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On a New Weighted Hilbert Inequality
Journal of Inequalities and Applications volume 2008, Article number: 637397 (2008)
Abstract
It is shown that a weighted Hilbert inequality for double series can be established by introducing a proper weight function. Thus, a quite sharp result of the classical Hilbert inequality for double series is obtained. And a similar result for the Hilbert integral inequality is also proved. Some applications are considered.
1. Introduction
Let and be two sequences of real numbers. It is all known that the inequality
is called Hilbert theorem for double series [1], where , and the constant factor in (1.1) is the best possible value. And the equality in (1.1) holds if and only if or is a zerosequence. The corresponding integral form of (1.1) is that
where , and the constant factor in (1.2) is the best possible value. Recently, various improvements and extensions of (1.1) and (1.2) appear in a great deal of papers (see [2–6], etc.). The aim of the present paper is to build some new inequalities by using the weight function method and the technique of analysis, and then to study some applications of them.
First we give some lemmas.
Lemma 1.1.
Let be a positive integer. Then
Proof.
Let be real numbers. Then
where is an arbitrary constant. This result is given in the paper (see [7]). Based on this indefinite integral it is easy to deduce that the equality (1.3) holds.
Lemma 1.2.
If and , where , then
(1) and are monotonously decreasing in interval ;

(2)
(1.5)
where the weight function is defined by
.
Proof.

(1)
At first, notice that , hence we can write in the form of: , where and . It is obvious that the functions and are monotonously decreasing in . So, is also monotonously decreasing in . In the next place, notice that , therefore we can write in the form of: , where and . It is clear that the functions and are monotonously decreasing in . So, is also monotonously decreasing in .

(2)
Below, we need only to compute the first integral,
(1.7)
By Lemma 1.1, we obtain the first integral of (1.5) at once after some simple computations and simplifications.
Similarly, the second integral of (1.5) can be gotten.
Lemma 1.3.
Let be a sequence of real numbers, and let be a real function and . If , then
Proof.
It is obvious that
We need only to show that
Let Then
Lemma 1.4.
Let be a real number, and let and be two real functions, and and , where . Then
.
Its proof is similar to that of Lemma 1.3. Hence, it is omitted.
2. Main Results
Theorem 2.1.
Let and be two sequences of real numbers. If and , then
where the weight function is defined by (1.6).
Proof.
Let be a real function and it satisfies condition First, we suppose that . By Lemma 1.3 and then applying Cauchy's inequality we have
where
It is easy to deduce that
Let . It is obvious that for and . Consider the function By Lemma 1.2, the function is monotonously decreasing in . Hereby, we have
Using (1.5), we can obtain immediately
Similarly, we have
It follows from (2.2), (2.6), and (2.7) that
where the weight function is defined by (1.6).
Next, consider the case for We can apply Schwarz's inequality to estimate the lefthand side of (2.1) as follows:
And then by using the inequality (2.8), the inequality (2.1) follows from (2.9) at once. It is obvious that the inequality (2.1) is a refinement of (1.1). Below, we give an extension of (1.2).
Theorem 2.2.
Let be a real number, and , and let and be two real functions, and and . Then
where the weight function is defined by
Specially, when , it is a refinement of (1.2).
Proof.
Let be a real function, and .
Firstly, we suppose that . Using Lemma 1.4 and then applying Cauchy's inequality we have
where
In the first place, we consider :
where
We need only to compute the weight function .
Let us select still . Then . Using (1.4), we have
Notice that . Hence, the function defined by (2.11) is just. So, we attain
Similarly, we have
We obtain from (2.12), (2.17), and (2.18) that
where the weight function is defined by (2.11).
Secondly, consider the case for . We can apply Schwarz's inequality to estimate the lefthand side of (2.10) as follows:
It follows from (2.19) and (2.20) that the inequality (2.10) is valid.
3. Applications
As applications, we will give some new refinements of HardyLittlewood's theorem and Widder's theorem below.
Let and for all . Define a sequence by
HardyLittlewood [1] proved that
where is the best constant that the inequality (3.2) keeps valid.
Theorem 3.1.
With the assumptions as the abovementioned, define a sequence by . Then
where is defined by (1.6).
Proof.
By our assumptions, we may write in the form of:
Apply Schwarz's inequality to estimate the lefthand side of (3.3) as follows:
It is known from (2.8) and (3.4) that the inequality (3.3) is valid. Theorem is therefore proved.
Let and . Then
This is the famous Widder theorem (see [1]).
Theorem 3.2.
With the assumptions as the abovementioned, then
where is defined by (2.11).
Proof.
First, we have the following relation:
Let . Then we have
where . By using (2.19), the inequality (3.6) follows from (3.8) at once.
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Acknowledgment
This work is a project supported by Scientific Research Fund of Hunan Provincial Education Department (07C520 and 06C657).
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Leping, H., Xuemei, G. & Mingzhe, G. On a New Weighted Hilbert Inequality. J Inequal Appl 2008, 637397 (2008). https://doi.org/10.1155/2008/637397
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Keywords
 Positive Integer
 Real Number
 Weight Function
 Function Method
 Simple Computation