Abstract and Applied Analysis

Abstract and Applied Analysis / 2021 / Article

Research Article | Open Access

Volume 2021 |Article ID 6690328 | https://doi.org/10.1155/2021/6690328

Oussama Kabbouch, Mustapha Najmeddine, "The Multiple -Riemann Integral", Abstract and Applied Analysis, vol. 2021, Article ID 6690328, 10 pages, 2021. https://doi.org/10.1155/2021/6690328

The Multiple -Riemann Integral

Academic Editor: Wing-Sum Cheung
Received27 Oct 2020
Accepted10 Mar 2021
Published22 Apr 2021

Abstract

The aim of this paper is to extend the notion of -Riemann integrability of functions defined over to functions defined over a rectangular box of . As a generalization of step functions, we introduce a notion of -step functions which allows us to give an equivalent definition of the -Riemann integrable functions.

1. Introduction

The motivation of this work results from the notion of the -Riemann integral of a function defined over an interval of recently introduced by Olbrys’ [1]. Our aim of this paper is to give an equivalent definition of the -Riemann integrability by the use of -step functions and extend this notion to functions defined over a rectangular box of .After introducing the notion of -multiple integral, we give its properties and we show a -version of Fubini’s theorem.

Throughout this paper, denotes a subfield of the field of real numbers . We denote the set of positive elements of by . For two real numbers , we denote If is a rectangular box of , we denote Following [1], we define the set of the -partitions of the interval by As in [1], we give a generalization of the radially -continuous and uniformly radially -continuous functions and propositions easy to prove.We say that is radially -continuous if it is radially -continuous at every point.

Definition 1. A function is called radially -continuous at a point if for every

Definition 2. We say that a function is uniformly radially -continuous if for any and , the mapping is uniformly continuous.

Every continuous and any uniformly continuous function in the usual sense is radially -continuous and uniformly radially -continuous, respectively.

Proposition 3. (i)A function is radially -continuous if and only if for every subrectangular box of , the function is continuous(ii)A function is uniformly radially -continuous if and only if for every subrectangular box of , the function is uniformly continuous

2. -Step Functions and -Riemann Integral

In the following definition, we introduce the notion of the -step functions which are a generalization of step functions.

Definition 4. A function is said to be a -step function on , if there exists a -partition of for which is constant on each set . In this case, we say that is a -compatible partition of on .

We denote the set of the -step functions on by .

Remark 5. (i)Every -step function on is bounded on .(ii)If , we get the usual definition of a step function.

Example 1. (i)Every constant function on is a -step function.(ii)Consider the following functions:It is easy to show that is a -step function but it is not a -step function on , and is a -step function, but it is not a -step function on .

Now, suppose that is a -step function on and let be a -compatible partition of , with whenever for all . Consider the following number:

We can easily show that the number does not depend on the choice of the -compatible partition.

Definition 6. Let be a -step function on , and a -compatible partition of . The real number is called the -integral of on and is denoted by

As for the introduction of the usual Riemann integral, we extend the definition of the -Riemann integral to a broader class of functions than -step functions.

Definition 7. A function is said to be -Riemann integrable on , if for every , there exist two -step functions on such that

We denote the set of the -Riemann integrable functions by .

Remark 8. One can easily verify that is a part of .

Let be a bounded function on and set

Note that and

Proposition 9. Let be a bounded function on . Then, if and only if

Proof. Suppose that . Given , then there exist such that Since then, Therefore, Conversely, let , then there are such that Then, Therefore, .

Definition 10. Let , the common value is called the -integral of on ; it is denoted by

In the case when , we will use the standard symbol instead of .

Now, we will show that our definition of the -integral is equivalent to that recently introduced by Olbrys’ ([1], Definition 7)

Proposition 11. A function is -Riemann integrable on in the sense of Definition 7 if and only if it is -Riemann integrable on in the sense of ([1], Definition 7) (meaning the two definitions are equivalents). Furthermore, when , we have where

Proof. Suppose that in the sense of ([1], Definition 7). Given , then there exists a partition of such that where and . We have thus built two functions such that where and are defined on by and Therefore, is -Riemann integrable on in the sense of Definition 7.
Conversely, suppose that is -Riemann integrable on in the sense of Definition 7. Then, there exist and such that Let be a common -compatible partition of and , and let Then, which shows This means that is -Riemann integrable on in the sense of ([1], Definition 7).
Finally, if , then the equalities (19) are ensured by Proposition 9 and the inequalities (26).

3. -Multiple Integral

This section is devoted to -multiple Riemann integral. For the theory of the classical multiple Riemann integral, see for instance [2].

Let be a closed rectangular box in , where are real numbers such that for all .

We denoted by

Definition 12. A rectangular box is said to be a -subrectangular box of if for all .

Example 2. is a -subrectangular box of .
is not a -subrectangular box of .

Definition 13. A -partition of is a finite collection of -subrectangular box of such that where is the interior of .

Definition 14. (i)Let be a bounded function over . is said to be a -step function on if there exists a -partition such that is constant on for all In this case, is said to be a -compatible partition of on We denoted the set of the -step functions on by . (ii)A -refinement of a -partition of is another -partition of such that each is a -subrectangular box of some

Let be a -step function and be a -compatible partition of such that for all . We define the number

We will first show that does not depend on the choice of the -compatible partition .

Lemma 15. Let be a -step function and be a -compatible partition of . Then, for every -refinement of , we have

Proof. Let be a -compatible partition of such that for all and be a -refinement of . For all , there is such that Then,

Corollary 16. Let be a -step function and be a -compatible partition of . The number does not depend on the choice of the -compatible partition .

Proof. Let and be two -compatible partitions of , then , where , is a -refinement of and . Therefore, by using Lemma 15, we get

Definition 17. Let be a -step function and be a -compatible partition of . We called the -Riemann integral of on and we denote it by

In the following proposition, we present some basic properties of -Riemann multiple integral of -step functions.

Proposition 18. Let be two -step functions on and . Then, (1)The function is a -step function on ; moreover,(2)If , then (3)The absolute value is a -step function on and (4)If is a -partition on , then

Proof. (1) Let be a -compatible partition of , then , where , is a -compatible partition of ; moreover, we have (2) and (3) are evident.
(4) Let be a -compatible partition of such that for all . Then, Since is a -compatible partition of on . Then,

3.1. -Riemann Integrable Functions Defined over

Definition 19. Let be a function, we said that is a -Riemann integrable function, if for every , there exist two -step functions and on such that

We denote the set of the -Riemann integrable functions by .

Remark 20. (i)Every -Riemann integrable function on is bounded on (ii) is a part of

Now, suppose that is a bounded function on . Set

Note that for all and

Theorem 21. Let be a bounded function on . Then, if and only if .

Proof. Suppose that . Given , there exists and in such that Then, which shows that for all Therefore, Conversely, suppose that . Let , by the definition of the greatest lower bound and the least upper bound, there are such that Then, Therefore,

Definition 22. Let , the common value is called the -Riemann integral of on , and we denote it by

In the case when , we will use the standard symbol instead of .

Proposition 23. Let be a function. Then, is -Riemann integrable on if and only if there exist two sequences of -step functions and such that

Moreover, we have Before presenting a version of the Fubini theorem for -Riemann integrable functions. We start by giving a lemma that is essential for the proof of this result.

Proof. It runs in a similar way as for the case of one variable. Suppose that is -integrable. Then, For all , there exist two -step functions and on such that Hence, . Now, let us show that Let . We have Then, Hence, The converse is trivial.

Lemma 24. Let be two subfields of and let be a -step function. Then, is a -Riemann integrable function. Moreover, we have

Proof. Let be a -compatible partition of such that for all . For all , there is a sequence of -subrectangular of such that Set where is the closure of . Then, there is a sequence of -subrectangular of such that Now, consider the functions We have is a -compatible partition of and on ; moreover, Since is a decreasing sequence, then then Hence, is -Riemann integrable. Moreover,

Theorem 25. Let be two subfields of . Then, and

Proof. Let , given , there exist such that By Lemma 24, there exist two sequences and of -step functions such that with Then, there is such that Hence, Therefore, and

Corollary 26. Let be a uniformly radially -continuous function, then it is -Riemann integrable on any subrectangular of .

Proof. Let be a subrectangular of . By Proposition 3, is uniformly continuous on . Since , where is the closure of , then there exists a unique continuous function such that . Since is Riemann integrable on then, using Theorem 25, is -Riemann integrable on ; moreover,

Lemma 27. Let be a -step function on . Then, there is a -partition of and a -partition of such that the function is a -step function and is a -compatible partition of whenever . Moreover, the function is a -step function on and

Proof. Let be a -partition of , then there exists a -refinement of such that and are two