# Inequalities for Generalized Logarithmic Means

## Abstract

For , the generalized logarithmic mean of two positive numbers and is defined as , for , , for , , , , for , , and , for , . In this paper, we prove that , and for all , and the constants , and cannot be improved for the corresponding inequalities. Here , and denote the arithmetic, geometric, and harmonic means of and , respectively.

## 1. Introduction

For , the generalized logarithmic mean and power mean of two positive numbers and are defined as

(1.1)
(1.2)

It is well known that and are continuous and increasing with respect to for fixed and . Let , , , , and be the arithmetic, identric, logarithmic, geometric, and harmonic means of and , respectively. Then

(1.3)

In [1], the following results are established: (1) implies that ; (2) implies that ; (3) implies that there exist such that ; (4) implies that there exist such that . Hence the question was answered: what are the least value and the greatest value such that the inequality holds for all ?

Stolarsky [2] proved that , with equality if and only if .

In [3], Pittenger proved that

(1.4)

for all , where

(1.5)

Here , are sharp and equality holds only if or or . The case reduces to Lin's results [1]. Other generalizations of Lin's results were given by Imoru [4].

Qi and Guo [5] established that

(1.6)

for all , and . The upper bound in (1.6) is the best possible.

In [6], Chu et al. established the following result:

(1.7)

for all , where the function is the logarithmic derivative of the gamma function.

Recently, some monotonicity results of the ratio between generalized logarithmic means were established in [79].

The purpose of this paper is to answer the following questions: what are the greatest values and , and the least value such that , , and for all ?

## 2. Main Results

Theorem 2.1.

for all , with inequality if and only if , and the constant cannot be improved.

Proof.

If , then from (1.1) we clearly see that Next, we assume that and , and then elementary computations yield

(2.1)

To prove that is the largest number for which the inequality holds, we take and , and we see that

(2.2)
(2.3)

where

Making use of the Taylor expansion, we have

(2.4)

Equations (2.3) and (2.4) imply that for any there exists such that for .

Theorem 2.2.

for all , with equality if and only if , and the constant cannot be improved.

Proof.

Simple computations yield

(2.5)

Next we prove that is the optimal value for which the inequality holds.

For and , elementary computations yield

(2.6)
(2.7)

where

Using Taylor expansion we get

(2.8)

Equations (2.7) and (2.8) imply that for any there exists such that for .

Theorem 2.3.

for all , with equality if and only if , and the constant cannot be improved.

Proof.

Form (1.1) we clearly see that if . If , then simple computations yield

(2.9)

To show that is the best possible constant for which the inequality holds, let and , and then

(2.10)

where

Using Taylor expansion we have

(2.11)

Equations (2.10) and (2.11) imply that for any there exists such that for .

Remark 2.4.

If , then

(2.12)

Therefore, we cannot get inequality for any and all

Remark 2.5.

It is easy to verify that for all

## References

1. Lin TP: The power mean and the logarithmic mean. The American Mathematical Monthly 1974, 81: 879–883. 10.2307/2319447

2. Stolarsky KB: The power and generalized logarithmic means. The American Mathematical Monthly 1980,87(7):545–548. 10.2307/2321420

3. Pittenger AO: Inequalities between arithmetic and logarithmic means. Univerzitet u Beogradu. Publikacije Elektrotehničkog Fakulteta. Serija Matematika i Fizika 1980, (678–715):15–18.

4. Imoru CO: The power mean and the logarithmic mean. International Journal of Mathematics and Mathematical Sciences 1982,5(2):337–343. 10.1155/S0161171282000313

5. Qi F, Guo B-N: An inequality between ratio of the extended logarithmic means and ratio of the exponential means. Taiwanese Journal of Mathematics 2003,7(2):229–237.

6. Chu YM, Zhang XM, Tang TM: An elementary inequality for psi function. Bulletin of the Institute of Mathematics. Academia Sinica 2008,3(3):373–380.

7. Li X, Chen C-P, Qi F: Monotonicity result for generalized logarithmic means. Tamkang Journal of Mathematics 2007,38(2):177–181.

8. Qi F, Chen S-X, Chen C-P: Monotonicity of ratio between the generalized logarithmic means. Mathematical Inequalities & Applications 2007,10(3):559–564.

9. Chen C-P: The monotonicity of the ratio between generalized logarithmic means. Journal of Mathematical Analysis and Applications 2008,345(1):86–89. 10.1016/j.jmaa.2008.03.071

## Acknowledgments

This research is partly supported by N S Foundation of China under Grant 60850005 and N S Foundation of Zhejiang Province under Grants D7080080 and Y7080185.

## Author information

Authors

### Corresponding author

Correspondence to Yu-Ming Chu.

## Rights and permissions

Reprints and Permissions

Chu, YM., Xia, WF. Inequalities for Generalized Logarithmic Means. J Inequal Appl 2009, 763252 (2010). https://doi.org/10.1155/2009/763252

• Accepted:

• Published:

• DOI: https://doi.org/10.1155/2009/763252

### Keywords

• Simple Computation
• Taylor Expansion
• Elementary Computation
• Monotonicity Result
• Generalize Logarithmic