# Multidimensional Hilbert-Type Inequalities with a Homogeneous Kernel

- Predrag Vuković
^{1}Email author

**2009**:130958

**DOI: **10.1155/2009/130958

© Predrag Vuković. 2009

**Received: **11 July 2009

**Accepted: **18 November 2009

**Published: **24 November 2009

## Abstract

We consider the Hilbert-type inequalities with nonconjugate parameters. The obtaining of the best possible constants in the case of nonconjugate parameters remains still open. Our generalization will include a general homogeneous kernel. Also, we obtain the best possible constants in the case of conjugate parameters when the parameters satisfy appropriate conditions. We also compare our results with some known results.

## 1. Introduction

The well-known Hardy-Hilbert's integral inequality (see [1]) is given by

and an equivalent form is given by

where the constant factors and are the best possible.

During the previous decades, the Hilbert-type inequalities were discussed by many authors, who either reproved them using various techniques or applied and generalized them in many different ways. For example, we refer to a paper of Yang (see [2]). If satisfy

where the constant factor is the best possible.

Our generalization will include a general homogeneous kernel , where , with being nonconjugate parameters. The techniques that will be used in the proofs are mainly based on classical real analysis, especially on the well-known Hölder's inequality and on Fubini's theorem. The obtaining of the best possible constants in the case of nonconjugate parameters seems to be a very difficult problem and it remains still open.

Let us recall the definition of nonconjugate exponents (see [3]). Let and be real parameters, such that

and let and , respectively, be their conjugate exponents, that is, and . Further, define

and note that for all and values as in (1.6). In particular, holds if and only if , that is, only when and are mutually conjugate. Otherwise, , and in such cases and will be referred to as nonconjugate exponents.

Considering , , and as in (1.6) and (1.7), Hardy et al. [1], proved that there exists a constant , dependent only on the parameters and , such that the following Hilbert-type inequality holds for all nonnegative functions and :

Conventions

## 2. Main Results

Before presenting our idea and results, we repeat the notion of general nonconjugate exponents from [3]. Let be the real parameters which satisfy

Further, the parameters , are defined by the equations

Since , , it is obvious that , . We define

It is easy to deduce that . Also, we introduce the parameters , , defined by the relations

In order to obtain our results we need to require

It is easy to see that the above conditions do not automatically apply (2.5). Further, it follows

Of course, if , then ; so the conditions (2.1)–(2.4) reduce to the case of conjugate parameters.

Results in this section will be based on the following general form of Hardy-Hilbert's inequality proven in [4]. All the measures are assumed to be -finite on some measure space.

Theorem 2.1.

In the same paper the authors discussed the case of equality in inequalities (2.7) and (2.8). They proved that the equality holds in (2.7) (and analogously in (2.8)) if and only if

In the following theorem we give the most important case where , the measures are Lebesgue measures, is a nonnegative homogeneous function of degree , and the functions represent the form where , . In order to obtain the generalizations of some known results we define

where we suppose that for and .

Due to technical reasons, we introduce real parameters satisfying

We also define

Theorem 2.2.

Proof.

for . This gives inequalities (2.14) with inequality sign . Condition (2.10) immediately gives that nontrivial case of equality in (2.14) leads to the divergent integrals. This completes the proof.

Remark 2.3.

where we used the well-known formula for gamma function (see, e.g., [5, Lemma ]). Now, by using Theorem 2.2 and (2.22) we obtain the result of Krnić et al. (see [6]).

## 3. The Best Possible Constants in the Conjugate Case

In this section we consider the inequalities in Theorem 2.2. In such a way we shall obtain the best possible constants for some general cases.

It follows easily that Theorem 2.2 in the conjugate case ( ) becomes as follows.

Theorem 3.1.

To obtain a case of the best inequality it is natural to impose the following conditions on the parameters :

In that case the constant from Theorem 3.1 is simplified to the following form:

where

Further, by using (3.4) and (3.5), the inequalities (3.1) with the parameters satisfying the relation (3.3), become

Theorem 3.2.

for some , then the constant is the best possible in inequalities (3.6) and (3.7).

Proof.

Let us suppose that the constant factor given by (3.4) is not the best possible in the inequality (3.6). Then, there exists a positive constant , such that (3.6) is still valid when we replace by .

We define the real functions by the formulas

Further, let denotes the left-hand side of the inequality (3.6), for the above choice of the functions By applying the -dimensional spherical coordinates and the substitutions we find

We conclude, by using (3.10), (3.12), and (3.16), that which is an obvious contradiction. It follows that the constant in (3.6) is the best possible.

Finally, the equivalence of the inequalities (3.6) and (3.7) means that the constant is also the best possible in the inequality (3.7). That completes the proof.

Remark 3.3.

If the kernel and the parameters satisfy the conditions from Theorem 3.2, then the constant is the best possible. For example, setting in the inequality (3.17), we obtain Yang's result (1.5) from introduction.

## Declarations

### Acknowledgment

This research is supported by the Croatian Ministry of Science, Education and Sports, Grant no. 058-1170889-1050.

## Authors’ Affiliations

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## Copyright

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