounded intervals by taking limits, so long as this doesn't yield an answer such as ∞ − ∞.
Integrating on structures other than Euclidean space. The Riemann integral is inextricably linked to the order structure of the real line.
Basic theorems of the Lebesgue integral
Two functions are said to be equal almost everywhere ( for short) if they coincide outside a subset of measure 0.
Measurability of the subset is not required.
 If f, g are nonnegative measurable functions (possibly assuming the value +∞) such that f = g almost everywhere, then
 To wit, the integral respects the equivalence relation of almosteverywhere equality.
 If f, g are functions such that f = g almost everywhere, then f is Lebesgue integrable if and only if g is Lebesgue integrable, and the integrals of f and g are the same if they exist.
 Linearity: If f and g are Lebesgue integrable functions and a and b are real numbers, then af + bg is Lebesgue integrable and
 Monotonicity: If f ≤ g, then
 Let be a measure space. Denote the algebra of Borel sets on . (By definition, contains the set and all Borel subsets of ). Consider a measurable nonnegative function . For a set , define
 Then is a Lebesgue measure on .
 Monotone convergence theorem: Suppose { f_{k}}_{k ∈ ℕ} is a sequence of nonnegative measurable functions such that
 Then, the pointwise limit f of f_{k} is Lebesgue measurable and
 The value of any of the integrals is allowed to be infinite.
 Fatou's lemma: If { f_{k}}_{k ∈ N} is a sequence of nonnegative measurable functions, then
 Again, the value of any of the integrals may be infinite.
 Dominated convergence theorem: Suppose { f_{k}}_{k ∈ N} is a sequence of complex measurable functions with pointwise limit f, and there is a Lebesgue integrable function g (i.e., g belongs to the space L^{1}) such that  f_{k}  ≤ g for all k.
 Then, f is Lebesgue integrable and
Alternative formulations
It is possible to develop the integral with respect to the Lebesgue measure without relying on the full machinery of measure theory. One such approach is provided by the Daniell integral.
There is also an alternative approach to developing the theory of integration via methods of functional analysis. The Riemann integral exists for any continuous function f of compact support defined on ℝ^{n} (or a fixed open subset). Integrals of more general functions can be built starting from these integrals.
Let C_{c} be the space of all realvalued compactly supported continuous functions of ℝ. Define a norm on C_{c} by
Then C_{c} is a normed vector space (and in particular, it is a metric space.) All metric spaces have Hausdorff completions, so let L^{1} be its completion. This space is isomorphic to the space of Lebesgue integrable functions modulo the subspace of functions with integral zero. Furthermore, the Riemann integral ∫ is a uniformly continuous functional with respect to the norm on C_{c}, which is dense in L^{1}. Hence ∫ has a unique extension to all of L^{1}. This integral is precisely the Lebesgue integral.
More generally, when the measure space on which the functions are defined is also a locally compact topological space (as is the case with the real numbers ℝ), measures compatible with the topology in a suitable sense (Radon measures, of which the Lebesgue measure is an example) an integral with respect to them can be defined in the same manner, starting from the integrals of continuous functions with compact support. More precisely, the compactly supported functions form a vector space that carries a natural topology, and a (Radon) measure is defined as a continuous linear functional on this space. The value of a measure at a compactly supported function is then also by definition the integral of the function. One then proceeds to expand the measure (the integral) to more general functions by continuity, and defines the measure of a set as the integral of its indicator function. This is the approach taken by Bourbaki (2004) and a certain number of other authors. For details see Radon measures.
Limitations of Lebesgue integral
The main purpose of the Lebesgue integral is to provide an integral notion where limits of integrals hold under mild assumptions. There is no guarantee that every function is Lebesgue integrable. But it may happen that improper integrals exist for functions that are not Lebesgue integrable. One example would be
over the entire real line. This function is not Lebesgue integrable, as
On the other hand, exists as an improper integral and can be computed to be finite; it is twice the Dirichlet integral.
See also
 Henri Lebesgue, for a nontechnical description of Lebesgue integration
 Null set
 Integration
 Measure
 Sigmaalgebra
 Lebesgue space
 Lebesgue–Stieltjes integration
 Riemann integral
 Henstock–Kurzweil integral
Notes
 Folland, Gerald B. (1984). Real Analysis: Modern Techniques and Their Applications. Wiley. p. 56.
 Lieb & Loss 2001
References
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