Abstract
Fractional integral operators are useful tools for generalizing classical integral inequalities. Convex functions play very important role in the theory of mathematical inequalities. This paper aims to investigate the Hadamard type inequalities for a generalized class of functions namely strongly -p-convex functions by using Riemann–Liouville fractional integrals. The results established in this paper give refinements of various well-known inequalities which have been published in the recent past.
Keywords:
Riemann–Liouville integrals; Hadamard inequality; strongly convex function; convex function MSC:
26A51; 26A33; 33E12
1. Introduction
Convex functions are very important in the study of mathematical inequalities and in solving the problems of optimization theory. Many well-known inequalities are direct consequences of these functions. Convex functions are further analyzed to define new classes of functions which are helpful in studying the extensions and generalizations of classical results. Because of their fascinating properties, convex functions are used in almost all areas of mathematics including analysis, optimization theory, and graph theory, etc. A refinement of convex function is the notion of strongly convex function introduced by Polyak in [1]. For more details one can see [1,2]. By using the definition of strongly convex functions it is quite possible to obtain refinements of inequalities which have been established for convex functions in the literature.
The Hadamard inequality is the most popular inequality which is studied for new classes of functions defined after motivating by analytic representation of convex functions given in (1). Consequently, a lot of generalizations, refinements, and extensions of the Hadamard inequality can be found in the literature.
Inspired by a rich literature dedicated to convex functions and the Hadamard inequality, in this paper we aim to define a new class of functions namely strongly -p-convex functions. This will generate many well known classes of functions such as; -convex [3], -convex [4], -convex [5], -convex [6], h-convex [7], p-convex [8], and harmonically convex functions [9] which are further linked with classical definitions of several types of convex functions. The newly defined class of functions also provide refinements of all aforementioned functions. Hence a strongly -p-convex function unifies several types of convexities and strongly convexities.
Moreover, it is used to establish new versions of the Hadamard type inequalities for Riemann–Liouville fractional integrals. These inequalities will work as generalizations as well as refinements of a lot of versions of this inequality which have been established in recent decades.
The Hadamard type inequalities which we have proved in this paper unify numerous published versions of such inequalities already exist in the literature of fractional integral inequalities. Following definitions and results will be useful to obtain connections and understandings with the findings of this paper.
Definition 1.
Let be an interval in . Then a real valued function is said to be convex function, if
for all and . If (1) holds in reverse direction, then f will be called concave function.
Every convex function f on an interval can be modified at the endpoints to become convex and continuous. An immediate consequence of this fact is the Riemann integrability of f. The Riemann integral of convex function f is estimated by the Hadamard inequality stated in the following theorem, see (Section 1.9 [10]).
Theorem 1.
Let be a convex function. Then the following inequality holds:
If f is concave, then the inequality (2) holds in reverse direction.
Definition 2
([11]). A function , is said to be m-convex, where if we have
for all and . We say that f is m-concave if is m-convex.
The definitions of strongly convex functions of different kinds are given as follows:
Definition 3
([1]). Let be a normed space and E be a convex subset of . A function will be called strongly convex function with modulus , if
holds .
Definition 4
([12]). A function is called strongly m-convex function with modulus if
for and
Definition 5
([13]). A function is said to be strongly -convex function, with modulus , for , if
holds for all and .
Definition 6
([14]). A function is said to be strongly -convex, where if
holds for all
Definition 7
([15]). Let be an interval containing and let be a non-negative function. A function is called strongly -convex function with modulus , if f is non-negative and for all and , one has
Several variants of inequality (2) in the form of generalizations, extensions and refinements have been published, see [6,16,17,18,19] and references therein. In [20], the notion of p-convex function is introduced and in [6] it is extended to the notion of -convex function. Likewise many such classes of functions are defined to establish the Hadamard type inequalities, see [17,21,22,23]. In recent decades, the Hadamard inequality has been studied for different kinds of fractional integral operators, see [16,24,25,26,27,28].
The classical Riemann–Liouville integrals of fractional order and the Hadamard inequalities for these integrals are given in the following definition and theorem, respectively:
Definition 8
([29]). Let . Then the left and right sided Riemann–Liouville integrals of the function f of fractional order are defined as follows:
and
The Hadamard inequality for Riemann–Liouville fractional integrals is composed in the following theorem:
Theorem 2
([30]). Let be positive and convex function on . Then, the following inequality holds:
with .
The definition of k-analogue of the Riemann–Liouville integrals is stated as follows:
Definition 9
([31]). Let . Then the left and right sided k-fractional Riemann–Liouville integrals of the function f of fractional order , are defined as follows:
and
where .
In the upcoming section, we define a generalize class of functions will be called strongly -p-convex functions, and discuss their consequences in the form of classical and new classes of functions. In Section 3, we prove Hadamard type inequalities for strongly -p-convex and related functions via integrals (8) and (9). Furthermore, we obtain refinements of some fractional versions of Hadamard inequalities proved in [6,9,20,23,26,27,28,30,32,33,34,35,36,37,38]. By using a parameter substitution, k-fractional versions of Hadamard inequalities which have proved in Section 3 can be obtained.
2. Some New Definitions
In this Section, we define a generalized class of functions, which reproduce several kinds of convex and strongly convex functions along with some new deduced definitions.
Definition 10.
Let be an interval containing and let be a non-negative function. A function , is called strongly (α, h-m)-p-convex with modulus , if f is non-negative and
holds for all , , and .
It is interesting to note that a number of already known definitions are direct consequences of the above definition. For specific settings of involved symbols and function h in the inequality (15), one can obtain easily strongly p-convex [36], strongly convex [1], strongly harmonic convex [37], p-convex [20], -convex [6], -p-convex [38], -convex [26], -p-Godunova–Levin [39], -p-convex, harmonically convex [9], P-function, harmonic s-convex [40], harmonically h-convex [41], -HA-convex [42], and reciprocally -convex [22] functions.
It can be noted that when , also for , we obtain the definition of star-shaped function. Some new induced definitions are given as follows:
We will say that the function f is strongly -p-convex with modulus , in the second sense if (15) is considered for .
We will say that the function f is strongly -p-convex with modulus , if (15) is considered for and .
We will say that the function f is strongly -convex with modulus , in the second sense if (15) is considered for .
We will say that the function f is strongly -function with modulus , in the second sense if (15) is considered for , .
We will say that the function f is Godunova–Levin type of strongly harmonic convex with modulus , in the second sense if (15) is considered for , and .
We will say that the function f is strongly -function with modulus , in the second sense if (15) is considered for , .
We will say that the function f is strongly harmonic s-convex with modulus , in the second sense if (15) is considered for , and .
We will say that the function f is strongly harmonic P-function with modulus , in the second sense if (15) is considered for and .
We will say that the function f is strongly -HA-convex with modulus , if (15) is considered for and .
We will say that the function f is strongly -HA-convex with modulus , if (15) is considered for , and .
We will say that the function f is Godunova–Levin type of strongly -HA-convex with modulus , if (15) is considered for , and .
3. Fractional Versions of Hadamard-Type Inequalities for Strongly --Convex Functions
In this section, we prove the Hadamard-type inequality for strongly -p-convex functions and further study its consequences.
Theorem 3.
Let f be a positive function such that . If f is a strongly -p-convex function on and . Then, for and , the following inequalities hold:
- (i)
- If ,with , for all .
- (ii)
- If ,with , for all .
Proof.
(i) The following inequality holds for strongly -p-convex function
By setting , in (18) and integrating the resulting inequality over the interval after multiplying with , we obtain
Now, let such that , that is, and let such that , that is, in (19), then multiplying by after applying Definition 8, we obtain the following inequality
From which one can obtain the first inequality of (16). Again using strongly -p-convexity of f and integrating the resulting inequality over the interval after multiplying with , we obtain
Again using substitution as considered in (19), the above inequality leads to the second inequality of (16).
(ii) Proof is similar to the proof of (i). □
The following remark establishes the connection with several already published inequalities.
Remark 1.
(i) If in (17), then one can obtain (Theorem 2.2 [38]). (ii) If , , and in (17), then one can obtain (Theorem 4 [34]).
(iii) If , , and in (17), then one can obtain (Theorem 2.4 [9]).
(iv) If and in (16), then one can obtain (Theorem 3.10 [38]).
(v) If and in (16), then one can obtain (Corollary 2.2 [26]).
(vi) If , and in (16), then Theorem 2 is obtained.
(vii) If , and in (16), then one can obtain the Hadamard inequality.
(viii) If , and in (16), then one can obtain (Theorem 2.1 [33]).
(ix) If , , and in (17), then one can obtain (Theorem 2.1 [20]).
(x) If , and in (16), then one can obtain (Theorem 2.1 [23]).
(xii) If and in (16), then one can obtain (Theorem 6 [32]).
(xiii) If and in (16), then one can obtain (Theorem 6 [35]).
(xiv) If , and in (17), then one can obtain (Theorem 2.1 [37]).
(xv) If and in (16), then one can obtain (Theorem 2.1 [28]).
Corollary 1.
The following inequalities hold for strongly -p-convex function:
- (i)
- If ,
- (ii)
- If ,
Proof.
Remark 2.
Corollary 2.
The following inequalities hold for strongly Godunova–Levin type -p-convex function:
- (i)
- If ,
- (ii)
- If ,
Remark 3.
Corollary 3.
The following inequalities hold for strongly -p-convex function in third sense:
- (i)
- If ,
- (ii)
- If ,
Proof.
Corollary 4.
The following inequalities hold for strongly -p-convex function:
- (i)
- If ,
- (ii)
- If ,
Proof.
Corollary 5.
The following inequality holds for strongly -HA-convex functions in second sense:
Proof.
If and in (17), then after some computations the above inequality can be obtained. □
Corollary 6.
The following inequality holds for strongly -HA-convex functions in second sense:
Proof.
If , and in (17), then after some computations the above inequality can be obtained. □
Corollary 7.
The following inequality holds for Godunova–Levin type of strongly -HA-convex functions:
Proof.
If , and in (17), then after some computations the above inequality can be obtained. □
Remark 4.
Lemma 1.
Let and be a differentiable mapping on . Furthermore, suppose that and . Then, the following identities hold:
- (i)
- If ,with , for all .
- (ii)
- If ,with , for all .
Proof.
Consider
Integrating by parts, we have
Setting in above equation, we have
Now we will evaluate
(ii) The proof is similar with (i). □
Remark 5.
(i) If , then one can obtain (Lemma 2 [30]).
(ii) If , then one can obtain (Lemma 2.4 [33]).
Theorem 4.
Let f be a positive function such that . If f is a strongly -p-convex function on and . Then, for and , the following inequalities hold:
- (i)
- ,with , for all .
- (ii)
- If ,with , for all .
Proof.
From Lemma 1 and using strongly -p-convexity of , we have
□
Remark 6.
(i) If , and in (37), then one can obtain (Theorem 3 [30]).
(ii) If , and in (37), then one can obtain (Theorem 2.2 [43]).
Corollary 8.
The following inequalities hold for strongly -p-convex function:
- (i)
- If ,
- (ii)
- If ,
Proof.
Corollary 9.
The following inequalities hold for strongly Godunova–Levin type -p-convex function:
- (i)
- If ,
- (ii)
- If ,
Proof.
Corollary 10.
The following inequalities hold for strongly -p-convex function in third sense:
- (i)
- If ,
- (ii)
- If ,
Proof.
Corollary 11.
The following inequalities hold for strongly -p-convex function:
- (i)
- If ,
Proof.
If in (37), then one can obtain the above inequality. □
- (ii)
- ,
Proof.
If in (38), then one can obtain the above inequality. □
Corollary 12.
The following inequality holds for strongly -HA-convex functions in second sense:
Proof.
If and in (38), then one can obtain the above inequality. □
Corollary 13.
The following inequality holds for strongly -HA-convex functions in second sense:
Proof.
If , and in (38), then one can obtain the above inequality. □
Corollary 14.
The following inequality holds for Godunova–Levin type of strongly -HA-convex functions:
Proof.
If , and in (38), then one can obtain the above inequality. □
4. Conclusions
The results obtained in this paper simultaneously produce the refinements of Hadamard inequalities for the Riemann–Liouville fractional integrals of -convex, -convex, -convex and -convex functions. The Hadamard type inequalities published in recent articles [9,20,23,26,28,30,33,34,35,37,38] are special cases of the results of this paper. Several new Hadamard inequalities are deducible for new classes of functions defined in Section 2, some of them are given in Corollaries 1–14. The new definition is applicable to extend the classical inequalities for convex functions.
Author Contributions
Conceptualization, T.Y., G.F., H.Y., S.H.S. and C.Y.J.; Investigation, H.Y., S.H.S. and C.Y.J.; Methodology, T.Y. and C.Y.J.; Supervision, G.F.; Writing—original draft, H.Y.; Writing—review & editing, T.Y., G.F., H.Y., S.H.S. and C.Y.J. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Not applicable.
Conflicts of Interest
The authors declare no conflict of interest.
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