Abstract
By using the way of weight functions and the idea of introducing parameters and by means of Hadamard’s inequality, we give a more accurate half-discrete Hilbert-type inequality with a best constant factor. We also consider its best extension with parameters, equivalent forms, operator expressions as well as some reverses.
MSC:26D15, 47A07.
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1 Introduction
If , , , such that and then we have the following famous Hilbert-type integral inequality (cf. [1]):
where the constant factor is the best possible. The integral analogue of inequality (1) is given as follows (cf. [1]). If , , and are non-negative real functions such that and , then
where the constant factor is the best possible. We named inequality (2) Hilbert-type integral inequality. In 2007, Yang proved the following more accurate Hilbert-type inequality (cf. [2]). If , , , , such that and , then
where the constant factor is still the best possible. Inequalities (1)-(3) are important in mathematical analysis and its applications [3]. There are lots of improvements, generalizations, and applications of inequalities (1)-(3); for more details, refer to [4–17].
At present, the research into half-discrete Hilbert-type inequalities is a new direction and has gradually heated up. We find a few results on the half-discrete Hilbert-type inequalities with the non-homogeneous kernel, which were published earlier (cf. [1], Th. 351 and [18]). Recently, Yang has given some half-discrete Hilbert-type inequalities (cf. [19–25]). Zhong proved a half-discrete Hilbert-type inequality with the non-homogeneous kernel as follows (cf. [26]). If , , , , is a measurable function in such that and , then
where the constant factor is the best possible.
In this paper, by using the way of weight functions and the idea of introducing parameters and by means of Hadamard’s inequality, we give a half-discrete Hilbert-type inequality with a best constant factor as follows:
The main objective of this paper is to consider its more accurate extension with parameters, equivalent forms, operator expressions as well as some reverses.
2 Some lemmas
Lemma 1 If , , define the following beta function (cf. [1]):
Lemma 2 Suppose that , , , , . Define the weight functions and as follows:
Setting , we have the following inequalities:
Proof Putting in (8), we have
For fixed , setting
in view of the conditions, we find and (cf. [27]). By Hadamard’s inequality (cf. [28]),
and putting , we obtain
where,
Since and , in view of the bounded properties of a continuous function, there exists such that (). For , we have
Hence, we proved that (9) and (10) are valid. □
Lemma 3 Suppose that , , (), , , , , and is a non-negative real measurable function in . Then
-
(i)
for , we have the following inequalities:
(15)
where and are defined by (7) and (8).
-
(ii)
for (), we have the reverses of (15) and (16).
Proof (i) By (7)-(10) and Hölder’s inequality [28], we have
By the Lebesgue term-by-term integration theorem [29] and (9), we obtain
Hence, (15) is valid. Using Hölder’s inequality, the Lebesgue term-by-term integration theorem, and (9) again, we have
Hence, (16) is valid.
-
(ii)
For () or (), using the reverse Hölder inequality, in the same way, we obtain the reverses of (15) and (16). □
Lemma 4 By the assumptions of Lemma 2 and Lemma 3, we set , , ,
(Note If , then and are normal spaces; if or , then both and are not normal spaces, but we still use the formal symbols in the following.)
For , setting , and as follows:
-
(i)
if , there exists a constant such that
(22)
then it follows
-
(ii)
if , there exists a constant such that
(24)
then it follows
-
(iii)
if , there exists a constant such that
(26)
then it follows
Proof We can obtain
-
(i)
For , then , , by (22), (28), and (29), we find
(31)
Setting , in the above integral, we have
and by the Fubini theorem [30], it follows
In view of (33) and (34), it follows that
Then by (31) and (35), (23) is valid.
-
(ii)
For , by (24) and (29), we find (notice that )
(36)
On the other hand, setting in , we have
By virtue of (36) and (37), (25) is valid.
-
(iii)
For , then , by (26) and (30), we find
(38)
Then by (37) and (38), (27) is valid. □
3 Main results and applications
Theorem 5 Suppose that , , , , , , , , , , satisfying , , , , then we have the following equivalent inequalities:
where the constant factor is the best possible.
Proof By the Lebesgue term-by-term integration theorem [29], we find that there are two expressions of I in (39). By (9), (15), and , we have (40). By Hölder’s inequality, we find
Then by (40), (39) is valid. On the other hand, assuming that (39) is valid, set
Then by (39), we have
By (9), (15), and , it follows that . If , then (40) is trivially valid; if , then . Thus, the conditions of applying (39) are fulfilled, and by (39), (44) takes a strict sign inequality. Thus, we find
Hence, (40) is valid, which is equivalent to (39).
By (9), (16), and , we obtain (41). By Hölder’s inequality again, we have
Hence, (39) is valid by using (41). On the other hand, assuming that (39) is valid, set
Then by (39), we find
By (9), (16), and , it follows that . If , then (41) is trivially valid; if , then , i.e., the conditions of applying (39) are fulfilled and by (48), we still have
Hence, (41) is valid, which is equivalent to (39). It follows that (39), (40), and (41) are equivalent.
If there exits a positive number such that (39) is still valid as we replace by k, then, in particular, (22) is valid ( are taken as (21)). Then we have (23). By (11), the Fatou lemma [30], and (23), we have
Hence, is the best value of (39). We confirm that the constant factor in (40) ((41)) is the best possible. Otherwise, we can get a contradiction by (42) ((46)) that the constant factor in (39) is not the best possible. □
Remark 6 (i) Define a half-discrete Hilbert operator as follows. For , we define satisfying
Then by (40), it follows , i.e., T is the bounded operator with . Since the constant factor in (40) is the best possible, we have .
-
(ii)
Define a half-discrete Hilbert operator in the following way. For , we define satisfying
Then by (41), it follows , i.e., is the bounded operator with . Since the constant factor in (41) is the best possible, we have .
Theorem 7 Suppose that , , , , , , , , , , satisfying , , , , then we have the following equivalent inequalities:
where the constant factor is the best possible.
Proof By (9), the reverse of (15), and , we have (50). Using the reverse Hölder inequality, we obtain the reverse form of (42) as follows:
Then by (50), (49) is valid.
On the other hand, if (49) is valid, set as (43), then (44) still holds with . By (49), we have
Then by (9), the reverse of (18), and , it follows that . If , then (50) is trivially valid; if , then , i.e., the conditions of applying (49) are fulfilled, and by (53), we still have
Hence, (50) is valid, which is equivalent to (49).
By the reverse of (16), in view of and , we have
Then (51) is valid. By the reverse Hölder inequality again, we have
Hence, (49) is valid by (51). On the other hand, if (49) is valid, set
Then by the reverse of (16) and , it follows that . If , then (51) is trivially valid; if , then , i.e., the conditions of applying (49) are fulfilled, and we have
Hence, (51) is valid, which is equivalent to (49). It follows that (49), (50), and (51) are equivalent.
If there exists a positive number such that (49) is still valid as we replace by k, then, in particular, (24) is valid. Hence, we have (25). For in (25), we obtain . Hence, is the best value of (49). We confirm that the constant factor in (50) ((51)) is the best possible. Otherwise, we can get a contradiction by (52) ((54)) that the constant factor in (49) is not the best possible. □
Theorem 8 If the assumption of in Theorem 5 is replaced by , then the reverses of (39), (40), and (41) are valid and equivalent. Moreover, the same constant factor is the best possible.
Proof In a similar way as in Theorem 7, we can obtain that the reverses of (39), (40), and (41) are valid and equivalent.
If there exists a positive number such that the reverse of (39) is still valid as we replace by k, then, in particular, (26) is valid. Hence, we have (27). For in (27), we obtain . Hence, is the best value of the reverse of (39). We confirm that the constant factor in the reverse of (40) ((41)) is the best possible. Otherwise, we can get a contradiction by the reverse of (42) ((46)) that the constant factor in the reverse of (39) is not the best possible. □
Remark 9 (i) For in (39), it follows
In particular, for , , (55) reduces to (5).
-
(ii)
For in (39), we have
(56)
which is more accurate than (55).
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Acknowledgements
This work is supported by the Emphases Natural Science Foundation of Guangdong Institution, Higher Learning, College and University (No. 05Z026), and Guangdong Natural Science Foundation (No. 7004344).
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AW carried out the study, and wrote the manuscript. BY participated in its design and coordination. All authors read and approved the final manuscript.
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Wang, A., Yang, B. A new more accurate half-discrete Hilbert-type inequality. J Inequal Appl 2012, 260 (2012). https://doi.org/10.1186/1029-242X-2012-260
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DOI: https://doi.org/10.1186/1029-242X-2012-260