Locally integrable function

Locally integrable function
In mathematics, a locally integrable function (sometimes also called locally summable function) is a function which is integrable (so its integral is finite) on every compact subset of its domain of definition. The importance of such functions lies in the fact that their function space is similar to spaces, but its members are not required to satisfy any growth restriction on their behavior at the boundary of their domain (at infinity if the domain is unbounded): in other words, locally integrable functions can grow arbitrarily fast at the domain boundary, but are still manageable in a way similar to ordinary integrable functions.
Definition
Standard definition
. Let be an open set in the Euclidean space and be a Lebesgue measurable function. If on is such that
i.e. its Lebesgue integral is finite on all compact subsets of , then is called locally integrable. The set of all such functions is denoted by :
where denotes the restriction of to the set .
The classical definition of a locally integrable function involves only measure theoretic and topological concepts and can be carried over abstract to complex-valued functions on a topological measure space : however, since the most common application of such functions is to distribution theory on Euclidean spaces, all the definitions in this and the following sections deal explicitly only with this important case.
An alternative definition
. Let be an open set in the Euclidean space . Then a function such that
for each test function is called locally integrable, and the set of such functions is denoted by . Here denotes the set of all infinitely differentiable functions with compact support contained in .
This definition has its roots in the approach to measure and integration theory based on the concept of continuous linear functional on a topological vector space, developed by the Nicolas Bourbaki school: it is also the one adopted by and by . This "distribution theoretic" definition is equivalent to the standard one, as the following lemma proves:
. A given function is locally integrable according to if and only if it is locally integrable according to , i.e.
Proof of
If part: Let be a test function. It is bounded by its supremum norm , measurable, and has a compact support, let's call it . Hence
by .
Only if part: Let be a compact subset of the open set . We will first construct a test function which majorises the indicator function of .
The usual set distance between and the boundary is strictly greater than zero, i.e.
hence it is possible to choose a real number such that (if is the empty set, take ). Let and denote the closed -neighborhood and -neighborhood of , respectively. They are likewise compact and satisfy
Now use convolution to define the function by
where is a mollifier constructed by using the standard positive symmetric one. Obviously is non-negative in the sense that , infinitely differentiable, and its support is contained in , in particular it is a test function. Since for all , we have that .
Let be a locally integrable function according to . Then
Since this holds for every compact subset of , the function is locally integrable according to . □
Generalization: locally p-integrable functions
. Let be an open set in the Euclidean space and be a Lebesgue measurable function. If, for a given with , satisfies
i.e., it belongs to for all compact subsets of , then is called locally -integrable or also -locally integrable. The set of all such functions is denoted by :
An alternative definition, completely analogous to the one given for locally integrable functions, can also be given for locally -integrable functions: it can also be and proven equivalent to the one in this section. Despite their apparent higher generality, locally -integrable functions form a subset of locally integrable functions for every such that .
Notation
Apart from the different glyphs which may be used for the uppercase "L", there are few variants for the notation of the set of locally integrable functions
adopted by , and .
adopted by and .
adopted by and .
Properties
Lp,loc is a complete metric space for all p ≥ 1
. is a complete metrizable space: its topology can be generated by the following metric:
where is a family of non empty open sets such that
, meaning that is compactly included in i.e. it is a set having compact closure strictly included in the set of higher index.
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, k ∈ is an indexed family of seminorms, defined as
In references and , this theorem is stated but not proved on a formal basis: a complete proof of a more general result, which includes it, is found in .
Lp is a subspace of L1,loc for all p ≥ 1
. Every function belonging to where is an open subset of , is locally integrable.
Proof. The case is trivial, therefore in the sequel of the proof it is assumed that . Consider the characteristic function of a compact subset of : then, for ,
where
is a positive number such that = for a given
is the Lebesgue measure of the compact set
Then for any belonging to , by Hölder's inequality, the product is integrable i.e. belongs to and
therefore
Note that since the following inequality is true
the theorem is true also for functions belonging only to the space of locally -integrable functions, therefore the theorem implies also the following result.
. Every function in is locally integrable, i. e. belongs to .
Note: If is an open subset of that is also bounded, then one has the standard inclusion which makes sense given the above inclusion . But the first of these statements is not true if is not bounded; then it is still true that for any , but not that . To see this, one typically considers the function , which is in but not in for any finite .
L1,loc is the space of densities of absolutely continuous measures
. A function is the density of an absolutely continuous measure if and only if .
The proof of this result is sketched by . Rephrasing its statement, this theorem asserts that every locally integrable function defines an absolutely continuous measure and conversely that every absolutely continuous measures defines a locally integrable function: this is also, in the abstract measure theory framework, the form of the important Radon–Nikodym theorem given by Stanisław Saks in his treatise.
Examples
The constant function defined on the real line is locally integrable but not globally integrable since the real line has infinite measure. More generally, constants, continuous functions and integrable functions are locally integrable.
The function for x ∈ (0, 1) is locally but not globally integrable on (0, 1). It is locally integrable since any compact set K ⊆ (0, 1) has positive distance from 0 and f is hence bounded on K. This example underpins the initial claim that locally integrable functions do not require the satisfaction of growth conditions near the boundary in bounded domains.
The function
is not locally integrable in : it is indeed locally integrable near this point since its integral over every compact set not including it is finite. Formally speaking, (\ 0): however, this function can be extended to a distribution on the whole as a Cauchy principal value.
The preceding example raises a question: does every function which is locally integrable in ⊊ admit an extension to the whole as a distribution? The answer is negative, and a counterexample is provided by the following function:
does not define any distribution on .
The following example, similar to the preceding one, is a function belonging to (\ 0) which serves as an elementary counterexample in the application of the theory of distributions to differential operators with irregular singular coefficients:
where and are complex constants, is a general solution of the following elementary non-Fuchsian differential equation of first order
Again it does not define any distribution on the whole , if or are not zero: the only distributional global solution of such equation is therefore the zero distribution, and this shows how, in this branch of the theory of differential equations, the methods of the theory of distributions cannot be expected to have the same success achieved in other branches of the same theory, notably in the theory of linear differential equations with constant coefficients.
Applications
Locally integrable functions play a prominent role in distribution theory and they occur in the definition of various classes of functions and function spaces, like functions of bounded variation. Moreover, they appear in the Radon–Nikodym theorem by characterizing the absolutely continuous part of every measure.
See also
Compact set
Distribution (mathematics)
Lebesgue's density theorem
Lebesgue differentiation theorem
Lebesgue integral
Lp space
Notes
References
. 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.
. Translated from the original 1958 Russian edition by Eugene Saletan, this is an important monograph on the theory of generalized functions, dealing both with distributions and analytic functionals.
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(available also as ).
(available also as ).
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. English translation by Laurence Chisholm Young, with two additional notes by Stefan Banach: the Mathematical Reviews number refers to the Dover Publications 1964 edition, which is basically a reprint.
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. A monograph on the theory of generalized functions written with an eye towards their applications to several complex variables and mathematical physics, as is customary for the Author.
External links
Measure theory
Integral calculus
Types of functions
Lp spaces