Research Article | Open Access

# Sharp Bounds for Neuman Means by Harmonic, Arithmetic, and Contraharmonic Means

**Academic Editor:**Shuangjie Peng

#### Abstract

We give several sharp bounds for the Neuman means
and ( and ) in terms of harmonic
mean *H* (contraharmonic mean *C*) or the geometric convex
combination of arithmetic mean *A* and harmonic mean *H*
(contraharmonic mean *C* and arithmetic mean *A*) and present a
new chain of inequalities for certain bivariate means.

#### 1. Introduction

For with , the Schwab-Borchardt mean [1â€“3] of and is defined as where and are the inverse cosine and inverse hyperbolic cosine functions, respectively.

The Schwab-Borchardt mean can be expressed by the symmetric elliptic integral [4] of the first kind as follows [5] (see also [6, (3.21)]): where .

Recently, the Schwab-Borchardt mean has been the subject of intensive research. In particular, many remarkable inequalities for the Schwab-Borchardt mean and its generated means can be found in the literature [1â€“3, 7â€“10].

Very recently, Neuman [11] found a new mean derived from the Schwab-Borchardt mean as follows:

Let , , , , , , , , and be, respectively, the geometric, harmonic, logarithmic, first Seiffert, arithmetic, Neuman-SÃ¡ndor, second Seiffert, quadratic, and contraharmonic means, and let be the Neuman means. Then Neuman [11] proved that for all with , and the double inequalities hold for all with if and only if , , , , , , , and .

Zhang et al. [12] presented the best possible parameters and such that the double inequalities hold for all with .

In [13], the authors found the greatest values , , , , , , , and and the least values , , , , , , , and such that the double inequalities hold for all with .

Let , , , , , and be the parameters such that , . Then He et al. [14] proved that

Let , , , and . Then we clearly see that for all with , and the functions and are, respectively, strictly increasing on the intervals and for fixed with .

The main purpose of this paper is to find the best possible parameters , , , , , , , , , and such that the double inequalities hold, for all with , and present a new chain of inequalities for certain bivariate means.

#### 2. Lemmas

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

Lemma 1 (see [15, Theorem 1.25]). *For , let be continuous on and differentiable on ; let on . If is increasing (decreasing) on , then so are
**
If is strictly monotone, then the monotonicity in the conclusion is also strict.*

Lemma 2 (see [16, Lemma 1.1]). *Suppose that the power series and have the radius of convergence and for all . If the sequence is (strictly) increasing (decreasing) for all , then the function is also (strictly) increasing (decreasing) on .*

Lemma 3. *The function
**
is strictly decreasing from onto .*

*Proof. *Making use of power series expansion we get

Let
Then
and is strictly decreasing for all .

Note that

Therefore, Lemma 3 follows easily from Lemma 2 and (18)â€“(21) together with the monotonicity of the sequence .

Lemma 4. *The function
**
is strictly increasing from onto .*

*Proof. *Let and . Then simple computations lead to
and is strictly increasing on .

Note that

Therefore, Lemma 4 follows from Lemma 1, (23), (24), and the monotonicity of .

Lemma 5. *The function
**
is strictly decreasing from onto .*

*Proof. *Let and . Then simple computations lead to

Let
Then it is not difficult to verify that
for all , where and .

Note that

It follows from Lemma 2 and (27)â€“(29) that the function is strictly decreasing on . Therefore, Lemma 5 follows from (26) and (30) together with Lemma 1 and the monotonicity of .

Lemma 6. *The function
**
is strictly decreasing from onto .*

*Proof. *Let , , , and . Then simple computations lead to

Since the function is strictly decreasing on , hence (34) leads to the conclusion that is strictly decreasing on .

Note that

Therefore, Lemma 6 follows easily from (32), (33), (35), and Lemma 1 together with the monotonicity of .

Lemma 7. *The function
**
is strictly increasing from onto .*

*Proof. *Let and . Then simple computations lead to

Let

Then
for all .

It follows from Lemma 2 and (38)â€“(40) that is strictly increasing on .

Note that

Therefore, Lemma 7 follows from Lemma 1, (37), and (41) together with the monotonicity of .

Lemma 8. *The function
**
is strictly increasing from onto .*

*Proof. *Let , , , and . Then simple computations lead to

Since the function is strictly decreasing on , hence Lemma 1 and (43) lead to the conclusion that is strictly increasing on .

Note that

Therefore, Lemma 8 follows easily from (44) and (45) together with the monotonicity of .

#### 3. Main Results

Theorem 9. *The double inequalities
**
hold for all with if and only if , , , , , , , and .*

*Proof. *Without loss of generality, we assume that . Let , , , , and be the parameters such that , . Then (9)â€“(12) lead to the conclusion that inequalities (46)â€“(49) are, respectively, equivalent to

Therefore, Theorem 9 follows easily from (50)â€“(53) and Lemmas 5â€“8.

Theorem 10. *Let . Then the double inequalities,
**
hold for all with if and only if , , , and .*

*Proof. *Without loss of generality, we assume that . Let , , , and be the parameters such that . Then from (9) and (10) we have

Therefore, Theorem 10 follows easily from (55) and (56) together with Lemmas 3 and 4.

Theorem 11. *Let . Then the double inequalities,
**
hold for all with if and only if , , , and .*

*Proof. *Without loss of generality, we assume that . Let , , , and be the parameters such that . Then from (11) and (12) one has
where the functions and are defined as in Lemmas 3 and 4, respectively.

Note that

Therefore, Theorem 11 follows easily from Lemmas 3 and 4 together with (58)-(59).

Theorem 12. *Let , , , and . Then the inequalities
**
hold for all with .*

*Proof. *It follows from [1, ] and [11, ] together with [9, , Theorems 2 and 5] that

Therefore, the second, fourth, ninth, and twelfth inequalities follow from (61) and the first, sixth, seventh, eighth, and eleventh inequalities follow from (62) immediately, while the fifth and thirteenth inequalities can be derived from and the fact that and are, respectively, the mean values of , , and ,