Locally integrable function

In mathematics, a locally integrable function (sometimes also called locally summable function) ^[1] 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 ${\textstyle L^{p}}$ spaces, but its members are not required to satisfy any growth restriction on their behaviour 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

Definition 1. ^[2] Let ${\textstyle \Omega }$ be an open set in the Euclidean space ${\textstyle \mathbb {R} ^{n}}$ and ${\textstyle f:\Omega \to {\mathbb {C}}}$ be a Lebesgue measurable function. If ${\textstyle f}$ on ${\textstyle \Omega }$ is such that

${\displaystyle \int _{K}|f|\,\mathrm {d} x<+\infty ,}$

i.e. its Lebesgue integral is finite on all compact subsets ${\textstyle K}$ of ${\textstyle \Omega }$, ^[3] then ${\textstyle f}$ is called locally integrable. The set of all such functions is denoted by ${\textstyle L_{1,{\text{loc}}}(\Omega )}$:

${\displaystyle L_{1,\mathrm {loc} }(\Omega )={\bigl \{}f\colon \Omega \to \mathbb {C} {\text{ measurable}}:f|_{K}\in L_{1}(K)\ \forall \,K\subset \Omega ,\,K{\text{ compact}}{\bigr \}},}$

where ${\textstyle \left.f\right|_{K}}$ denotes the restriction of ${\textstyle f}$ to the set ${\textstyle K}$.

An alternative definition

Definition 2. ^[4] Let ${\textstyle \Omega }$ be an open set in the Euclidean space ${\textstyle \mathbb {R} ^{n}}$. Then a function ${\textstyle f:\Omega \to \mathbb {C} }$ such that

${\displaystyle \int _{\Omega }|f\varphi |\,\mathrm {d} x<+\infty ,}$

for each test function ${\textstyle \varphi \in C_{c}^{\infty }(\Omega )}$ is called locally integrable, and the set of such functions is denoted by ${\textstyle L_{1,{\text{loc}}}(\Omega )}$. Here, ${\textstyle C_{c}^{\infty }(\Omega )}$ denotes the set of all infinitely differentiable functions ${\textstyle \varphi \colon \Omega \to {\mathbb {R}}}$ with compact support contained in ${\textstyle \Omega }$.

This definition has its roots in the approach to measure and integration theory based on the concept of a continuous linear functional on a topological vector space, developed by the Nicolas Bourbaki school. ^[5] It is also the one adopted by Strichartz (2003) and by Maz'ya & Shaposhnikova (2009 , p. 34). ^[6] This "distribution theoretic" definition is equivalent to the standard one, as the following lemma proves:

Lemma 1. A given function ${\textstyle f:\Omega \to \mathbb {C} }$ is locally integrable according to Definition 1 if and only if it is locally integrable according to Definition 2 , i.e.,

${\displaystyle \int _{K}|f|\,\mathrm {d} x<+\infty \quad \forall \,K\subset \Omega ,\,K{\text{ compact}}\quad \Longleftrightarrow \quad \int _{\Omega }|f\varphi |\,\mathrm {d} x<+\infty \quad \forall \,\varphi \in C_{\mathrm {c} }^{\infty }(\Omega ).}$

Proof of Lemma 1

If part: Let ${\textstyle \varphi \in C_{c}^{\infty }(\Omega )}$ be a test function. It is bounded by its supremum norm ${\textstyle \lVert \varphi \rVert _{\infty }}$, measurable, and has a compact support, let's call it ${\textstyle K}$. Hence,

${\displaystyle \int _{\Omega }|f\varphi |\,\mathrm {d} x=\int _{K}|f|\,|\varphi |\,\mathrm {d} x\leq \|\varphi \|_{\infty }\int _{K}|f|\,\mathrm {d} x<\infty }$

by Definition 1 .

Only if part: Let ${\textstyle K}$ be a compact subset of the open set ${\textstyle \Omega }$. We will first construct a test function ${\textstyle \varphi _{K}\in C_{c}^{\infty }(\Omega )}$ which majorises the indicator function ${\textstyle \chi _{K}}$ of ${\textstyle K}$. The usual set distance ^[7] between ${\textstyle K}$ and the boundary ${\textstyle \partial \Omega }$ is strictly greater than zero, i.e.,

${\displaystyle \Delta$ :=d(K,\partial \Omega )>0,}

hence it is possible to choose a real number ${\textstyle \delta }$ such that ${\textstyle \Delta >2\delta >0}$ (if ${\textstyle \partial \Omega }$ is the empty set, take ${\textstyle \Delta =\infty }$). Let ${\textstyle K_{\delta }}$ and ${\textstyle K_{2\delta }}$ denote the closed ${\textstyle \delta }$-neighborhood and ${\textstyle 2\delta }$-neighborhood of ${\textstyle K}$, respectively. They are likewise compact and satisfy

${\displaystyle K\subset K_{\delta }\subset K_{2\delta }\subset \Omega ,\qquad d(K_{\delta },\partial \Omega )=\Delta -\delta >\delta >0.}$

Now use convolution to define the function ${\textstyle \varphi _{K}:\Omega \to \mathbb {R} }$ by

${\displaystyle \varphi _{K}(x)={\chi _{K_{\delta }}\ast \varphi _{\delta }(x)}=\int _{\mathbb {R} ^{n}}\chi _{K_{\delta }}(y)\,\varphi _{\delta }(x-y)\,\mathrm {d} y,}$

where ${\textstyle \varphi _{\delta }}$ is a mollifier constructed by using the standard positive symmetric one. Obviously ${\textstyle \varphi _{K}}$ is non-negative in the sense that ${\textstyle \varphi _{K}\geq 0}$, infinitely differentiable, and its support is contained in ${\textstyle K_{2\delta }}$. In particular, it is a test function. Since ${\textstyle \varphi _{K}(x)=1}$ for all ${\textstyle x\in K}$, we have that ${\textstyle \chi _{K}\leq \varphi _{K}}$.

Let ${\textstyle f}$ be a locally integrable function according to Definition 2 . Then

${\displaystyle \int _{K}|f|\,\mathrm {d} x=\int _{\Omega }|f|\chi _{K}\,\mathrm {d} x\leq \int _{\Omega }|f|\varphi _{K}\,\mathrm {d} x<\infty .}$

Since this holds for every compact subset ${\textstyle K}$ of ${\textstyle \Omega }$, the function ${\textstyle f}$ is locally integrable according to Definition 1 . □

General definition of local integrability on a generalized measure space

The classical Definition 1 of a locally integrable function involves only measure theoretic and topological ^[8] concepts and thus can be carried over abstract to complex-valued functions on a topological measure space ${\textstyle (X,\Sigma ,\mu )}$. ^[9] Nevertheless, the concept of a locally integrable function can be defined even on a generalised measure space ${\textstyle (X,{\mathcal {C}},\mu )}$, where ${\textstyle {\mathcal {C}}}$ is no longer required to be a sigma-algebra but only a ring of sets and, notably, ${\textstyle X}$ does not need to carry the structure of a topological space.

Definition 1A. ^[10] Let ${\textstyle (X,{\mathcal {C}},\mu )}$ be an ordered triple where ${\textstyle X}$ is a nonempty set, ${\textstyle {\mathcal {C}}}$ is a ring of sets, and ${\textstyle \mu }$ is a positive measure on ${\textstyle {\mathcal {C}}}$. Moreover, let ${\textstyle f}$ be a function from ${\textstyle X}$ to a Banach space ${\textstyle B}$ or to the extended real number line ${\textstyle {\overline {\mathbb {R} }}}$. Then ${\textstyle f}$ is said to be locally integrable with respect to${\textstyle \mu }$ if for every set ${\textstyle K\in {\mathcal {C}}}$, the function ${\textstyle f\cdot \chi _{k}}$ is integrable with respect to ${\textstyle \mu }$.

The equivalence of Definition 1 and Definition 1A when ${\textstyle X}$ is a topological space can be proven by constructing a ring of sets ${\textstyle {\mathcal {C}}}$ from the set ${\textstyle {\mathcal {K}}}$ of compact subsets of ${\textstyle X}$ by the following steps.

1. It is evident that ${\textstyle \emptyset \in {\mathcal {K}}}$ and, moreover, the operations of union ${\textstyle \cup }$ and intersection ${\textstyle \cap }$ make ${\textstyle {\mathcal {K}}}$ a lattice with least upper bound ${\textstyle \vee \equiv \cup }$ and greatest lower bound ${\textstyle \wedge \equiv \cap }$. ^[11] .
2. The class of sets ${\textstyle {\mathcal {D}}}$ defined as ${\textstyle {\mathcal {D}}\triangleq \{A\setminus B\mid A,B\in {\mathcal {K}}\}}$ is a semiring of sets ^[11] such that ${\textstyle {\mathcal {D}}\supset {\mathcal {K}}}$ because of the condition ${\textstyle \emptyset \in {\mathcal {K}}}$.
3. The class of sets ${\textstyle {\mathcal {C}}}$ defined as ${\textstyle {\mathcal {C}}\triangleq \{\cup _{i=1}^{n}A_{i}\mid A_{i}\in {\mathcal {D}}{\text{ and }}A_{i}\cap A_{j}=\emptyset {\text{ if }}i\neq j\}}$, i.e., the class formed by finite unions of pairwise disjoint sets of ${\textstyle {\mathcal {D}}}$, is a ring of sets, precisely the minimal one generated by ${\textstyle {\mathcal {K}}}$. ^[12]

By means of this abstract framework, Dinculeanu (1966 , pp. 163–188) lists and proves several properties of locally integrable functions. Nevertheless, even if working in this more general framework is possible, since the most commonly seen applications of such functions are to distribution theory on Euclidean spaces, ^[2] all the definitions and properties presented in the following sections deal explicitly only with this latter important case.

Generalization: locally p-integrable functions

Definition 3. ^[13] Let ${\textstyle \Omega }$ be an open set in the Euclidean space ${\textstyle \mathbb {R} ^{n}}$ and ${\textstyle f:\Omega \to \mathbb {C} }$ be a Lebesgue measurable function. If, for a given ${\textstyle p}$ with ${\textstyle 1\leq p\leq +\infty }$, ${\textstyle f}$ satisfies

${\displaystyle \int _{K}|f|^{p}\,\mathrm {d} x<+\infty ,}$

i.e., it belongs to ${\textstyle L_{p}(K)}$ for all compact subsets ${\textstyle K}$ of ${\textstyle \Omega }$, then ${\textstyle f}$ is called locally${\textstyle p}$-integrable or also ${\textstyle p}$-locally integrable. ^[13] The set of all such functions is denoted by ${\textstyle L_{p,{\text{loc}}}(\Omega )}$:

${\displaystyle L_{p,\mathrm {loc} }(\Omega )=\left\{f:\Omega \to \mathbb {C} {\text{ measurable }}\left|\ f|_{K}\in L_{p}(K),\ \forall \,K\subset \Omega ,K{\text{ compact}}\right.\right\}.}$

An alternative definition, completely analogous to the one given for locally integrable functions, can also be given for locally ${\textstyle p}$-integrable functions: it can also be and proven equivalent to the one in this section. ^[14] Despite their apparent higher generality, locally ${\textstyle p}$-integrable functions form a subset of locally integrable functions for every ${\textstyle p}$ such that ${\textstyle 1<p\leq +\infty }$. ^[15]

Notation

Apart from the different glyphs which may be used for the uppercase "L", ^[16] there are few variants for the notation of the set of locally integrable functions

• ${\textstyle L_{\mathrm {loc} }^{p}(\Omega ),}$ adopted by ( Hörmander 1990 , p. 37), ( Strichartz 2003 , pp. 12–13) and ( Vladimirov 2002 , p. 3).
• ${\textstyle L_{p,\mathrm {loc} }(\Omega ),}$ adopted by ( Maz'ya & Poborchi 1997 , p. 4) and Maz'ya & Shaposhnikova (2009 , p. 44).
• ${\textstyle L_{p}(\Omega ,\mathrm {loc} ),}$ adopted by ( Maz'ja 1985 , p. 6) and ( Maz'ya 2011 , p. 2).

Properties

L_p,loc is a complete metric space for all p ≥ 1

Theorem 1. ^[17] ${\textstyle L_{p,{\text{loc}}}}$ is a complete metrizable space: its topology can be generated by the following metric:

${\displaystyle d(u,v)=\sum _{k\geq 1}{\frac {1}{2^{k}}}{\frac {\Vert u-v\Vert _{p,\omega _{k}}}{1+\Vert u-v\Vert _{p,\omega _{k}}}}\qquad u,v\in L_{p,\mathrm {loc} }(\Omega ),}$

where ${\textstyle \{\omega _{k}\}_{k\geq 1}}$ is a family of non empty open sets such that

• ${\textstyle \omega _{k}\Subset \omega _{k+1}}$, meaning that ${\textstyle \omega _{k}}$is compactly contained in${\textstyle \omega _{k+1}}$ i.e. each of them is a set whose closure is compact and strictly included in the set of higher index. ^[18]
• ${\textstyle \cup _{k}\omega _{k}=\Omega }$ and finally
• ${\textstyle {\Vert \cdot \Vert }_{p,\omega _{k}}\to \mathbb {R} ^{+}}$, ${\displaystyle k\in \mathbb {N} }$ is an indexed family of seminorms, defined as

${\displaystyle {\Vert u\Vert }_{p,\omega _{k}}=\left(\int _{\omega _{k}}|u(x)|^{p}\,\mathrm {d} x\right)^{1/p}\qquad \forall \,u\in L_{p,\mathrm {loc} }(\Omega ).}$

In ( Gilbarg & Trudinger 2001 , p. 147), ( Maz'ya & Poborchi 1997 , p. 5), ( Maz'ja 1985 , p. 6) and ( Maz'ya 2011 , p. 2), this theorem is stated but not proved on a formal basis: ^[19] a complete proof of a more general result, which includes it, can be found in ( Meise & Vogt 1997 , p. 40).

L_p is a subspace of L_1,loc for all p ≥ 1

Theorem 2. Every function ${\textstyle f}$ belonging to ${\textstyle L_{p,{\text{loc}}}(\Omega )}$, ${\textstyle 1\leq p\leq +\infty }$, where ${\textstyle \Omega }$ is an open subset of ${\textstyle \mathbb {R} ^{n}}$, is locally integrable.

Proof. The case ${\textstyle p=1}$ is trivial, therefore in the sequel of the proof it is assumed that ${\textstyle 1<p\leq +\infty }$. Consider the characteristic function ${\textstyle \chi _{K}}$ of a compact subset ${\textstyle K}$ of ${\textstyle \Omega }$: then, for ${\textstyle p\leq +\infty }$,

${\displaystyle \left|{\int _{\Omega }|\chi _{K}|^{q}\,\mathrm {d} x}\right|^{1/q}=\left|{\int _{K}\mathrm {d} x}\right|^{1/q}=|K|^{1/q}<+\infty ,}$

where

• ${\textstyle q}$ is a positive number such that ${\textstyle 1/p+1/q=1}$ for a given ${\textstyle 1\leq p\leq +\infty }$,
• ${\textstyle \vert K\vert }$ is the Lebesgue measure of the compact set ${\textstyle K}$.

Then for any ${\textstyle f}$ belonging to ${\textstyle L_{p}(\Omega )}$ the product by ${\textstyle f\chi _{K}}$ is integrable by Hölder's inequality i.e. belongs to ${\textstyle L_{1}(\Omega )}$ and

${\displaystyle {\int _{K}|f|\,\mathrm {d} x}={\int _{\Omega }|f\chi _{K}|\,\mathrm {d} x}\leq \left|{\int _{\Omega }|f|^{p}\,\mathrm {d} x}\right|^{1/p}\left|{\int _{K}\mathrm {d} x}\right|^{1/q}=\|f\|_{p}|K|^{1/q}<+\infty ,}$

therefore

${\displaystyle f\in L_{1,\mathrm {loc} }(\Omega ).}$

Note that since the following inequality is true

${\displaystyle {\int _{K}|f|\,\mathrm {d} x}={\int _{\Omega }|f\chi _{K}|\,\mathrm {d} x}\leq \left|{\int _{K}|f|^{p}\,\mathrm {d} x}\right|^{1/p}\left|{\int _{K}\mathrm {d} x}\right|^{1/q}=\|f\chi _{K}\|_{p}|K|^{1/q}<+\infty ,}$

the theorem is true also for functions ${\textstyle f}$ belonging only to the space of locally ${\textstyle p}$-integrable functions, therefore the theorem implies also the following result.

Corollary 1. Every function ${\textstyle f}$ in ${\textstyle L_{p,{\text{loc}}}(\Omega )}$, ${\textstyle 1<p\leq +\infty }$, is locally integrable, i. e. belongs to ${\textstyle >L_{1,{\text{loc}}}(\Omega )}$.

Note: If ${\textstyle \Omega }$ is an open subset of ${\textstyle \mathbb {R} ^{n}}$ that is also bounded, then one has the standard inclusion ${\displaystyle L_{p}(\Omega )\subset L_{1}(\Omega )}$ which makes sense given the above inclusion ${\displaystyle L_{1}(\Omega )\subset L_{1,{\text{loc}}}(\Omega )}$. But the first of these statements is not true if ${\displaystyle \Omega }$ is not bounded; then it is still true that ${\displaystyle L_{p}(\Omega )\subset L_{1,{\text{loc}}}(\Omega )}$ for any ${\displaystyle p}$, but not that ${\displaystyle L_{p}(\Omega )\subset L_{1}(\Omega )}$. To see this, one typically considers the function ${\displaystyle u(x)=1}$, which is in ${\displaystyle L_{\infty }(\mathbb {R} ^{n})}$ but not in ${\displaystyle L_{p}(\mathbb {R} ^{n})}$ for any finite ${\displaystyle p}$.

L_1,loc is the space of densities of absolutely continuous measures

Theorem 3. A function ${\textstyle f}$ is the density of an absolutely continuous measure if and only if ${\displaystyle f\in L_{1,{\text{loc}}}}$.

The proof of this result is sketched by ( Schwartz 1998 , p. 18). 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. ^[20]

Examples

• The constant function 1 defined on the real line is locally integrable but not globally integrable since the real line has infinite measure. More generally, constants, continuous functions ^[21] and integrable functions are locally integrable. ^[22]
• The function ${\textstyle f(x)=1/x}$ for ${\textstyle x\in (0,1)}$ is locally but not globally integrable on ${\textstyle (0,1)}$. It is locally integrable since any compact set ${\textstyle K\subset (0,1)}$ has positive distance from ${\textstyle 0}$ and ${\textstyle f}$ is hence bounded on ${\textstyle K}$. 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

${\displaystyle f(x)={\begin{cases}1/x&x\neq 0,\\0&x=0,\end{cases}}\quad x\in \mathbb {R} }$

is not locally integrable at ${\textstyle x=0}$: it is indeed locally integrable near this point since its integral over every compact set not including it is finite. Formally speaking, ${\textstyle 1/x\in L_{1,loc}(\mathbb {R} \setminus 0)}$: ^[23] however, this function can be extended to a distribution on the whole ${\textstyle \mathbb {R} }$ as a Cauchy principal value. ^[24]

• The preceding example raises a question: does every function which is locally integrable in ${\textstyle \Omega \subsetneq \mathbb {R} }$ admit an extension to the whole ${\textstyle \mathbb {R} }$ as a distribution? The answer is negative, and a counterexample is provided by the following function:

${\displaystyle f(x)={\begin{cases}e^{1/x}&x\neq 0,\\0&x=0,\end{cases}}}$

does not define any distribution on ${\textstyle \mathbb {R} }$. ^[25]

• The following example, similar to the preceding one, is a function belonging to ${\textstyle L_{1,{\text{loc}}}(\mathbb {R} ,0)}$ which serves as an elementary counterexample in the application of the theory of distributions to differential operators with irregular singular coefficients:

${\displaystyle f(x)={\begin{cases}k_{1}e^{1/x^{2}}&x>0,\\0&x=0,\\k_{2}e^{1/x^{2}}&x<0,\end{cases}}}$

where ${\displaystyle k_{1}}$ and ${\displaystyle k_{2}}$ are complex constants, is a general solution of the following elementary non-Fuchsian differential equation of first order

${\displaystyle x^{3}{\frac {\mathrm {d} f}{\mathrm {d} x}}+2f=0.}$

Again it does not define any distribution on the whole ${\displaystyle \mathbb {R} }$, if ${\textstyle k_{1}}$ or ${\textstyle k_{2}}$ 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. ^[26]

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

1. According to Gel'fand & Shilov (1964 , p. 3).
2. See for example ( Schwartz 1998 , p. 18) and ( Vladimirov 2002 , p. 3).
3. Another slight variant of this definition, chosen by Vladimirov (2002 , p. 1), is to require only that ${\textstyle K\Subset \Omega }$ (or, using the notation of Gilbarg & Trudinger (2001 , p. 9), ${\textstyle K\subset \subset \Omega }$), meaning that ${\textstyle K}$is strictly included in${\textstyle \Omega }$ i.e. it is a set having compact closure strictly included in the given ambient set.
4. See for example ( Strichartz 2003 , pp. 12–13).
5. This approach was praised by Schwartz (1998 , pp. 16–17) who remarked also its usefulness, however using Definition 1 to define locally integrable functions.
6. Note that Maz'ya and Shaposhnikova define only the "localized" version of the Sobolev space ${\textstyle W^{k,p}(\Omega )}$, nevertheless explicitly asserting that the same method is used to define local versions of all other Banach spaces used in the cited book. In particular, ${\textstyle L_{1,{\text{loc}}}(\Omega )}$ is introduced on page 44.
7. Not to be confused with the Hausdorff distance.
8. The notion of compactness must obviously be defined on the given abstract measure space.
9. This is the approach developed for example by Cafiero (1959 , pp. 285–342) and by Saks (1937 , chapter I), without dealing explicitly with the locally integrable case.
10. ( Dinculeanu 1966 , p. 163).
11. ( Dinculeanu 1966 , p. 7).
12. ( Dinculeanu 1966 , pp. 8−9).
13. See for example ( Vladimirov 2002 , p. 3) and ( Maz'ya & Poborchi 1997 , p. 4).
14. As remarked in the previous section, this is the approach adopted by Maz'ya & Shaposhnikova (2009), without developing the elementary details.
15. Precisely, they form a vector subspace of ${\textstyle L_{p,{\text{loc}}}(\Omega )}$: see Corollary 1 to Theorem 2 .
16. See for example ( Vladimirov 2002 , p. 3), where a calligraphic ℒ is used.
17. See ( Gilbarg & Trudinger 2001 , p. 147), ( Maz'ya & Poborchi 1997 , p. 5) for a statement of this results, and also the brief notes in ( Maz'ja 1985 , p. 6) and ( Maz'ya 2011 , p. 2).
18. In turn this simply means that the boundaries of two sets of the family with different index do not touch.
19. Gilbarg & Trudinger (2001 , p. 147) and Maz'ya & Poborchi (1997 , p. 5) only sketch very briefly the method of proof, while in ( Maz'ja 1985 , p. 6) and ( Maz'ya 2011 , p. 2) it is assumed as a known result, from which the subsequent development starts.
20. According to Saks (1937 , p. 36), "If ${\textstyle E}$ is a set of finite measure, or, more generally the sum of a sequence of sets of finite measure ${\textstyle \mu }$, then, in order that an additive function of a set ${\textstyle {\boldsymbol {\mathfrak {X}}}}$ on ${\textstyle E}$ be absolutely continuous on ${\textstyle E}$, it is necessary and sufficient that this function of a set be the indefinite integral of some integrable function of a point of ${\textstyle E}$". Assuming ${\textstyle \mu }$ to be the Lebesgue measure, the two statements can be seen to be equivalent.
21. See for example ( Hörmander 1990 , p. 37).
22. See ( Strichartz 2003 , p. 12).
23. See ( Schwartz 1998 , p. 19).
24. See ( Vladimirov 2002 , pp. 19–21).
25. See ( Vladimirov 2002 , p. 21).
26. For a brief discussion of this example, see ( Schwartz 1998 , pp. 131–132).

References

• Cafiero, Federico (1959), Misura e integrazione, Monografie matematiche del Consiglio Nazionale delle Ricerche (in Italian), vol. 5, Roma: Edizioni Cremonese, pp. VII+451, MR   0215954, Zbl   0171.01503 . 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.
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• Maz'ya, Vladimir G.; Poborchi, Sergei V. (1997), Differentiable Functions on Bad Domains, Singapore–New Jersey–London–Hong Kong: World Scientific, pp. xx+481, ISBN   981-02-2767-1, MR   1643072, Zbl   0918.46033 .
• Maz'ya, Vladimir G.; Shaposhnikova, Tatyana O. (2009), Theory of Sobolev multipliers. With applications to differential and integral operators, Grundlehren der Mathematischen Wissenschaft, vol. 337, Heidelberg: Springer-Verlag, pp. xiii+609, ISBN   978-3-540-69490-8, MR   2457601, Zbl   1157.46001 .
• Meise, Reinhold; Vogt, Dietmar (1997), Introduction to Functional Analysis, Oxford Graduate Texts in Mathematics, vol. 2, Oxford: Clarendon Press, pp. x+437, ISBN   0-19-851485-9, MR   1483073, Zbl   0924.46002 .
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• Schwartz, Laurent (1998) [1966], Théorie des distributions, Publications de l'Institut de Mathématique de l'Université de Strasbourg (in French) (Nouvelle ed.), Paris: Hermann Éditeurs, pp. xiii+420, ISBN   2-7056-5551-4, MR   0209834, Zbl   0149.09501 .
• Strichartz, Robert S. (2003), A Guide to Distribution Theory and Fourier Transforms (2nd printing ed.), River Edge, NJ: World Scientific Publishers, pp. x+226, ISBN   981-238-430-8, MR   2000535, Zbl   1029.46039 .
• Vladimirov, V. S. (2002), Methods of the theory of generalized functions, Analytical Methods and Special Functions, vol. 6, London–New York: Taylor & Francis, pp. XII+353, ISBN   0-415-27356-0, MR   2012831, Zbl   1078.46029 . 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

• Rowland, Todd. "Locally integrable". MathWorld .
• Vinogradova, I.A. (2001) [1994], "Locally integrable function", Encyclopedia of Mathematics , EMS Press

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