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Refinements and Sharpening of some Huygens and Wilker Type Inequalities

Wei-Dong Jiang, Qiu-Ming Luo, Feng Qi*

College of Mathematics, Inner Mongolia University for Nationalities, Tongliao City, Inner Mongolia Autonomous Region, China

Abstract: In the article, some Huygens and Wilker type inequalities involving trigonometric and hyperbolic functions are refined and sharpened.

Keywords: refinement, sharpening, Huygens inequality, Wilker inequality, trigonometric function, hyperbolic function

Received June 30, 2014; Revised August 16, 2014; Accepted August 21, 2014

Citation: Wei-Dong Jiang, Qiu-Ming Luo, Feng Qi, Refinements and Sharpening of some Huygens and Wilker Type Inequalities. Turkish Journal of Analysis and Number Theory, 2 (4). 134-139. January 2014. doi:10.12691/tjant-2-4-6

1. Introduction

The famous Huygens inequality for the sine and tangent functions states that for

[picture] | (1.1)

The hyperbolic counterpart of (1.1) was established in [10] as follows: For

[picture] | (1.2)

The inequalities (1.1) and (1.2) were respectively refined in [[10], Theorem 2.6] as

[picture] | (1.3)

for and

In [8] the inequality (1.2) was improved as

[picture] | (1.4)

In [10], the following inequality is given

[picture] | (1.5)

For more information in this area, please refer to [13,16], [[14], Section 1.7 and Section 7.3] and closely related references therein.

In [18], Wilker proved

[picture] | (1.6)

and proposed that there exists a largest constant c such that

[picture] | (1.7)

for In [17], the best constant c in (1.7) was found and it was proved that

for The constants and in the above inequality are the best possible. For more information on this topic, please see [4,5,19], [[14], pp. 38-40, Section 8] and closely related references therein.

Recently the inequalities (1.3) and (1.6) were respectively refined in [9] as

and

[picture] | (1.8)

The hyperbolic counterparts of the last two inequalities in (1.8) were also given in [9] as follows:

The aim of this paper is to refine and sharpen some of the above-mentioned Huygens and Wilker type inequalities.

2. Some Lemmas

In order to attain our aim, we need several lemmas below.

Lemma 2.1. The Bernoulli numbers for have the property

[picture] | (2.1)

where the Bernoulli numbers for are defined by

Proof. In [[2], p. 16 and p. 56], it is listed that for

[picture] | (2.2)

where is the Riemann zeta function defined by

[picture] | (2.3)

From (2.2), the formula (2.1) follows.

Lemma 2.2. For we have

[picture] | (2.4)

Proof. This is an easy consequence of combining the equality

[picture] | (2.5)

see [[1], p. 75, 4.3.68], with Lemma 2.1.

Lemma 2.3 ([[1], p. 75, 4.3.70]). For

[picture] | (2.6)

Lemma 2.4. For

[picture] | (2.7)

Proof. Since

the formula (2.7) follows from differentiating (2.6).

Lemma 2.5. For

[picture] | (2.8)

Proof. This follows from differentiating on both sides of (2.5) and using (2.1).

Lemma 2.6. For

[picture] | (2.9)

and

[picture] | (2.10)

Proof. Combining

with Lemma 2.5, the identity (2.5), and Lemma 2.1 gives (2.9).

The equality (2.10) follows from combination of

with Lemma 2.4.

Lemma 2.7. Let f and g be continuous on and differentiable in such that in If is increasing (or decreasing) in , then the functions and are also increasing (or decreasing) in

The above Lemma 2.7 can be found, for examples, in [[3], p. 292, Lemma 1], [[6], p. 57, Lemma 2.3], [[11], p. 92, Lemma 1], and [[12], p. 161, Lemma 2.3].

Lemma 2.8. Let and for be real numbers and the power series

[picture] | (2.11)

be convergent on for some If and the ratio is strictly increasing for then the function is also strictly increasing on

The above Lemma 2.8 can be found, for examples, in [[3], p. 292, Lemma 2], [[15], p. 71, Lemma 1], and [[20], Lemma 2.2].

3. Main Results

Now we are in a position to state and prove our main results, refinements and sharpening of some Huygens and Wilker type inequalities mentioned in the first section.

Theorem 3.1. For we have

[picture] | (3.1)

The scalars and in (3.1) are the best possible.

Proof. Let

for . By virtue of (2.4), (2.7), and (2.8), we

have

So the function is strictly increasing on . Moreover, it is easy to obtain

The proof of Theorem 3.1 is complete.

Theorem 3.2. For

[picture] | (3.2)

The constants and in (3.2) are the best possible.

Proof. By using (2.4), (2.7), (2.9), and (2.10), the function

may be expanded as

Since and

for the sequence is increasing and Thus, the function is increasing on Moreover,

and

The proof of Theorem 3.2 is complete.

Theorem 3.3. For x > 0, we have

[picture] | (3.3)

The constant is the best possible.

Proof. Let

and let

and

From the power series expansions

[picture] | (3.4)

and

[picture] | (3.5)

it follows that

and

It is easy to see that the quotient

satisfies

for This means that the sequence is increasing. By Lemma 2.8, the function is increasing on , and so, by Lemma 2.7, the function is increasing on Moreover, it is not difficult to obtain Theorem 3.3 is thus proved.

Theorem 3.4. For x > 0,

[picture] | (3.6)

The number in (3.6) is the best possible.

Proof. Let

and let

and

By the power series expansions in (3.4), we obtain

and

The ratio

satisfies

and

Furthermore, when by a simple computation, we have

where

and

for . Hence, the sequence is increasing. By Lemma 2.8, the function is increasing. Finally, it is easy to see that The proof of Theorem 3.4 is complete.

Theorem 3.5. For we have

[picture] | (3.7)

The numbers and are the best possible.

Proof. Let

By (2.4), (2.6), and (2.7), we have

This shows that the function is increasing on . Moreover, it is straightforward to obtain

and

The proof of Theorem 3.5 is complete.

Remark 3.1. This paper is a slightly revised version of the preprint [7].

Acknowledgements

The first author was partially supported by the Project of Shandong Province Higher Educational Science and Technology Program under grant No. J11LA57, China. The second author was partially supported by the NNSF under Grant No. 11361038, by the Natural Science Foundation Project of Chongqing City under Grant No. CSTC2011JJA00024, by the Research Project of Science and Technology of Chongqing Education Commission under Grant No. KJ120625, and by the Fund of Chongqing Normal University, China under Grant No. 10XLR017 and 2011XLZ07, China

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