Abstract
In this research, we aim to explore generalizations of Hardy-type inequalities using nabla Hölder’s inequality, nabla Jensen’s inequality, chain rule on nabla calculus and leveraging the properties of convex and submultiplicative functions. Nabla calculus on time scales provides a unified framework that unifies continuous and discrete calculus, making it a powerful tool for studying various mathematical problems on time scales. By utilizing this approach, we seek to extend Hardy-type inequalities beyond their classical continuous or discrete settings to a more general time scale domain. As specific instances of our discoveries, we have the integral inequalities previously established in the existing literature.
Keywords:
Hardy’s inequality; Hölder’s inequality; Jensen’s inequality; convex functions; submultiplicative functions; nabla calculus; time scales MSC:
26D10; 26D15; 34N05; 47B38; 39A12
1. Introduction
In the work [1], Hardy introduced a discrete inequality:
where for and is sharp.
In [2], the author used the calculus of variations to show the integral form of (1). Specifically, he showed that if and integrable over and then
Since the first Hardy-type inequalities were presented, more and more researchers have expanded upon these inequalities and used them in a variety of fields, see the papers [3,4,5] and the books [6,7,8].
In [9], the authors demonstrated that, for the reversed sign applies to (2). They explicitly mentioned that if such that then
where is sharp.
In [10], Levinson proved that (2) still holds for parameters and That is, the inequality
is valid for
In [11], G. H. Hardy showcased an overarching generalization of (2) as, if and , then
and
In [12], the author found that
This formulation is recognized as the Knopp inequality in mathematical literature, while its discrete counterpart is referred to as the Carleman inequality:
In [13], S. Kaijser et al. showed that
where is a convex function and is a locally integrable function.
Later, in [14] authors generalized (7) as follows: Let s.t. be locally integrable in , be convex on Then,
holds s.t. and
In [15], Sulaiman proved that if are nondecreasing functions and then
In the same paper [15], he proved that if and s.t. is non-increasing, then
Also, he established if and s.t. is convex, then
where
In the last few years, there has been significant attention given to the study of inequalities (1) and (2) on a time scale , which is referred to as an arbitrary nonempty closed subset of . The emergence of this novel idea has motivated scholars to investigate Hardy-type inequalities on . The initial exploration of this direction is credited to P. Řehák [16]. In fact, he demonstrated that
Given that and . If, furthermore, as then is sharp. Refer to Section 2 for the notations used here and in the upcoming content.
In [17], the authors derived the subsequent Jensen’s inequality over
where is a convex function.
Furthermore, the authors ([18], Theorem 7.1.3) utilized the aforementioned outcome to establish that
where is continuous and convex such that is a integrable and
In [19], the authors demonstrated an extension of the inequality described in (12) as follows:
Here represent two time scale measure spaces, is a kernel, ,
is a convex function and s.t. are integrable functions.
Recently, the authors [20] presented a time scale form of (9)–(11) by using delta calculus, respectively, as follows: If are nondecreasing functions, then
and if s.t. is non-increasing, , then
Also, if s.t. is convex, then
where is a constant s.t.
For more information about Hardy’s inequality on time scales delta calculus, we suggest reading [21,22,23,24,25,26,27,28,29,30].
In continuation of this growing trend and with the aim of advancing the exploration of dynamic inequalities over , we shall present numerous novel of Hardy inequalities within this mathematical framework. The research findings will contribute to the understanding of mathematical inequalities and their applications on time scales, presenting a valuable addition to the existing body of knowledge in this field. Furthermore, these results may have potential implications in various mathematical and applied areas where time scales play a crucial role, such as difference equations, dynamic systems, and more.
While numerous outcomes exist in the realm of calculus involving the delta derivative, there remains a notable scarcity of research concerning the nabla derivative. As a result, the primary goal of this study is to expand (9)–(11) for nabla time scale calculus. The foundational principles of these principal theorems draw inspiration from the work presented in the paper [20], wherein analogous findings were delineated within the domain of time scale delta calculus.
Our discussions are organized as follows. Basic ideas and a number of lemmas related to time scale calculus are presented in the section that follows. The main findings are presented in the final section.
2. Preliminaries
Before delving into the main results, we will introduce some preliminary concepts and background information to set the foundation for our study.
In [31], the authors introduced a definition for the backward jump operator and the backward graininess function v as follows: and , respectively.
In the context of any function , signifies the value of at . Additionally, we define as .
Definition 1
([32]). Γ is left-dense continuous (continuous), if it exhibits continuity at each left-dense point in and possesses well-defined right-sided limits at right-dense points within . The set encompassing all such ld-continuous functions is denoted as .
Definition 2
([32]). Γ is differentiable at ς, if it is exists within a neighborhood V of ς and is unique (which is referred to as the nabla derivative of Γ at ς), s.t. is a neighborhood of ς s.t. and
Theorem 1
([32]). Assuming κ and χ are differentiable at , then
- 1.
- The product is differentiable at s andholds.
- 2.
- If , then is differentiable at s andholds.
Lemma 1
([33]). Assuming is continuously differentiable, is continuous and differentiable, then
Definition 3
([32]). Λ is considered a antiderivative of κ if true for . The nabla integral of κ is defined as
Theorem 2
([32]). Let and . Then
- 1.
- ,
- 2.
- ,
- 3.
- If , then
- 4.
- If , then
Lemma 2
(The integration by parts [32]). If and , then
Lemma 3
(Hölder’s inequality [34]). Let and such that Then,
Definition 4
([35]). is termed convex if
and s.t.
Definition 5
([36]). is submultiplicative if
Lemma 4
(Jensen’s inequality [34]). Assume and . If is continuous and convex, and then
Lemma 5
([35]). Suppose is a continuous function. If exists on and then ψ is convex.
For more information about some properties about convexity, concavity and submultiplicative functions, see [37,38,39,40,41].
3. Principal Findings
Throughout our study, we will operate under the assumption that the functions involved are ld-continuous functions and we will consider integrals that exist within the context.
Theorem 3.
Given and considering non-decreasing functions , we have
where
Proof.
Remark 1.
When and we get (9), which is established in [15].
Theorem 4.
Assume and such that is non-increasing, and Consider the definition of F as provided in (23). Additionally, suppose be a constant s.t.
then,
Proof.
From (23), we see that
Applying (20) on with we obtain
Substituting (30) into (29), we obtain
Applying (18) on for we see that
Since , we see that
and then
Using(32) in (31), we get
Applying (19) on with and we observe that
where
Since we have
Since the function is non-increasing, we get
Using (36) in (35), we find that
Applying (18) on we see that
where Since and it is evident that
and then we get
By integrating the previous inequality over from to (taking into account that and ), we derive
Using (39) in (37), we obtain
Using (40) in (33), we observe that
Using (27) in (41), we see that
which is (28). □
Theorem 5.
Assume and s.t. ψ is convex. Consider the definition of F as provided in (23). Additionally, suppose be a constant s.t.
then,
Proof.
Note that
Applying (21), we find that
Substituting (45) into (44), we have that
Applying (30) on with we find that
Substituting (47) into (46), we find that
Applying (18) on we find that
where Since and we observe that
and then we have from (49) that
Integrating (50) over from to s, we see that
Substituting (51) into (48), we observe that
Applying (19) on with and we see that
where
Since we have
Substituting (54) into (52), we obtain
Applying (18) on we get
where Since and we see that
and then we have from (56) that
Integrating (57) over from to considering that yields
Using (58) in (55), we have
Using (42), the last inequality becomes
which is (43). □
Remark 3.
Now, the upcoming lemma is both novel and crucial for demonstrating our fundamental outcomes.
Lemma 6.
Let and be convex and submultiplicative on such that Then, is non-decreasing.
Proof.
Theorem 6.
Assume and . Additionally, consider , where ψ is both convex and submultiplicative, and it satisfies we can establish that
where F is given in (23).
Proof.
It’s important to note that
As is a submultiplicative, we can deduce that
Applying (21), we obtain
Using (68) in (67), we get
Applying (18) on with and then
where
Since we have from (70) that
Since we have and
Subsequently, we can conclude from (71) that
By employingLemma 6, taking into account the non-decreasing nature of , we can deduce and thus
Form (73) and (72), we get
Applying (18), on we have that
where Since and we get
Integrating (75) over from to we obtain
Using (76) in (74), we get
Using (77) in (69), we obtain
which is (65). □
4. Conclusions
In this research, we have successfully demonstrated several dynamic inequalities of Hardy type by utilizing nabla calculus, specifically for convex, submultiplicative functions, and monotone functions. Looking ahead, we intend to extend our exploration by presenting similar inequalities using diamond- calculus for as well as quantum calculus. It is intriguing to consider the prospect of introducing analogous inequalities on time scales through Riemann–Liouville-type fractional integrals. Moreover, there’s potential for us to generalize the dynamic inequalities discussed in this article to two or more dimensions, incorporating symmetry in both the functions and variables. The concept of symmetry has various implications for convex functions, submultiplicative functions and Hardy-type inequalities, impacting their properties, behavior, and generalization. Recognizing and utilizing symmetry can aid in proving inequalities and understanding their solutions and applications. By addressing the suggested future research directions, scholars can continue to deepen their understanding of Hardy-type inequalities and their broader implications. This research is a stepping stone towards further advancements in the field.
Author Contributions
Investigation, software and writing—original draft, H.M.R. and A.I.S.; supervision, writing—review editing and funding, O.S.B. and M.I.M. All authors have read and agreed to the published version of the manuscript.
Funding
This research project was supported by the Researchers Supporting Project Number (RSPD2023R1004), King Saud University, Riyadh, Saudi Arabia.
Data Availability Statement
Not applicable.
Acknowledgments
This research project was supported by the Researchers Supporting Project Number (RSPD2023R1004), King Saud University, Riyadh, Saudi Arabia.
Conflicts of Interest
The authors declare no conflict of interest.
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