Journal of Applied Mathematics

Volume 2013 (2013), Article ID 645263, 6 pages

http://dx.doi.org/10.1155/2013/645263

Research Article

Refinements of Generalized Aczél's Inequality and Bellman's Inequality and Their Applications

Jing-Feng Tian¹ and Shu-Yan Wang²

¹College of Science and Technology, North China Electric Power University, Baoding, Hebei Province 071051, China

²Department of Mathematics and Computer, Baoding University, Baoding, Hebei Province 071000, China

Received 13 October 2012; Accepted 30 December 2012

Academic Editor: Shanhe Wu

Copyright © 2013 Jing-Feng Tian and Shu-Yan Wang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

We give some refinements of generalized Aczél's inequality and Bellman's inequality proposed by Tian. As applications, some refinements of integral type of generalized Aczél's inequality and Bellman's inequality are given.

1. Introduction

The famous Aczél’s inequality [1] states that if , are real numbers such that and , then

It is well known that Aczél’s inequality plays an important role in the theory of functional equations in non-Euclidean geometry. In recent years, various attempts have been made by many authors to improve and generalize the Aczél’s inequality (see [2–19] and references therein). We state here some improvements of Aczél’s inequality.

One of the most important results in the references mentioned above is an exponential extension of (1), which is stated in the following theorem.

Theorem A. Let and be real numbers such that , , and and let , be positive numbers such that and . If , then

If , then the reverse inequality in (2) holds.

Remark 1. The case of Theorem A was proved by Popoviciu [8]. The case was given in [15] by Vasić and Pečarić.

Vasić and Pečarić [16] presented a further extension of inequality (1).

Theorem B. Let , , , , , and let . Then

In a recent paper [18], Wu and Debnath established an interesting generalization of Aczél’s inequality [1] as follows.

Theorem C. Let , , , , , and let . Then

In 2012, Tian [10] presented the following reversed version of inequality (4).

Theorem D. Let , , , , , , and let . Then

Therefore, applying the above inequality, Tian gave the reversed version of inequality (3) as follows.

Theorem E. Let , , , , , and , . Then

Moreover, in [10], Tian obtained the following integral form of inequality (5).

Theorem F. Let , , , , and let be positive Riemann integrable functions on such that . Then

Bellman inequality [20] related with Aczél’s inequality is stated as follows.

Theorem G. Let , be positive numbers such that and . If , then

If  , then the reverse inequality in (8) holds.

Remark 2. The case of Theorem G was proposed by Bellman [20]. The case was proved in [15] by Vasić and Pečarić.

The main purpose of this work is to give refinements of inequalities (5) and (8). As applications, some refinements of integral type of inequality (5) and (8) are given.

2. Refinements of Generalized Aczl’s Inequality and Bellman’s Inequality

Theorem 3. Let , , , , , , and let . Then

where

Proof

Case (I). When , then . On the one hand, we split the left-hand side of inequality (9) as follows:

where

From this hypothesis, it is immediate to obtain the inequality

Thus, by using inequality (6), we have

On the other hand, by using inequality (6) again, we obtain the inequality

Combining inequalities (11), (14), and (15) we can get inequality (9).

Case (II). When , using Case (I) with , we have

where , , , .

Hence, taking , into (16), we obtain

that is,

Therefore, repeating the foregoing arguments, we get

Combining inequalities (18) and (19) leads to inequality (9) immediately. The proof of Theorem 3 is completed.

If we set , then from Theorem 3, we obtain the following refinement of inequality (6).

Corollary 4. Let , , , , , , and . Then

Putting , , , , and in Theorem 3, we obtain the refinement and generalization of Theorem A for .

Corollary 5. Let , , , , , , and . Then, the following inequality holds:

Based on the mathematical induction, it is easy to see that the following generalized Bellman’s inequality is true.

Theorem 6. Let , , , , and let . Then

Next, we give a refinement of generalized Bellman’s inequality (22) as follows.

Theorem 7. Let , , , , and let . Then

Proof. The proof of Theorem 7 is similar to the one of Theorem 3. Applying generalized Bellman’s inequality (22) twice, we can deduce the inequality (23).

3. Application

In this section, we show two applications of the inequalities newly obtained in Section 2.

Firstly, we give an improvement of inequality (7) by using Theorem 3.

Theorem 8. Let , let , , , and let be positive integrable functions defined on with . Then, for any , one has

Proof. We need to prove only the left side of inequality (24). The proof of the right side of inequality (24) is similar. For any positive integers and , we choose an equidistant partition of and , respectively, as

Noting that , we have

Consequently, there exists a positive integer , such that

for all and .

By using Theorem 3, for any , the following inequality holds:

Since

we have

Noting that are positive Riemann integrable functions on , we know that and are also integrable on . Letting on both sides of inequality (30), we get the left side of inequality (24). The proof of Theorem 8 is completed.

We give here a direct consequence from Theorem 8. Putting , , , , , , and in (24), we obtain a special important case as follows.

Corollary 9. Let and be real numbers such that , , and , let , and let be positive integrable functions defined on with and . Then, for any , one has

Finally, we present a refinement of integral type of generalized Bellman’s inequality.

Theorem 10. Let , let , and let be positive integrable functions defined on with . Then, for any , one has

Proof. The proof of Theorem 10 is similar to the proof of Theorem 8.

A special case to the last theorem is as follows.

Corollary 11. Let , let , and let be positive integrable functions defined on with and . Then, for any , one has

Acknowledgments

The authors would like to express hearty thanks to the anonymous referees for their great efforts to improve this paper. This work was supported by the NNSF of China (no. 61073121), the Natural Science Foundation of Hebei Province of China (no. F2012402037), the Natural Science Foundation of Hebei Education Department (no. Q2012046), and the Fundamental Research Funds for the Central Universities (no. 11ML65).

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