In this paper, we introduce a new class of -convex functions which generalize P-functions and convex, -convex, Godunova-Levin functions, and we give some properties of the functions. Moreover, we establish the corresponding Schur, Jensen, and Hadamard types of inequalities.
MSC:35K65, 35B33, 35B40.
Hinweise
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
All authors contributed equally to the manuscript and read and approved the final manuscript.
1 Introduction
Let I and J be intervals in R. To motivate our work, let us recall the definitions of some special classes of functions.
A function is said to be a Godunova-Levin function or belongs to the class if f is non-negative and
for all and .
The class was firstly described in [1] by Godunova and Levin. Some further properties of it are given in [2, 3]. It has been known that non-negative convex and monotone functions belong to this class of functions.
Let be a fixed real number. A function is said to be an s-convex function (in the second sense) or belongs to the class , if
for all and .
An s-convex function was introduced by Breckner [4] and a number of properties and connections with s-convexity (in the first sense) were discussed in [5]. Of course, s-convexity means just convexity when .
Let be a non-negative and non-zero function. We say that is an h-convex function or that f belongs to the class , if f is non-negative and
for all and .
The h- and p-convex functions were introduced by Varšanec, Zhang and Wan, and a number of properties and Jensen’s inequalities of the functions were established (cf. [8]). As one can see, the definitions of the P-function, convex, -convex, Godunova-Levin functions have similar forms. This observation leads us to generalize these varieties of convexity.
2 Definitions and basic results
In this section, we give new definitions and properties of the -convex function. Throughout this paper, we assume that , f and h are real non-negative functions defined on I and J, respectively, and the set I is p-convex when or . We first give a definition of the new class of convex functions.
Definition 6 Let be a non-negative and non-zero function. We say that is a -convex function or that f belongs to the class , if f is non-negative and
(2.1)
for all and . Similarly, if the inequality sign in (2.1) is reversed, then f is said to be a -concave function or belong to the class .
Remark 2 It can be obviously seen that if , then all non-negative p-convex and p-concave functions belong to and , respectively; if and , then all non-negative convex functions belong to ; if and , then ; if , , and , then ; if and , then , and if , then .
Example 1 Let , where and . If f is a function defined as , where p is an odd number and , we then have
and hence, f belongs to .
Next, we discuss some interesting properties of -convex (concave) functions, which include linearity, product, composition properties, and an ordered property of h and p. In addition, we give some interesting properties of the -convex function, when h is a super(sub)-multiplicative function.
Property 1Ifand , then . Similarly, ifand , then .
Proof The proof immediately follows from the definitions of the classes and . □
Property 2Letandbe non-negative functions defined on an intervalJwithin . If , then . Similarly, if , then .
Proof If , then for any and we have
and hence, . □
Property 3Let .
(a)
For , iffis monotone increasing (monotone decreasing), andor , and ( or ), then .
(b)
For , iffis monotone increasing (monotone decreasing), andor , and ( or ), then .
Let .
(c)
For , iffis monotone increasing (monotone decreasing), andor , and ( or ), then .
(d)
For , iffis monotone increasing (monotone decreasing), andor , and ( or ), then .
Proof (a) Setting , we have
When and , we have , and so . We then obtain
since f is monotone increasing and . Therefore, we get .
The results of (b), (c), and (d) follow by similar arguments as above. □
A function is called a super-multiplicative function if
(2.3)
for all .
If the inequality sign in (2.3) is reversed, then h is said to be a sub-multiplicative function, and if the equality holds in (2.3), then h is called a multiplicative function.
Example 2 Let . If , then h is a multiplicative function. If , then h is a sub-multiplicative function, and if , then h is a super-multiplicative function.
Property 5LetIbe an interval such that . We then have the following.
(a)
If , , andhis super-multiplicative, then the inequality
(2.4)
holds for alland allsuch that .
(b)
Lethbe a non-negative function withfor some . Iffis a non-negative function satisfying (2.4) for alland allwith , then .
(c)
If , , andhis sub-multiplicative, then the inequality
(2.5)
holds for alland allsuch that .
(d)
Lethbe a non-negative function withfor some . Iffis a non-negative function satisfying (2.5) for alland allwith , then .
Proof (a) Let , , and let a and b be numbers such that and . We then have and
(b) If , then . Setting in (2.4), we get
By setting , where , and dividing both sides of the inequality above by , we obtain for all , which is a contradiction to the assumption for some , and so .
The results of (c) and (d) follow by using similar arguments as above, and so we omit the proofs here. □
Corollary 1Let , where , and let . For all , inequality (2.4) holds for allwithif and only if . For all , inequality (2.5) holds for allwithif and only if .
Proof Let , , and let a and b be positive numbers such that and . We then have and
Setting in (2.4), we get , while by the definition of the -convex function, and hence . □
Property 6Suppose that , , are functions such thatandfor all , and thatandare functions with , , and .
Ifis a super-multiplicative function, , andfis increasing (decreasing) and (), then the composite functionbelongs to . Ifis a sub-multiplicative function, , andfis increasing (decreasing) and (), then the composite functionbelongs to .
Proof If and f is an increasing function, then we have
for all and . Using Property 5(a) with , we obtain
which implies that belongs to . □
If f is a convex or concave function, then we may give a similar statement on the composite function of f and g.
Property 7Letandbe functions with . If the functionfis convex and increasing (decreasing), and () withfor , thenbelongs to . If the functionfis concave and increasing (decreasing), and () withfor , thenbelongs to .
Proof If and f is an increasing function, we then have
for all and . Since and f is convex, we obtain
which implies that belongs to . □
3 Schur-type inequalities
In this section, we establish Schur-type inequalities of -convex functions.
Theorem 1Letbe a non-negative super-multiplicative function and letbe a function such that . Then for allsuch thatand , the following inequality holds:
(3.1)
If the functionhis sub-multiplicative and , then the inequality sign in (3.1) is reversed.
Proof Let and let be the numbers stated in this theorem. Then one can easily see that
We also have
and
Setting , , and in (2.1), we have and
(3.2)
Assuming and multiplying both sides of the inequality above by , we obtain inequality (3.1). □
Remark 3 In fact, if , , , , and , then inequality (3.1) gives the Schur inequality, see [[10], p.177].
The following corollary gives a Schur-type inequality for the -convex function.
Corollary 2Ifbelongs to the classand , then we have the inequality
(3.3)
for allwith . If , then the inequality sign in (3.3) is reversed. If , , and , , thenand inequality (3.3) gives the Schur inequality.
4 Jensen-type inequalities
In this section, we introduce some Jensen-type inequalities of -convex functions.
Theorem 2Letbe positive real numbers with . Ifhis a non-negative super-multiplicative function and ifand , then we have the inequality
(4.1)
Ifhis sub-multiplicative and , then the inequality sign in (4.1) is reversed.
Proof When , inequality (4.1) holds by (2.1) with . Assuming inequality (4.1) holds for , we obtain
and, hence, the result follows by mathematical induction. □
Remark 4 For and , inequality (4.1) becomes the classical Jensen inequality.
Theorem 3Letbe positive real numbers and letbe an interval inI. Ifis a non-negative super-multiplicative function and , then for allwe have the inequality
(4.2)
Ifhis a non-negative sub-multiplicative function and , then the inequality sign in (4.2) is reversed.
Proof Setting , , and in (3.2), we get the inequalities
Multiplying both sides of the above inequality with and adding all inequalities side by side for , we obtain (4.2). □
Let K be a finite nonempty set of positive integers and let F be an index set function defined by
Theorem 4Letbe a non-negative function, and letMandKbe finite nonempty sets of positive integers such that . Ifhis super-multiplicative andbelongs to the class , then for , , we have the inequality
(4.3)
Ifhis sub-multiplicative and , then the inequality sign in (4.3) is reversed.
Proof Setting , , and in (2.1), we obtain the inequality
Multiplying both sides of the above inequality with , we get the inequality
Subtracting from both sides of the inequality above and using the identity , we obtain (4.3). □
A simple consequence of Theorem 4 is stated in the following corollary without proof.
Corollary 3Letbe a non-negative super-multiplicative function. If , , and , then forwe have
(4.4)
and
(4.5)
Ifhis sub-multiplicative and , then the inequality signs in (4.4) and (4.5) are reversed, and min is replaced with max.
Remark 5 Some results obtained from Theorem 4 and Corollary 3 are given in [[11], p.7], when , , and h is a convex or concave function.
5 Hadamard-type inequalities
In this section, we give some Hadamard-type inequalities of -convex functions.
Theorem 5Ifforwith , then we have
(5.1)
Proof Setting , we get
By using inequality (2.1) we obtain
and hence, by integrating the above inequality over , we have
which gives the second inequality.
Setting , we obtain
and, hence, the first inequality follows. □
Remark 6 If and , then inequality (5.1) gives the classical Hadamard inequality.
Theorem 6Suppose thatfandgare functions such that , , , andwithand . We then have
(5.2)
whereand .
Proof Since and , we have
for all . Because f and g are non-negative, we get the inequality
Integrating both sides of the above inequality over , we obtain the inequality
Setting , we get
□
Theorem 7Let , be functions such thatand , and letwith . We then have
(5.3)
Proof Since , we have
Integrating the above inequality over , we obtain
□
Theorem 8Letandbe functions such that , , and letwith . We then have the inequality
(5.4)
Proof Since and , we have
for all . Because f and g are non-negative, we get the inequality
Multiplying both sides of the above inequality with and integrating the result over and , we obtain the inequality
By (5.1), we have the inequality
□
Theorem 9Let , be functions such that , , and letwith . We then have the inequality
(5.5)
Proof Since and , we have the inequalities
for all . Because f and g are non-negative, we get the inequality
Multiplying both sides of the inequality above with and integrating the result over and , we obtain
By inequality (5.1), we have
□
Acknowledgements
This work is supported by the Natural Science Foundation of Shandong Province of China (ZR2012AM018) and the Fundamental Research Funds for the Central Universities (No. 201362032). The authors would like to deeply thank all the reviewers for their insightful and constructive comments.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
All authors contributed equally to the manuscript and read and approved the final manuscript.