- Research
- Open Access
- Published:
Cauchy type means for some generalized convex functions
Journal of Inequalities and Applications volume 2021, Article number: 114 (2021)
Abstract
In this paper, we establish Jensen’s inequality for s-convex functions in the first sense. By using Jensen’s inequalities, we obtain some Cauchy type means for p-convex and s-convex functions in the first sense. Also, by using Hermite–Hadamard inequalities for the respective generalized convex functions, we find new generalized Cauchy type means.
Introduction
Cauchy mean value theorem is of huge importance in mathematical analysis. Mercer [18] and Pečarić [21] made connection between Cauchy type means and Jensen’s inequality. These are given both in discrete and in integral form and have many applications. A meaningful advancement in theory of Cauchy type means is given in [1–5, 18–21]. Also see [8–11, 15–17] for more information about means. The following result is given in [19], which involves Jensen’s inequality both in numerator and denominator.
Theorem 1.1
([19])
Let \(G\subseteq \mathbb{R}\) be an interval and \(r_{i}>0\) for all \(1\leq i\leq n\) such that \(\Sigma ^{n}_{i=1}r_{i}=S_{n}\) and \(c_{1},\ldots,c_{n}\in G\) not all the same. Consider the twice differentiable functions \(\zeta _{1},\zeta _{2}:G\rightarrow \mathbb{R}\) such that
Then
Here our aim is to find some Cauchy type means for p-convex and s-convex functions in the first sense using Jensen’s and Hermite–Hadamard inequalities, respectively.
Let M, N be two bivariable means defined in a real interval G, and let \(J\subseteq G \) be a subinterval of G. According to Aumann [6], a function \(\zeta :J\rightarrow G\) is convex with respect to the pair of means \((M,N)\) if
and ζ is convex with respect to M if
These notions generalize the classical notions of convexity. Moreover, taking for M the weighted power mean, i.e.,
and for N the weighted arithmetic mean
one gets the following definition.
Definition 1.1
Let \(G\subset (0, \infty )\) be a real interval and \(p\in \mathbb{R\backslash }\{0\}\). A function \(\zeta :G\rightarrow \mathbb{R}\) is said to be a p-convex function if
for all \(g_{1},g_{2}\in G\) and \(r\in [0,1]\). If inequality (2) is reversed, then ζ is called p-concave function.
Definition 1.2
([12])
Let \(s\in (0,1]\). A function \(\zeta :[0,\infty )\rightarrow \mathbb{R}\) is called an s-convex function (in the first sense) or \(\zeta \in K^{1}_{s}\) if
for all \(g_{1},g_{2}\in \mathbb{R}^{+}=[0,\infty )\) and \(r_{1},r_{2}\geq 0\) with \(r_{1}^{s}+r_{2}^{s}=1\).
Cauchy type means for p-convex functions in Jensen’s sense
Toplu et al. [22] proved Jensen’s inequality for p-convex functions as follows.
Theorem 2.1
([22])
Let \(p\in \mathbb{R\backslash }\{0\}\) and \(\zeta :G\subset (0,\infty )\rightarrow \mathbb{R}\) be a p-convex function. Let \(g_{i}\in G\) and \(r_{i}\in [0,1]\), \(0\leq i\leq n \), then the following inequality holds:
where \(\sum^{n}_{1}r_{i}=1\).
Now, by using Theorem 2.1, we state and prove the following theorem, which gives the Cauchy type mean for p-convex function.
Theorem 2.2
Let \(G\subset (0,\infty )\) be an interval, \(p\in \mathbb{R\backslash }\{0\}\), and \(r_{i}\in [0,1]\). Let \(\zeta _{1},\zeta _{2}\in C^{2}(G)\) be p-convex functions. Then there exist some \(\chi \in G\) such that the following equality holds:
with each \(r_{i}\in [0,1]\) such that \(\sum^{n}_{1}r_{i}=1\) and provided that the denominators are non-zero.
Proof
Let us define
and
where \(\lambda \in [0,1]\). Similarly, we define \((T\zeta _{2})(\lambda )\).
Note that
and
Now consider a function \(Q(\lambda )\) defined as follows:
such that we have
Then from two applications of mean value theorem, we have \(\upsilon \in G\) so that
It implies that
For some fixed υ, the expression in the square brackets in (6) is a continuous function of \(g_{i}\), so it vanishes. Corresponding to that value of \(g_{i}\), we can have a number
such that
This gives equality (5). □
Corollary 2.3
Let \(G\subset (0,\infty )\) be an interval, \(p\in \mathbb{R\backslash }\{0\}\), and \(r_{i}\in [0,1]\). Let \(\zeta _{1},\zeta _{2}\in C^{2}(G)\) be p-convex functions such that \(\frac{\zeta ^{\prime \prime }_{1}}{\zeta ^{\prime \prime }_{2}}\) is invertible. Then there exist some \(\chi \in G\) such that the following equality holds:
with each \(r_{i}\in [0,1]\) such that \(\sum^{n}_{1}r_{i}=1\) and provided that the denominators are non-zero.
Corollary 2.4
Let \(G\subset (0,\infty )\) be an interval, \(p\in \mathbb{R\backslash }\{0\}\), and \(r_{i}\in [0,1]\). Let \(\zeta \in C^{2}(G)\) be a p-convex function. Then there exist some \(\chi \in G\) such that the following equality holds:
with each \(r_{i}\in [0,1]\) such that \(\Sigma ^{n}_{1}r_{i}=1\).
Proof
By letting \(\zeta _{1}=\zeta \) and \(\zeta _{2}(w)=w^{2}\), where \(w\in (0,\infty )\), in Theorem 2.2, we achieve equality (8). □
Cauchy type means for p-convex functions in the Hermite–Hadamard sense
Let \(\zeta :G\subset (0,\infty )\rightarrow \mathbb{R}\) be a p-convex function, \(p\in \mathbb{R}\backslash \{0\}\), and \(g_{1},g_{2}\in G\) with \(g_{1}< g_{2}\). If \(\zeta \in L_{1}[g_{1},g_{2}]\), then we have (e.g., see [13])
By using the right half of inequality (9), we have following result.
Theorem 3.1
Let \(G\subset (0,\infty )\) be an interval, \(p\in \mathbb{R}\backslash \{0\}\), and \(g_{1},g_{2}\in G\) with \(g_{1}< g_{2}\). Let \(\zeta _{1},\zeta _{2}\in C^{2}(G)\) be p-convex functions. Then there exists some \(\chi \in G\) such that the following equality holds:
provided that the denominators are non-zero.
Proof
Let
and
where \(\lambda \in [0,1]\). Similarly, we can define \((T\zeta _{2})(\lambda )\).
Observe that
and
Now consider the function \(Q(\lambda )\) defined by
such that we have
Then, from two applications of mean value theorem, we find \(\upsilon \in G\) such that
It implies
For any fixed υ, the expression in the square brackets in (11) is a continuous function of w, so it vanishes. Corresponding to that value of w, we get a number
such that
This gives equality (10). □
Corollary 3.2
If \(\frac{\zeta ^{\prime \prime }_{1}}{\zeta ^{\prime \prime }_{2}}\) is invertible, then we have
Corollary 3.3
By taking \(\zeta _{2}(w)={w}^{2}\) and \(\zeta _{1}=\zeta \) in Theorem 3.1, we have
Cauchy type means for s-convex functions in Jensen’s sense
Here first we prove Jensen’s inequality for s-convex function.
Lemma 4.1
Let \(s\in (0,1]\) and \(\zeta :G\subset \mathbb{R}^{+}\rightarrow \mathbb{R}\) be an s-convex function. Let \(\sum^{n}_{1}r_{i}g_{i}\) be convex combinations of points \(g_{i}\in G\) with coefficients \(r_{i}\in [0,1]\). Then each s-convex function (in the first sense) satisfies the inequality
where \(\sum^{n}_{1}r_{i}^{s}=1\).
Proof
We apply induction on the number of points in convex combination.
Basis step: for \(n=1\), equality (14) is true since
where \(r^{s}_{1}=1\) since \(r_{1}=1\).
Induction step: suppose that (14) holds for all convex combinations of points containing less than or equal to \(n-1\) points. Let \(r_{n}\neq 1\) and
where the sum \(\sum_{1}^{n-1} ( \frac{r_{i}}{1-r_{n}} ) g_{i}\in G\). Then, by induction hypothesis, we have
Thus we get (14). □
Remark 4.1
By taking \(s=1\) in Lemma 4.1 we can get Jensen’s inequality for convex function.
Now, by using the above lemma, we state and prove the following theorem, which gives the Cauchy type means for s-convex function.
Theorem 4.1
Let \(s\in (0,1]\) and \(r_{i}\in [0,1]\). Let \(\zeta _{1},\zeta _{2}\in C^{2}(G\subset [0,\infty ))\) be s-convex functions (in the first sense). Then there exist some \(\chi \in G\) such that the following equality holds:
with each \(r_{i}\in [0,1]\) such that \(\sum^{n}_{1}r_{i}^{s}=1\) and provided that the denominators are non-zero.
Proof
Define
and
where \(\lambda \in [0,1]\). Accordingly, we can define \((T\zeta _{2})(\lambda )\).
Note that
and
Now consider the function \(Q(\lambda )\) defined by
such that we have
Then, from two applications of mean value theorem, we find \(\upsilon \in G\) such that
It follows that
For any fixed υ, the expression in the square brackets in (18) is a continuous function of \(g_{i}\), so it vanishes. Corresponding to that value of \(g_{i}\), we get a number
so that
This gives equality (17). □
Corollary 4.2
Let \(s\in (0,1]\). Let \(\zeta _{1},\zeta _{2}\in C^{2}(G\subset [0,\infty ))\) be s-convex functions (in the first sense) such that \(\frac{\zeta ^{\prime \prime }_{1}}{\zeta ^{\prime \prime }_{2}}\) is invertible. Then there exist some \(\chi \in G\) such that the following equality holds:
with each \(r_{i}\in [0,1]\) such that \(\sum^{n}_{1}r_{i}^{s}=1\) and provided that the denominators are non-zero.
Corollary 4.3
Let \(s_{1},s_{2}\in (0,1)\). Let \(\zeta _{1},\zeta _{2}\in C^{2}((0,\infty ))\) be an \(s_{1}\)-convex function and an \(s_{2}\)-convex function (in the first sense), respectively, defined as \(\zeta _{1}(w)=w^{s_{1}}\) and \(\zeta _{2}(w)=w^{s_{2}}\). Then, from Theorem 4.1, we get
Cauchy type means for s-convex functions in the Hermite–Hadamard sense
Drgomir and Fitzpatrick [7] gave the following result.
Theorem 5.1
Suppose that \(\zeta :[0,\infty )\rightarrow \mathbb{R}\) is an s-convex function in the first sense, where \(s\in (0,1)\), and let \(g_{1},g_{2}\in [0,\infty )\), \(g_{1}\leq g_{2}\). Then the following inequality holds:
The above inequalities are sharp.
From inequality (21) we give the following result.
Theorem 5.2
Suppose that \(\zeta _{1},\zeta _{2}:[0,\infty )\rightarrow \mathbb{R}\) is an s-convex function in the first sense, where \(s\in (0,1)\), and let \(g_{1},g_{2}\in [0,\infty )\), \(g_{1}\leq g_{2}\). Let \(\zeta _{1},\zeta _{2}\in C^{2}([g_{1},g_{2}])\). Then there exist some \(\chi \in [g_{1},g_{2}]\) such that the following equality holds:
provided that the denominators are non-zero.
Proof
Let
and
where \(\lambda \in [0,1]\). Accordingly, we can define \((T\zeta _{2})(\lambda )\).
We can have
and
Now consider the function \(Q(\lambda )\) defined by
such that we have
Then, from two applications of mean value theorem, we find \(\upsilon \in [g_{1},g_{2}]\) such that
It implies
For some fixed υ, the expression in the square brackets in (23) is a continuous function of w, so it vanishes. Corresponding to that value of w, we get a number
such that
Thus we get (22). □
Corollary 5.3
If \(\frac{\zeta _{1}^{\prime \prime }}{\zeta _{2}^{\prime \prime }}\) is invertible, then we have
Corollary 5.4
Let \(s_{1},s_{2}\in (0,1)\). By taking \(\zeta _{1}(w)=w^{s_{1}}\) and \(\zeta _{2}(w)={w}^{s_{2}}\), where \(w\in (0,\infty )\), in Theorem 5.2we have
Now we define the following definition.
Definition 5.1
Let \(s\in (0,1)\) and \(g_{1},g_{2}\in [0,\infty )\), \(g_{1}\leq g_{2}\). Then quasi-arithmetic mean for the strictly monotonic function Φ defined on \([g_{1},g_{2}]\) is as follows:
Theorem 5.5
Let \(s\in (0,1)\) and \(g_{1},g_{2}\in [0,\infty )\), \(g_{1}\leq g_{2}\). Let \(\varPhi _{1},\varPhi _{2},\varPhi _{3}\in C^{2}([g_{1},g_{2}])\) be strictly monotonic real-valued functions. Then
for some υ, provided that the denominators are non-zero.
Proof
Let us choose functions \(\zeta _{1}=\varPhi _{1}\circ \varPhi ^{-1}_{3}\), \(\zeta _{2}=\varPhi _{2}\circ \varPhi ^{-1}_{3}\), \(w=\varPhi _{3}(w)\), and \(\frac{g_{1}+g_{2}}{2}=\frac{1}{g_{2}-g_{1}}\int ^{g_{2}}_{g_{1}} \varPhi _{3}(w)\,dw\) in Theorem 5.2, we observe that there exists some \(\upsilon \in [g_{1},g_{2}]\) such that
Then, by letting \(\varPhi ^{-1}_{3}(\chi )=\upsilon \), we notice that we have \(\upsilon \in [g_{1},g_{2}]\) such that
□
Again from inequality (21) we have following result.
Theorem 5.6
Suppose that \(\zeta _{1},\zeta _{2}:[0,\infty )\rightarrow \mathbb{R}\) is an s-convex function in the first sense, where \(s\in (0,1)\), and let \(g_{1},g_{2}\in [0,\infty )\), \(g_{1}\leq g_{2}\). Let \(\zeta _{1},\zeta _{2}\in C^{2}([g_{1},g_{2}])\). Then there exist some \(\chi \in [g_{1},g_{2}]\) such that the following equality holds:
provided that the denominators are non-zero.
Proof
Consider the function
Similarly, we can define \(T\zeta _{2}(w)\).
Note that
and
We observe that
Now we define \(D(w)\) as follows:
Then note that
Thus, by application of the mean-value theorem, we get
for some \(\chi \in [g_{1},g_{2}]\). Consequently, this completes the proof of the theorem. □
Corollary 5.7
If \(\frac{\zeta _{1}^{\prime \prime }}{\zeta _{2}^{\prime \prime }}\) is invertible, then we have
Corollary 5.8
Let \(s_{1},s_{2}\in (0,1)\). By taking \(\zeta _{1}(w)=w^{s_{1}}\) and \(\zeta _{2}(w)={w}^{s_{2}}\), where \(w\in (0,\infty )\), in Theorem 5.6, we have
Conclusion
In Sect. 2, we proved Cauchy type mean for p-convex functions. In Sect. 3, Cauchy type theorem in the Hermite–Hadamard sense was obtained for p-convex functions. In Sect. 4, we proved Jensen’s inequality for s-convex functions in the first sense, and then a Cauchy type theorem was obtained. In Sect. 5, a Cauchy type theorem in the Hermite–Hadamard sense was obtained for s-convex functions in the first sense.
Availability of data and materials
The data and material used to support the findings of this study are included within the article.
References
- 1.
Antczak, T.: Mean value in invexity analysis. Nonlinear Anal. 60, 1471–1484 (2005)
- 2.
Anwar, M., Pečarić, J.: New Cauchy’s means. J. Inequal. Appl. 2008, 10 (2008)
- 3.
Anwar, M., Pečarić, J.: Cauchy’s means of Levinson type. J. Inequal. Pure Appl. Math. 9(4), 1–8 (2008)
- 4.
Anwar, M., Pečarić, J.: Cauchy means of Mercer’s type. Util. Math. 84, 201–208 (2011)
- 5.
Anwar, M., Pečarić, J., Lipanović, M.R.: Cauchy-type means for positive linear functionals. Tamakang J. Math. 42(4), 511–530 (2011)
- 6.
Aumann, G.: Konvexe Funktionen und Induktion bei Ungleichungen zwischen Mittelverten. Bayer. Akad. Wiss. Munchen. 109, 405–413 (1933)
- 7.
Drgomir, S.S., Fitzpatrick, S.: The Hadamard’s inequality for s-convex functions in second sense. Demonstr. Math. 32, 687–696 (1999)
- 8.
Guessab, A.: Direct and converse results for generalized multivariate Jensen-type inequalities. J. Nonlinear Convex Anal. 13(4), 777–797 (2012)
- 9.
Guessab, A., Schmeisser, G.: Sharp integral inequalities of the Hermite-Hadamard type. J. Approx. Theory 115(2), 260–288 (2002)
- 10.
Guessab, A., Schmeisser, G.: Convexity results and sharp error estimates in approximate multivariate integration. Math. Comput. 73(247), 1365–1384 (2004)
- 11.
Guessab, A., Schmeisser, G.: Sharp error estimates for interpolatory approximation on convex polytopes. SIAM J. Numer. Anal. 43(3), 909–923 (2005)
- 12.
Hudzik, H., Maligranda, L.: Some remark on s-convex functions. Aequ. Math. 48, 100–111 (1994)
- 13.
İşcan, I.: Hermite-Hadamard type inequalities for p-convex functions. Int. J. Anal. Appl. 11(2), 137–145 (2016)
- 14.
İşcan, I.: Ostrowski type inequalities for p-convex functions. New Trends Math. Sci. 4(3), 140–150 (2016)
- 15.
Matković, A., Pečarić, J.: A variant of Jensen’s inequality for convex functions of several variables. J. Math. Inequal. 1(1), 45–51 (2007)
- 16.
Matković, A., Pečarić, J., Perić, I.: A variant of Jensen’s inequality of Mercer’s type for operators with applications. Linear Algebra Appl. 418(2–3), 551–564 (2006)
- 17.
Matković, A., Pečarić, J., Perić, I.: Refinements of Jensen’s inequality of Mercer’s type for operator convex functions. Math. Inequal. Appl. 11(1), 113–126 (2008)
- 18.
Mercer, A.McD.: Some new inequalities involving elementary mean values. J. Math. Anal. Appl. 229, 677–681 (1999)
- 19.
Mitrinovic, D.S., Pečarić, J., Fink, A.M.: Classical and New Inequalities in Analysis. Kluwer Academic, Dordrecht (1993)
- 20.
Pečarić, J., Lipanović, M.R., Srivastav, H.M.: Some mean-value theorems of the Cauchy type. Fract. Calc. Appl. Anal. 9, 143–158 (2006)
- 21.
Pečarić, J., Perić, I., Srivastava, H.: A family of the Cauchy type mean-value theorems. J. Math. Anal. Appl. 306, 730–739 (2005)
- 22.
Toplu, T., İşcan, I., Maden, S.: Lazhar type inequalities for p-convex functions. In: International Conference on Mathematics and Mathematics Education (ICMME-2018), Ordu University, 27–29 June 2018, Ordu (2018)
Acknowledgements
We thank the anonymous referees and editor for their careful reading of the manuscript and many insightful comments to improve the results.
Funding
This research article is supported by the National University of Sciences and Technology(NUST), Islamabad, Pakistan.
Author information
Affiliations
Contributions
Both authors contributed equally to this work. Both authors read and approved the final manuscript.
Corresponding author
Ethics declarations
Competing interests
The authors declare that they have no competing interests.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Mehreen, N., Anwar, M. Cauchy type means for some generalized convex functions. J Inequal Appl 2021, 114 (2021). https://doi.org/10.1186/s13660-021-02647-2
Received:
Accepted:
Published:
Keywords
- Cauchy mean value theorem
- Jensen’s inequality
- Hermite–Hadamard inequality
- p-convex function
- s-convex function in the first sense