#### Abstract

We present the best possible lower and upper bounds for the Neuman-Sándor mean in terms of the convex combinations of either the harmonic and quadratic means or the geometric and quadratic means or the harmonic and contraharmonic means.

#### 1. Introduction

For with , the Neuman-Sándor mean [1] is defined by where is the inverse hyperbolic sine function.

Recently, the theory of bivariate means have been the subject of intensive research [2–17]. In particular, many remarkable inequalities for the Neuman-Sándor mean can be found in the literature [1, 18–20].

Let , , , , , , and be the harmonic, geometric, logarithmic, first Seiffert, arithmetic, second Seiffert, quadratic, and contraharmonic means of and , respectively. Then it is well known that the inequalities hold for all with .

In [1, 18], Neuman and Sándor proved that the double inequalities hold for all with .

Let , with , and . Then the following Ky Fan inequalities were presented in [1].

The double inequality for all with was established by Li et al. in [19], where (), and is the th generalized logarithmic mean of and , and is the unique solution of the equation .

Neuman [20] proved that the double inequalities hold for all with if and only if , , and .

The main purpose of this paper is to find the least values , , , and the greatest values , , , such that the double inequalities hold true for all with .

Our main results are presented in Theorems 1.1–1.3.

Theorem 1.1. *The double inequality
**
holds for all with if and only if and .*

Theorem 1.2. *The double inequality
**
holds for all with if and only if and .*

Theorem 1.3. *The double inequality
**
holds for all with if and only if and .*

#### 2. Lemmas

In order to prove our main results we need two Lemmas, which we present in this section.

Lemma 2.1 (see [21, Lemma 1.1]). *Suppose that the power series and have the radius of convergence and for all . Let , then the following statements are true.*(1)*If the sequence is (strictly) increasing (decreasing), then is also (strictly) increasing (decreasing) on .*(2)*If the sequence is (strictly) increasing (decreasing) for and (strictly) decreasing (increasing) for , then there exists such that is (strictly) increasing (decreasing) on and (strictly) decreasing (increasing) on . *

Lemma 2.2. *Let , and
**
Then and for all .*

*Proof. *From (2.1), one has
where

We divide the proof into two cases. *Case **1.* (). Then (2.5) leads to
where
We clearly see that the function is strictly decreasing in . Then from (2.8), we get
for .

Therefore, for all follows easily from (2.2), (2.4), (2.6), (2.7), and (2.9). *Case **2.* (). Then (2.3) and (2.5) yield
where
We divide the discussion of this case into two subcases, and all computations are carried out using MATHEMATICA software. *Subcase A. *. Then from (2.12) and the fact that
we know that

for . *Subcase B. *. Then from (2.12), one has
where
We conclude that
for all . Indeed, if , then (2.17) follows from (2.16) and the inequality
If , then (2.17) follows from (2.16) and the inequalities
From (2.15) together with (2.17) we clearly see that there exists such that for and for .

Subcases A and B lead to the conclusion that for and for . Thus from (2.11), we know that is strictly increasing in and strictly decreasing in .

It follows from (2.4) and (2.10) together with the piecewise monotonicity of that there exists such that is strictly increasing in and strictly decreasing in .

Therefore, for follows from (2.2) and (2.10) together with the piecewise monotonicity of .

#### 3. Proof of Theorems 1.1–1.3

*Proof of Theorem 1.1. *Since , and are symmetric and homogeneous of degree 1. Hence, without loss of generality, we assume that . Let and . Then , , , , and

Making use of power series and , we can express (3.1) as follows:
Let and Then . Moreover, by a simple calculation, we see that
for .

Equations (3.1) and (3.2) together with inequality (3.3) and Lemma 2.1 lead to the conclusion that is strictly decreasing in . This in turn implies that

Therefore, Theorem 1.1 follows from (3.1) and (3.4) together with the monotonicity of .

*Proof of Theorem 1.2. *Since , and are symmetric and homogeneous of degree 1. Hence, without loss of generality, we assume that . Let , and . Then making use of gives
Moreover, we obtain

We take the difference between the additive convex combination of , , and as follows:
where is defined as in Lemma 2.2.

Therefore, for all with follows from (3.8) and Lemma 2.2. This conjunction with the following statement gives the asserted result. (i)If , then (3.5) and (3.6) imply that there exists such that for all with .(ii)If , then (3.5) and (3.7) imply that there exists such that for all with .

*Proof of Theorem 1.3. *We will follow, to some extent, lines in the proof of Theorem 1.1. First we rearrange terms of (1.9) to obtain
Use of followed by a substitution gives
where

Since the function is an even function, it suffices to investigate its behavior on the interval .

Using power series of and , then (3.11) can be rewritten as

Let and . Then
It follows from (3.13) that the sequence is strictly increasing for .

Equations (3.12) and (3.13) together with Lemma 2.1 and the monotonicity of lead to the conclusion that is strictly increasing in . Moreover,

Making use of (3.14) and (3.10) together with the monotonicity of gives the asserted result.

#### Acknowledgments

This research was supported by the Natural Science Foundation of China under Grants 11071069 and 11271105, and Innovation Team Foundation of the Department of Education of Zhejiang Province under Grant T200924.