By means of weight functions and the improved Euler-Maclaurin summation formula, a more accurate half-discrete Hilbert-type inequality with a non-homogeneous kernel and a best constant factor is given. A best extension, some equivalent forms, the operator expressions as well as some particular cases are also considered.
MSC:26D15, 47A07.
Hinweise
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
BY carried out the molecular genetic studies participated in the sequence alignment and drafted the manuscript. XL conceived of the study and participated in its design and coordination. All authors read and approved the final manuscript.
1 Introduction
Assuming that , , , we have the following Hilbert’s integral inequality (cf. [1]):
(1)
where the constant factor π is best possible. If , , , then we have the following analogous discrete Hilbert’s inequality:
(2)
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with the same best constant factor π. Inequalities (1) and (2) are important in analysis and its applications (cf. [2‐4]).
In 1998, by introducing an independent parameter , Yang [5] gave an extension of (1). For generalizing the results from [5], Yang [6] gave some best extensions of (1) and (2) as follows. If , , , is a non-negative homogeneous function of degree −λ satisfying , , , , , and , , then
(3)
where the constant factor is best possible. Moreover, if is finite and is decreasing for (), then for , , and , , , we have
(4)
where the constant is still the best value. Clearly, for , , , , (3) reduces to (1), while (4) reduces to (2).
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Some other results about integral and discrete Hilbert-type inequalities can be found in [7‐16]. On half-discrete Hilbert-type inequalities with the general 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 best possible. In 2005, Yang [17] gave a result with the kernel by introducing a variable and proved that the constant factor is best possible. Very recently, Yang [18] and [19] gave the following half-discrete Hilbert’s inequality with the best constant factor:
(5)
Chen [20] and Yang [21] gave two more accurate half-discrete Mulholland’s inequalities by using Hadamard’s inequality.
In this paper, by means of weight functions and the improved Euler-Maclaurin summation formula, a more accurate half-discrete Hilbert-type inequality with a non-homogeneous kernel and a best constant factor is given as follows. For , , ,
(6)
Moreover, a best extension of (6), some equivalent forms, the operator expressions as well as some particular inequalities are considered.
2 Some lemmas
Lemma 1If , , (), () are decreasing continuous functions satisfying , , define a functionas follows:
Then there existssuch that
(7)
whereis a Bernoulli function of the first order. In particular, for , , we haveand
(8)
for , , if , then it followsand
(9)
Proof Define a decreasing continuous function as
Then it follows
Since , is a non-constant decreasing continuous function with , by the improved Euler-Maclaurin summation formula (cf. [6], Theorem 2.2.2), it follows
and then in view of the above results and by simple calculation, we have (7). □
Lemma 2If , , , andandare weight functions given by
(10)
(11)
then we have
(12)
Proof Substituting in (10), and by simple calculation, we have
For fixed , we find
By the Euler-Maclaurin summation formula (cf. [6]), it follows
(13)
(i)
For , we obtain , and
Setting , wherefrom , and
then by (7), we find
In view of (11) and the above results, since for , namely , it follows
(ii)
For , we obtain , and
Since for , , by the improved Euler-Maclaurin summation formula (cf. [6]), it follows
In view of (13) and the above results, for , we find
Hence, for , we have , and then (12) follows. □
Lemma 3Let the assumptions of Lemma 2 be fulfilled and, additionally, let , , , , be a non-negative measurable function in . Then we have the following inequalities:
(14)
(15)
Proof Setting , by Hölder’s inequality (cf. [22]) and (12), it follows
Then by the Lebesgue term-by-term integration theorem (cf. [23]), we have
Hence, (14) follows. By Hölder’s inequality again, we have
By the Lebesgue term-by-term integration theorem, we have
and in view of (12), inequality (15) follows. □
Lemma 4Let the assumptions of Lemma 2 be fulfilled and, additionally, let , , . Setting , ; , , and , , then we have
(16)
(17)
Proof We find
and then (16) is valid. We obtain
and so (17) is valid. □
3 Main results
We introduce the functions
wherefrom and .
Theorem 5If , , , , , , , , and , then we have the following equivalent inequalities:
(18)
(19)
(20)
where the constantis the best possible in the above inequalities.
Proof The two expressions for I in (18) follow from the Lebesgue term-by-term integration theorem. By (14) and (12), we have (19). By Hölder’s inequality, we have
Then by (19), we have (18). On the other hand, assume that (18) is valid. Setting
where . By (14), we find . If , then (19) is trivially valid; if , then by (18) we have
therefore ; that is, (19) is equivalent to (18). On the other hand, by (12) we have . Then in view of (15), we have (20). By Hölder’s inequality, we find
Then by (20), we have (18). On the other hand, assume that (18) is valid. Setting
then . By (15), we find . If , then (20) is trivially valid; if , then by (18), we have
therefore ; that is, (20) is equivalent to (18). Hence, (18), (19) and (20) are equivalent.
If there exists a positive number k () such that (18) is valid as we replace with k, then, in particular, it follows that . In view of (16) and (17), we have
and (). Hence, is the best value of (18).
By the equivalence of the inequalities, the constant factor in (19) and (20) is the best possible. □
Remark 1 (i) Define the first type half-discrete Hilbert-type operator as follows. For , we define by
Then by (19), and so is a bounded operator with . Since by Theorem 5 the constant factor in (19) is best possible, we have .
(ii)
Define the second type half-discrete Hilbert-type operator as follows. For , we define by
Then by (20), and so is a bounded operator with . Since by Theorem 5 the constant factor in (20) is best possible, we have .
Remark 2 (i) For , (18) reduces to (6). Since we find
then for in (18), we have the following inequality:
(21)
Hence, (18) is a more accurate inequality of (21).
(ii)
For in (18), we have , , , and
(22)
for in (18), we have , , , and
(23)
for in (18), we have , , , and
(24)
Acknowledgements
This work is supported by Guangdong Natural Science Foundation (No. 7004344).
Open AccessThis 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.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
BY carried out the molecular genetic studies participated in the sequence alignment and drafted the manuscript. XL conceived of the study and participated in its design and coordination. All authors read and approved the final manuscript.