# Optimal Power Mean Bounds for the Weighted Geometric Mean of Classical Means

- Bo-Yong Long
^{1, 2}and - Yu-Ming Chu
^{3}Email author

**2010**:905679

https://doi.org/10.1155/2010/905679

© B.-Y. Long and Y.-M. Chu. 2010

**Received: **13 November 2009

**Accepted: **25 February 2010

**Published: **8 March 2010

## Abstract

For , the power mean of order of two positive numbers and is defined by , for , and , for . In this paper, we answer the question: what are the greatest value and the least value such that the double inequality holds for all and with ? Here , , and denote the classical arithmetic, geometric, and harmonic means, respectively.

## Keywords

## 1. Introduction

For , the power mean of order of two positive numbers and is defined by

Recently, the power mean has been the subject of intensive research. In particular, many remarkable inequalities for can be found in literatures [1–12]. It is well known that is continuous and increasing with respect to for fixed and . Let , and be the classical arithmetic, geometric, and harmonic means of two positive numbers and , respectively. Then

In [13], Alzer and Janous established the following sharp double inequality (see also [14, page 350]):

In [15], Mao proved

for all , and is the best possible lower power mean bound for the sum .

The following sharp bounds for and in terms of power mean are proved in [16]:

The purpose of this paper is to answer the question: what are the greatest value and the least value such that the double inequality

## 2. Main Result

In order to establish our main results we need the following lemma.

Lemma 2.1.

Proof.

Therefore, Lemma 2.1( ) follows from (2.1)–(2.3) and (2.5) together with (2.7).

Therefore, Lemma 2.1(2) follows from (2.1)–(2.3) and (2.5) together with (2.8).

Theorem 2.2.

(2) for , and for , each equality occurs if and only if , and and are the best possible power mean bounds for the product .

( ) If and , then we clearly see that

If and , without loss of generality, we assume that . Let and , then , and simple computations lead to

Therefore, for follows from (2.11) and Lemma 2.1(1) together with (2.12), and for follows from (2.11) and Lemma 2.1(2) together with (2.12).

Next, we prove that and are the best possible power mean bounds for the product .

Firstly, we prove that is the best possible upper power mean bound for the product if .

Let , then the Taylor expansion leads to

Equations (2.13) and (2.14) imply that if , then for any there exists , such that for .

Secondly, we prove that is the best possible lower power mean bound for the product if .

From (2.15) and , we clearly see that

Equation (2.16) implies that if , then for any there exists , such that for .

Thirdly, we prove that is the best possible lower power mean bound for the product if .

Let , then the Taylor expansion leads to

Equations (2.17) and (2.18) imply that if , then for any there exists , such that for .

Finally, we prove that is the best possible upper power mean bound for the product if .

From (2.19) and we clearly see that

Equation (2.20) implies that if , then for any there exists , such that for .

## Declarations

### Acknowledgments

This work is partly supported by the National Natural Science Foundation of China (Grant no. 60850005) and the Natural Science Foundation of Zhejiang Province (Grant no. D7080080, Y607128).

## Authors’ Affiliations

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