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A kind of extension of the famous Young inequality
Journal of Inequalities and Applications volume 2013, Article number: 437 (2013)
Abstract
Young inequality, extended in (Geometry of Orlicz Spaces, 1986; Geometry of Orlicz Spaces, 1996), has extensive use and great effort in mathematical analysis. By the kind of extended Young inequality, we can get the famous Holder inequality and the Minkowski inequality. But until now, we have not found its strict proof of analysis. In (Geometry of Orlicz Spaces, 1986; Geometry of Orlicz Spaces, 1996), only the probable pattern description was found. In this paper, we will get the strict proof of analysis of a kind of extension of Young inequality with the approximation method.
MSC:46B20, 46B02, 46A22.
1 Introduction
The original Young inequality [1] has been proposed in an integral form by Young in 1912. Suppose that is a strictly increasing and continuous function defined in , is the inverse function of , , , . Then
where the equality holds if and only if .
Young inequality has an extensive use and a great effort in mathematical analysis. Now Young inequality was extended as follows.
Let and be complementary N-function with each other (see Definition 2.1 and Definition 2.2), then the kind of Young inequality holds, and the equality holds if and only if or for all .
By the kind of Young inequality, we can get the famous Holder inequality and the Minkowski inequality (see references [2] and [3]). But until now, we have not found its strict proof of analysis. In references [2] and [3], only the probable pattern description was found. Some other decisions can be found in [4–8].
In this paper, we will get its strict proof of analysis with the approximation method.
2 Preliminaries
Definition 2.1 [2]
The mapping is called an N-function if it has the following properties:
-
(i)
is even, continuous, convex and .
-
(ii)
for all .
-
(iii)
and .
Lemma 2.1 [2]
is an N-function if and only if there exists with the following properties:
-
(i)
is right-continuous and nondecreasing;
-
(ii)
whenever ;
-
(iii)
and , .
Record 2.1 [2]
is the right-derivative of N-function .
Lemma 2.2 Let be the left-derivative of N-function , then , and .
Proof From the proof process of Theorem 1.4 in reference [2], we know is left continuous, and for all , .
Hence, for , we have .
Therefore,
On the other hand, since and is left continuous, we get
Therefore, we have
Since for all , , then we have
That is,
Let , by the property (i) of in Definition 2.1, we have
 □
Definition 2.2 [2]
Suppose that is an N-function. Let be the right derivative of . Let , called the right-inverse function of . By Theorem 1.5 in reference [2], we know that also satisfies the three properties of Lemma 2.1, and is called the complementary N-function of . It is obvious, the left derivative of satisfies .
Lemma 2.3 [2]
, ; , .
Lemma 2.4 [2]
is strictly convex if and only if is strictly increasing, that is, is continuous.
Lemma 2.5 [2]
For any N-function and , there exists a strictly convex N-function , such that
where and are the right derivatives of and , respectively.
Record 2.2 Lemma 2.5 is Theorem 1.10 in reference [2], but it reverses the old conclusion ‘,’ for the new conclusion ‘.’ From the construction process of , in the proof in reference [3], we know if is continuous, then is also continuous.
Lemma 2.6 Suppose that and , then or if and only if . By the symmetry, we get another necessary and sufficient condition, that is, .
Proof Sufficiency.
Suppose that .
-
(i)
If , it is clear that .
-
(ii)
If , then . If , then the conclusion holds.
If , we need only to prove that .
From Definition 2.2, we have
Since , then for any , we get
Let , since is right continuous, then we have
On the other hand, from , we get
So, we have
Necessity.
If , it is clearly established.
If , then from
and
We have
 □
The next two lemmas are about the change of variable of integral and distribute integral.
Lemma 2.7 [9]
Suppose that and are defined on the interval , and the Stieltjes integral of about exists. Suppose that is a strictly increasing and continuous function on the interval , and and , then
Lemma 2.8 [9]
Suppose that and are defined on the interval , and the Stieltjes integral of about exists, then
3 Main result
Theorem 3.1 Suppose that is an N-function, and is the complementary N-function of , then Young inequality holds, and holds if and only if or .
Proof Suppose that and .
Firstly, we will prove the necessity of the equality.
Suppose that there exist and satisfying
Let
From Young inequality, we have learned that for all u and v, .
From , we have and we can get the minimum 0 in .
If , from , we get that , then or , that is, the necessity of the equality holds.
If , then is the minimum of the on the interval .
Therefore, the left derivative of is less than or equal to zero on the point , and the right derivative of is more than or equal to zero on the point .
That is,
Then
From Lemma 2.6, we get or .
That is, the necessity of the equality holds.
Secondly, we will get the proof of the Young inequality and the sufficiency of the equality in three steps.
Step I. Suppose that and are all strictly convex. From Lemma 2.4, the right derivative and are all strictly increasing, continuous, and are the right inverse-function of each other. From the reference [9], we have that the Stieltjes integral exists.
From Lemma 2.7 and Lemma 2.8, we have
-
(i)
If , then .
Hence, by expression (1), we have
-
(ii)
If , then .
Hence, by expression (1), we have
-
(iii)
If , then .
From expression (1), we have .
That is, the sufficiency of the equality holds.
Step II. Suppose that is strictly convex, then from Lemma 2.4, the right derivative is strictly increasing, and the right-inverse function is continuous and nondecreasing.
From Lemma 2.5 and Record 2.2, , we can construct a function strictly increasing and continuous such that
Hence,
Let be the right-inverse function of , then is strictly increasing and continuous.
In the following, we will get the relation of and .
In expression (2), let , we have
That is,
From Lemma 2.3 and expression (3), we get
Since is nondecreasing, by expression (4), we get
From the result in Step I, we get
Therefore,
Let , we have
In the following, we will prove the sufficiency of the equality.
If , from Lemma 2.3 and expression (3), for above, we have
In expression (6), let , by Lemma 2.2, we get
On the other hand, in expression (5), let , we get
Therefore,
By Lemma 2.2, we get
Now we need to prove that
In fact, if , from Definition 2.2, since is strictly increasing, then we have . If , from Lemma 2.6, we get . Therefore, we have .
By the result in Step I, we have
From expressions (9) and (11), we get
Let , we have
On the other hand, we have got the inequality .
Let , we have
Therefore, together with expression (12), we have
That is, the sufficiency of the equality holds.
Step III for any N-function , suppose that its complementary N-function is , is the right-inverse function of , and is the right-inverse function of . From Lemma 2.5, for above, we can find a strictly convex N-function and its right-derivative such that
Suppose that is the complementary N-function of , is the right derivative of .
In the following, we will get the relation of and for above. In expression (13), let , we have
From Lemma 2.3, we have that
Therefore, by expressions (14) and (15), we have
Then, by Lemma 2.3, together with expression (16), we have
Since is nondecreasing, then by expression (17), we get
From the result in Step II, we get
Let , we have
In the following, we will prove sufficiency of the equality.
By the result in Step II, we have
Therefore,
Let , together with expression (13), we get , , and since is continuous.
Therefore,
On the other hand, we have got the inequality .
Let , we have
Therefore, together with expression (20), we have
That is, the sufficiency of the equality holds. □
References
Pei LW: Typical Problems and Methods in Mathematical Analysis. Higher Education Press, Beijing; 2000. (in Chinese)
Wu CX, Wang TQ, Chen ST, Wang YW: Geometry of Orlicz Spaces. Harbin Institute of Technology Press, Harbin; 1986. (in Chinese)
Chen, ST: Geometry of Orlicz Spaces. Dissertations Mathematicae Warszawa, Warszawa (1996)
Chen S, He X, Hudzik H, Kaminska A: Monotonicity and best approximation in Orlicz-Sobolev spaces with the Luxemburg norm. J. Math. Anal. Appl. 2008, 344: 687–698. 10.1016/j.jmaa.2008.02.015
Gong W, Shi Z: Drop proper ties and approximative compactness in Orlicz-Bochner function spaces. J. Math. Anal. Appl. 2008, 344: 748–756. 10.1016/j.jmaa.2008.03.024
Shi Z, Gong W: Monotone points in Orlicz-Bochner function spaces. Math. Appl. 2010, 23(2):376–383.
Liu CY, Shi ZR: U Properties in Orlicz spaces. J. Math. Phys. 2011, 31(2):328–334. (in Chinese)
Shi ZR, Liu CY: Noncreasy and uniformly noncreasy Orlicz-Bochner function spaces. Nonlinear Anal. 2011, 74: 6153–6161. 10.1016/j.na.2011.05.094
Jang ZJ: Theory of Functions of a Real Variable. Higher Education Press, Beijing; 1994. (in Chinese)
Acknowledgements
This research was partially supported by the National Natural Science Foundation of China (Grant No: 11271245, and Grant No: 11301397), and the Natural Science Foundation Guangdong Province of China (2012KJCX0101).
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LX conceived of the study, XZ participated in its design and study. All authors read and approved the final manuscript.
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Xianqiang, L., Zhiping, X. A kind of extension of the famous Young inequality. J Inequal Appl 2013, 437 (2013). https://doi.org/10.1186/1029-242X-2013-437
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DOI: https://doi.org/10.1186/1029-242X-2013-437