- Research
- Open access
- Published:
A half-discrete Hilbert-type inequality with a homogeneous kernel and an extension
Journal of Inequalities and Applications volume 2011, Article number: 124 (2011)
Abstract
Using the way of weight functions and the technique of real analysis, a half-discrete Hilbert-type inequality with a general homogeneous kernel is obtained, and a best extension with two interval variables is given. The equivalent forms, the operator expressions, the reverses and some particular cases are considered.
2000 Mathematics Subject Classification: 26D15; 47A07.
1 Introduction
Assuming that p > 1, , f (≥ 0) ∈ Lp (R+), g(≥ 0) ∈ Lq (R+), , || g || q > 0, we have the following Hardy-Hilbert's integral inequality [1]:
where the constant factor is the best possible. If a m , b n ≥ 0, , , , || b || q > 0, then we still have the following discrete Hardy-Hilbert's inequality with the same best constant factor :
For p = q = 2, the above two inequalities reduce to the famous Hilbert's inequalities. Inequalities (1) and (2) are important in analysis and its applications [2–4].
In 1998, by introducing an independent parameter λ ∈ (0, 1], Yang [5] gave an extension of (1) for p = q = 2. Refinement and generalizing the results from [5], Yang [6] gave some best extensions of (1) and (2) as follows: If λ1, λ2 ∈ R, λ1 + λ2 = λ, kλ(x, y) is a non-negative homogeneous function of degree - λ satisfying for any x, y, t > 0, kλ(tx, ty) = t-λ kλ (x, y), , , , , g(≥ 0) ∈ L q , ψ , || f || p , ϕ , || g || q , ψ > 0, then we have
where the constant factor k(λ1) is the best possible. Moreover, if kλ(x, y) is finite and is decreasing with respect to x > 0(y > 0), then for a m ,b n ≥ 0, , , || a || p , ϕ , || b || q , Ψ > 0, we have
with the best constant factor k(λ1). Clearly, for λ = 1, , , (3) reduces to (1), and (4) reduces to (2). Some other results about Hilbert-type inequalities are provided by [7–15].
On half-discrete Hilbert-type inequalities with the non-homogeneous kernels, Hardy et al. provided a few results in Theorem 351 of [1]. But they did not prove that the constant factors are the best possible. And, Yang [16] gave a result by introducing an interval variable and proved that the constant factor is the best possible. Recently, Yang [17] gave the following half-discrete Hilbert's inequality with the best constant factor B(λ1, λ2)(λ, λ1 > 0, 0 < λ2 ≤ 1, λ1 + λ2 = λ):
In this article, using the way of weight functions and the technique of real analysis, a half-discrete Hilbert-type inequality with a general homogeneous kernel and a best constant factor is given as follows:
which is a generalization of (5). A best extension of (6) with two interval variables, some equivalent forms, the operator expressions, the reverses and some particular cases are considered.
2 Some lemmas
We set the following conditions:
Condition (i) v(y)(y ∈ [n0 - 1, ∞)) is strictly increasing with v(n0 - 1) ≥ 0 and for any fixed x ∈ (b, c), f(x, y) is decreasing for y ∈ (n0 - 1, ∞) and strictly decreasing in an interval of (n0 - 1, ∞).
Condition (ii) is strictly increasing with and for any fixed x ∈ (b, c), f(x, y) is decreasing and strictly convex for .
Condition (iii) There exists a constant β ≥ 0, such that v(y)(y ∈ [n0 - β, ∞)) is strictly increasing with v(n0 - β) ≥ 0, and for any fixed x ∈ (b, c), f(x, y) is piecewise smooth satisfying
where is Bernoulli function of the first order.
Lemma 1 If λ1, λ2 ∈ R, λ1 + λ2 = λ, kλ(x, y) is a non-negative finite homogeneous function of degree - λ in and v(y)(y ∈ [n0, ∞), n0 ∈ N) are strictly increasing differential functions with u(b+) = 0, v(n0) > 0, u(c-) = v(∞) = ∞, setting K(x, y) = kλ(u(x), v(y)), then we define weight functions ω(n) and ϖ(x) as follows:
It follows
Moreover, setting , if k(λ1) ∈ R+ and one of the above three conditions is fulfilled, then we still have
Proof . Setting in (7), by calculation, we have (9).
-
(i)
If Condition (i) is fulfilled, then we have
-
(ii)
If Condition (ii) is fulfilled, then by Hadamard's inequality [18], we have
-
(iii)
If Condition (iii) is fulfilled, then by Euler-Maclaurin summation formula [6], we have
The lemma is proved. ■
Lemma 2 Let the assumptions of Lemma 1 be fulfilled and additionally, p > 0(p ≠ 1), a n ≥ 0, n ≥ n0(n ∈ N), f (x) is a non-negative measurable function in (b, c). Then, (i) for p > 1, we have the following inequalities:
(ii) for 0 < p < 1, we have the reverses of (11) and (12).
Proof(i) By Hölder's inequality with weight [18] and (9), it follows
Then, by Lebesgue term-by-term integration theorem [19], we have
and (11) follows.
Still by Hölder's inequality, we have
Then, by Lebesgue term-by-term integration theorem, we have
and then in view of (9), inequality (12) follows.
-
(ii)
By the reverse Hölder's inequality [18] and in the same way, for q < 0, we have the reverses of (11) and (12). ■
3 Main results
We set , , wherefrom , .
Theorem 1 Suppose that λ1, λ2 ∈ R, λ1 + λ2 = λ, kλ(x, y) is a non-negative finite homogeneous function of degree - λ¸ in and v(y)(y ∈ [n0, ∞), n0 ∈ N are strictly increasing differential functions with u(b+) = 0, v(n0) > 0, u(c-) = v(∞) = ∞, ϖ(x) < k (λ1) ∈ R+(x ∈ (b, c)). If , f ( x ), a n ≥ 0, f ∈ L p Φ ( b , c ), || f || p ,Φ > 0 and || a || q ,Ψ > 0, then we have the following equivalent inequalities:
Moreover, if is decreasing and there exist constants δ < λ1 and M > 0, such that, then the constant factor k(λ1) in the above inequalities is the best possible.
Proof By Lebesgue term-by-term integration theorem, there are two expressions for I in (13). In view of (11), for ϖ(x) < k(λ1) ∈ R+, we have (14). By Hölder's inequality, we have
Then, by (14), we have (13). On the other hand, assuming that (13) is valid, setting
then Jp-1 = || a || q ,Ψ . By (11), we find J < ∞. If J = 0, then (14) is naturally valid; if J > 0, then by (13), we have
and we have (14), which is equivalent to (13).
In view of (12), for [ϖ(x) ]1-q> [k(λ1)]1-q, we have (15). By Hölder's in equality, we find
Then, by (15), we have (13). On the other hand, assuming that (13) is valid, setting
then Lq-1 = || f || p Φ . By (12), we find L < ∞. If L = 0, then (15) is naturally valid; if L > 0, then by (13), we have
and we have (15) which is equivalent to (13).
Hence, inequalities (13), (14) and (15) are equivalent.
There exists an unified constant d ∈ (b, c), satisfying u(d) = 1. For 0 < ε < p(λ1 - δ), setting , x ∈ (b, d); , x ∈ (d, c), and , n ≥ n0, if there exists a positive number k(≤ k(λ1)), such that (13) is valid as we replace k(λ1) by k, then in particular, we find
For , we find
namely A(ε) = O(1)(ε → 0+). Hence, by (18) and (19), it follows
By Fatou Lemma [19], we have , then by (20), it follows k(λ1) ≤ k(ε → 0+). Hence, k = k(λ1) is the best value of (12).
By the equivalence, the constant factor k(λ1) in (14) and (15) is the best possible, otherwise we can imply a contradiction by (16) and (17) that the constant factor in (13) is not the best possible. ■
Remark 1 (i) Define a half-discrete Hilbert's operator T : as: for f ∈ L p Φ (b, c), we define , satisfying
Then, by (14), it follows and then T is a bounded operator with || T || ≤ k(λ1). Since, by Theorem 1, the constant factor in (14) is the best possible, we have || T || = k(λ1).
-
(ii)
Define a half-discrete Hilbert's operator as: for a ∈ l q Ψ , we define , satisfying
Then, by (15), it follows and then is a bounded operator with . Since, by Theorem 1, the constant factor in (15) is the best possible, we have .
In the following theorem, for 0 < p < 1, or p < 0, we still use the formal symbols of and ||a|| q ,Ψ and so on. ■
Theorem 2 Suppose that λ1, λ2 ∈ R, λ1 + λ2 = λ, kλ(x, y) is a non-negative finite homogeneous function of degree -λ in , u(x)(x ∈ (b, c), -∞ ≤ b < c ≤ ∞) and v(y)(y ∈ [n0, ∞), n0 ∈ N) are strictly increasing differential functions with u(b+) = 0, v(n0) > 0, u(c-) = v(∞) = ∞, k(λ1) ∈ R+, θλ(x) ∈ (0, 1), k(λ1)(1 - θλ(x)) < ϖ(x) < k(λ1)(x ∈ (b, c)). If 0 < p < 1, , f(x), a n ≥ 0, , and 0 < ||a|| q ,Ψ < ∞. Then, we have the following equivalent inequalities:
Moreover, if is decreasing and there exist constants δ, δ0 > 0, such that and k(λ1 - δ0) ∈ R+, then the constant factor k(λ1) in the above inequalities is the best possible.
Proof. In view of (9) and the reverse of (11), for ϖ(x) > k(λ1)(1 - θλ(x)), we have (22). By the reverse Hölder's inequality, we have
Then, by (22), we have (21). On the other hand, assuming that (21) is valid, setting a n as Theorem 1, then Jp-1 = ||a|| q Ψ. By the reverse of (11), we find J > 0. If J = ∞, then (24) is naturally valid; if J < ∞, then by (21), we have
and we have (22) which is equivalent to (21).
In view of (9) and the reverse of (12), for [ϖ(x)]1-q> [k(λ1)(1 - θ λ (x))]1-q(q < 0), we have (23). By the reverse Hölder's inequality, we have
Then, by (23), we have (21). On the other hand, assuming that (21) is valid, setting
then . By the reverse of (12), we find . If , then (23) is naturally valid; if , then by (21), we have
and we have (23) which is equivalent to (21). ■
Hence, inequalities (21), (22) and (23) are equivalent.
For 0 < ε < pδ0, setting and as Theorem 1, if there exists a positive number k(≥ k(λ1)), such that (21) is still valid as we replace k(λ1) by k, then in particular, for q < 0, in view of (9) and the conditions, we have
Since we have , t ∈ (0, 1] and
then by Lebesgue control convergence theorem [19], it follows
By (26) and (27), we have
and then k(λ1) ≥ k(ε → 0+). Hence, k = k(λ1) is the best value of (21).
By the equivalence, the constant factor k(λ1) in (22) and (23) is the best possible, otherwise we can imply a contradiction by (24) and (25) that the constant factor in (21) is not the best possible. ■
In the same way, for p < 0, we also have the following theorem.
Theorem 3 Suppose that λ1, λ2 ∈ R, λ1 + λ2 = λ, kλ(x, y) is a non-negative finite homogeneous function of degree -λ¸ in , u(x)(x ∈ (b, c), -∞ ≤ b < c ≤ ∞) and v(y)(y ∈ [n0, ∞), n0 ∈ N) are strictly increasing differential functions with u(b+) = 0, v(n0) > 0, u(c-) = v(∞) = ∞, ϖ(x) < k(λ1) ∈ R + (x ∈ (b, c)). If p < 0, , f(x), a n ≥ 0, 0 < || f || p Φ < ∞ and 0 < ||a|| q ,Ψ < ∞. Then, we have the following equivalent inequalities:
Moreover, if is decreasing and there exists constant δ0 > 0, such that k(λ1 + δ0) ∈ R+, then the constant factor k(λ1) in the above inequalities is the best possible.
Remark 2 (i) For n0 = 1, b = 0, c = ∞, u(x) = v(x) = x, if
then (13) reduces to (6). In particular, for ,(6) reduces to (5).
-
(ii)
For n0 = 1, b = 0, c = ∞, u(x) = v(x) = xα(α > 0), , since
is decreasing for y ∈ (0, ∞) and strictly decreasing in an interval of (0, ∞), then by Condition (i), it follows
Since for , , then by (13), we have the following inequality with the best constant factor :
-
(iii)
For n0 = 1, b = β, c = ∞, , , since for any fixed x ∈ (β, ∞),
is decreasing and strictly convex for , then by Condition (ii), it follows
Since for , , then by (13), we have the following inequality with the best constant factor :
-
(iv)
For n0 = 1, b = 1 - β = γ, c = ∞, u(x) = v(x) = (x - γ), , , , we have
and
Hence, v(y)(y ∈ [γ, ∞)) is strictly increasing with v(1 - β) = v(γ) = 0, and for any fixed x ∈ (γ, ∞), f(x, y) is smooth with
We set
For x ∈ (γ, ∞), 0 < λ ≤ 4, by (33) and the following improved Euler-Maclaurin summation formula [6]:
we have
Since for , i.e. ,
then h(x) is strictly decreasing and R(x) > h(x) > h(∞) = 0.
Then, by Condition (iii), it follows . For , it follows
and by (13), we have the following inequality with the best constant factor :
where .
References
Hardy GH, Littlewood JE, Pólya G: Inequalities. Cambridge University Press, Cambridge, UK; 1934.
Mitrinović DS, Pečarić JE, Fink AM: Inequalities Involving Functions and their Integrals and Derivatives. Kluwer Academic Publishers, Boston, MA; 1991.
Yang B: Hilbert-Type Integral Inequalities. In Bentham Science Publishers Ltd. Arabia Unites a Tribal Chief Country; 2009.
Yang B: Discrete Hilbert-Type Inequalities. In Bentham Science Publishers Ltd. Arabia Unites a Tribal Chief Country; 2011.
Yang B: On Hilbert's integral inequality. J Math Anal Appl 1998, 271: 778–785.
Yang B: The Norm of Operator and Hilbert-Type Inequalities. Science Press, Beijing, China; 2009.
Yang B, Brnetić I, Krnić M, Pečarić J: Generalization of Hilbert and Hardy-Hilbert integral inequalities. Math Inequal Appl 2005,8(2):259–272.
Krnić M, Pečarić J: Hilbert's inequalities and their reverses. Publ Math Debrecen 2005,67(3–4):315–331.
Jin J, Debnath L: On a Hilbert-type linear series operator and its applications. J Math Anal Appl 2010, 371: 691–704. 10.1016/j.jmaa.2010.06.002
Azar L: On some extensions of Hardy-Hilbert's inequality and applications. J Inequal Appl 2006,2009(Article ID 546829):12.
Yang B, Rassias T: On the way of weight coefficient and research for Hilbert-type inequalities. Math Inequal Appl 2003,6(4):625–658.
Arpad B, Choonghong O: Best constant for certain multilinear integral operator. J Inequal Appl 2006,2006(Article ID 28582):12.
Kuang J, Debnath L: On Hilbert's type inequalities on the weighted Orlicz spaces. Pacific J Appl Math 2007,1(1):95–103.
Zhong W: The Hilbert-type integral inequality with a homogeneous kernel of Lambda-degree. J Inequal Appl 2008,2008(Article ID 917392):13.
Li Y, He B: On inequalities of Hilbert's type. Bull Aust Math Soc 2007,76(1):1–13. 10.1017/S0004972700039423
Yang B: A mixed Hilbert-type inequality with a best constant factor. Int J Pure Appl Math 2005,20(3):319–328.
Yang B: A half-discrete Hilbert's inequality. J Guangdong Univ Edu 2011,31(3):1–7.
Kuang J: Applied Inequalities. Shangdong Science Technic Press, Jinan, China; 2004.
Kuang J: Introduction to Real Analysis. Hunan Education Press, Chan-sha, China; 1996.
Acknowledgements
This study was supported by the Guangdong Science and Technology Plan Item (No. 2010B010600018) and Guangdong Modern Information Service Industry Develop Particularly item 2011 (No. 13090).
Author information
Authors and Affiliations
Corresponding author
Additional information
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
BY wrote and reformed the article. QC conceived of the study, and participated in its design and coordination. All authors read and approved the final manuscript.
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
About this article
Cite this article
Yang, B., Chen, Q. A half-discrete Hilbert-type inequality with a homogeneous kernel and an extension. J Inequal Appl 2011, 124 (2011). https://doi.org/10.1186/1029-242X-2011-124
Received:
Accepted:
Published:
DOI: https://doi.org/10.1186/1029-242X-2011-124