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On Hardy-type integral inequalities with the gamma function
Journal of Inequalities and Applications volume 2017, Article number: 131 (2017)
Abstract
By means of real analysis and weight functions, we obtain a few equivalent conditions of two kinds of Hardy-type integral inequalities with the non-homogeneous kernel and parameters. The constant factors related to the gamma function are proved to be the best possible. We also consider the operator expressions and some cases of homogeneous kernel.
1 Introduction
If \(0 < \int_{0}^{\infty } f^{2}(x)\,dx < \infty \) and \(0 < \int_{0} ^{\infty } g^{2}(y)\,dy < \infty \), then we have the following Hilbert’s integral inequality (cf. [1]):
where the constant factor π is the best possible. In 1925, by introducing one pair of conjugate exponents (p, q), Hardy [2] gave an extension of (1) as follows: For \(p > 1\), \(\frac{1}{p} + \frac{1}{q} = 1\), \(f(x)\), \(g(y) \ge 0\), \(0 < \int_{0}^{\infty } f^{p}(x)\,dx < \infty\) and \(0 < \int_{0}^{\infty } g^{q}(y)\,dy < \infty\), we have
where the constant factor \(\frac{\pi }{\sin (\pi / p)}\) is the best possible. Inequalities (1) and (2) are important in analysis and its applications (cf. [3, 4]).
In 1934, Hardy et al. gave an extension of (2) as follows: If \(k_{1}(x,y)\) is a non-negative homogeneous function of degree −1, \(k_{p} = \int_{0}^{\infty } k_{1}(u,1)u^{\frac{ - 1}{p}}\,du \in \mathbf{R}_{ +} = (0,\infty)\), then
where the constant factor \(k_{p}\) is the best possible (cf. [3], Theorem 319). Additionally, a Hilbert-type integral inequality with the non-homogeneous kernel is proved as follows: If \(h(u) > 0\), \(\phi (\sigma) = \int_{0}^{\infty } h(u)u^{\sigma - 1}\,du \in \mathbf{R}_{ +}\), then
where the constant factor \(\phi (\frac{1}{p})\) is still the best possible (cf. [3], Theorem 350).
In 1998, by introducing an independent parameter \(\lambda > 0\), Yang gave a best extension of (1) with the kernel \(\frac{1}{ ( x + y )^{\lambda }}\) (cf. [5, 6]). In 2004, by introducing another pair conjugate exponents (r, s), Yang [7] gave an extension of (2) as follows: If \(\lambda > 0\), \(r > 1,\frac{1}{r} + \frac{1}{s} = 1\), \(f(x)\), \(g(y) \ge 0\), \(0 < \int_{0}^{\infty } x^{p ( 1 - \frac{\lambda }{r} ) - 1}f^{p}(x)\,dx < \infty \) and \(0 < \int_{0}^{\infty } y^{q (1 -\frac{\lambda }{s} ) - 1}g^{q}(y)\,dy < \infty \), then
where the constant factor \(\frac{\pi }{\lambda \sin ( \pi / r ) }\) is the best possible. For \(\lambda = 0\), \(r = q\), \(s = p\), (5) reduces to (2); For \(\lambda = 1\), \(r = p\), \(s = q\), (5) reduces to the dual form of (2) as follows:
For \(p = q = 2\), both (2) and (7) reduce to (1).
In 2005, in [8] one also gave an extension of (2) and (5) with the kernel \(\frac{1}{ ( x + y ) ^{\lambda }}\). Krnić et al. [9–14] provided some extensions and particular cases of (2), (3) and (4) with parameters.
In 2009, Yang gave an extension of (3) and (5) as follows (cf. [15, 16]): If \(\lambda_{1} + \lambda_{2} = \lambda \in \mathbf{R} = (- \infty ,\infty)\), \(k_{\lambda } (x,y)\) is a non-negative homogeneous function of degree −λ, satisfying
and
then
where the constant factor \(k(\lambda_{1})\) is the best possible. For \(\lambda = 1\), \(\lambda_{1} = \frac{1}{q}\), \(\lambda_{2} = \frac{1}{p}\), (7) reduces to (3). Additionally, an extension of (4) was given as follows:
where the constant factor \(\phi (\sigma)\) is the best possible (cf. [17]). For \(\sigma = \frac{1}{p}\), (8) reduces to (4).
Some equivalent inequalities of (7) and (8) are considered by [16]. In 2013, Yang [17] studied the equivalency between (7) and (8). In 2017, Hong [18] studied an equivalent condition between (7) with a few parameters.
Remark 1
cf. [17]
If \(h(xy) = 0\), for \(xy > 1\), \(\phi (\sigma) = \int_{0}^{1} h(u)u^{\sigma - 1}\,du = \phi_{1}(\sigma) \in \mathbf{R}_{ +}\), then (8) reduces to the following Hardy-type integral inequality with the non-homogeneous kernel:
if \(h(xy) = 0\), for \(xy < 1\), \(\phi (\sigma) = \int_{1}^{\infty } h(u)u ^{\sigma - 1}\,du = \phi_{2}(\sigma) \in \mathbf{R}_{ +}\), then (8) reduces to the following another kind of Hardy-type integral inequality with the non-homogeneous kernel:
In this paper, by real analysis and the weight functions, we obtain a few equivalent conditions of two kinds of Hardy-type integral inequalities with the non-homogeneous kernel and parameters as \(\frac{ ( \min \{ xy,1 \} ) ^{\alpha } \vert \ln xy \vert ^{\beta }}{ ( \max \{ xy,1 \} ) ^{\lambda + \alpha }}\). The constant factors related to the gamma function are proved to be the best possible. We also consider the operator expressions and some cases of homogeneous kernel.
2 Two lemmas
For \(\beta > - 1\), \(\sigma + \mu = \lambda \in \mathbf{R}\), we set
Then, for \(\sigma > - \alpha \), setting \(v = - (\sigma + \alpha)\ln u\), we find
For \(\mu > - \alpha \), we find
where \(\Gamma ( \eta ) : = \int_{0}^{\infty } v^{\eta - 1}e ^{ - v}\,dv\) (\(\eta > 0\)) is the gamma function (cf. [19]).
Lemma 1
If \(p > 1\), \(\frac{1}{p} + \frac{1}{q} = 1\), \(\sigma_{1} \in R\), \(\beta > - 1\), \(\sigma > - \alpha \), there exists a constant \(M_{1}\), such that, for any non-negative measurable functions \(f(x)\) and \(g(y)\) in (0, ∞), the following inequality:
holds true, then we have \(\sigma_{1} = \sigma \), and then \(M_{1} \ge k_{1}(\sigma)\).
Proof
If \(\sigma_{1} > \sigma \), then, for \(n \ge \frac{1}{ \sigma_{1} - \sigma}\) (\(n \in \mathbf{N}\)), we set the following functions:
and find
Setting \(u = xy\), we obtain
Then by (13), we have
Since \(( \sigma_{1} - \sigma ) - \frac{1}{n} \ge 0\), it follows that \(\int_{1}^{\infty } y^{ ( \sigma_{1} - \sigma ) - \frac{1}{n} - 1}\,dy = \infty \). By (14), in view of \(\int_{0}^{ \frac{1}{y}} \frac{ ( \min \{ u,1 \} ) ^{\alpha } ( - \ln u ) ^{\beta }}{ ( \max \{ u,1 \} ) ^{\lambda + \alpha }} u^{\sigma + \frac{1}{pn} - 1}\,du > 0\), we find \(\infty < \infty \), which is a contradiction.
If \(\sigma_{1} < \sigma \), then, for \(n \ge \frac{1}{\sigma - \sigma _{1}}\) (\(n \in \mathbf{N}\)), we set the following functions:
and find
Setting \(u = xy\), we obtain
Then by the Fubini theorem (cf. [20]) and (13), we have
Since \(( \sigma_{1} - \sigma ) - \frac{1}{n} \ge 0\), it follows that \(\int_{1}^{\infty } x^{ ( \sigma - \sigma_{1} ) - \frac{1}{n} - 1}\,dx = \infty \). By (15), in view of
we find \(\infty < \infty \), which is a contradiction.
Hence, we conclude that \(\sigma_{1} = \sigma \).
For \(\sigma_{1} = \sigma \), we reduce (15) as follows:
Since
is non-negative and increasing in (0, 1], by Levi theorem (cf. [20]), we find
The lemma is proved. □
Lemma 2
If \(p > 1\), \(\frac{1}{p} + \frac{1}{q} = 1\), \(\sigma_{1} \in R\), \(\beta > - 1\), \(\mu = \lambda - \sigma > - \alpha \), there exists a constant \(M_{2}\), such that, for any non-negative measurable functions \(f(x)\) and \(g(y)\) in (0, ∞), the following inequality:
holds true, then we have \(\sigma_{1} = \sigma \), and then \(M_{2} \ge k_{1}(\mu)\).
Proof
If \(\sigma_{1} < \sigma \), then, for \(n \ge \frac{1}{ \sigma - \sigma_{1}}\) (\(n \in \mathbf{N}\)), we set two functions \(\tilde{f}_{n}(x)\) and \(\tilde{g}_{n}(y)\) as in Lemma 1, and find
Setting \(u = xy\), we obtain
and then by (17), we obtain
Since \(( \sigma_{1} - \sigma ) + \frac{1}{n} \le 0\), it follows that \(\int_{1}^{\infty } y^{ ( \sigma_{1} - \sigma ) + \frac{1}{n} - 1}\,dy = \infty \). By (18), in view of
we have \(\infty < \infty\), which is a contradiction.
If \(\sigma_{1} > \sigma \), then, for \(n \ge \frac{1}{\sigma_{1} -\sigma}\) (\(n \in \mathbf{N}\)), we set two functions \(f_{n}(x)\) and \(g_{n}(y)\) as in Lemma 1, and we find
Setting \(u = xy\), we obtain
and then by Fubini theorem (cf. [20]) and (17), we have
Since \(( \sigma - \sigma_{1} ) + \frac{1}{n} \le 0\), it follows that \(\int_{0}^{1} x^{ ( \sigma - \sigma_{1} ) + \frac{1}{n} - 1}\,dx = \infty \). By (19), in view of
we have \(\infty < \infty \), which is a contradiction.
Hence, we conclude that \(\sigma_{1} = \sigma \).
For \(\sigma_{1} = \sigma \), we reduce (19) as follows:
Since
is non-negative and increasing in \([ 1, \infty)\), still by the Levi theorem (cf. [20]), we have
The lemma is proved. □
3 Main results and corollaries
Theorem 1
If \(p > 1\), \(\frac{1}{p} + \frac{1}{q} = 1\), \(\sigma _{1} \in R\), \(\beta > - 1\), \(\sigma > - \alpha \), then the following conditions are equivalent:
-
(i)
There exists a constant \(M_{1}\), such that, for any \(f(x) \ge 0\), satisfying
$$ 0 < \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f^{p}(x)\,dx < \infty , $$we have the following Hardy-type integral inequality of the first kind with the non-homogeneous kernel:
$$\begin{aligned} J&: = \biggl\{ \int_{0}^{\infty } y^{p\sigma_{1} - 1} \biggl[ \int_{0} ^{\frac{1}{y}} \frac{ ( \min \{ xy,1 \} ) ^{ \alpha } \vert \ln xy \vert ^{\beta }}{ ( \max \{ xy,1 \} ) ^{\lambda + \alpha }} f(x)\,dx \biggr] ^{p}\,dy \biggr\} ^{\frac{1}{p}} \\ &< M_{1} \biggl[ \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f ^{p}(x) \,dx \biggr] ^{\frac{1}{p}}. \end{aligned}$$(21) -
(ii)
There exists a constant \(M_{1}\), such that, for any \(f(x)\), \(g(y) \ge 0\), satisfying \(0 < \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f^{p}(x)\,dx < \infty \), and \(0 < \int_{0}^{\infty } y^{q ( 1 - \sigma_{1} ) - 1}g^{q}(y)\,dy < \infty \), we have the following inequality:
$$\begin{aligned} I&: = \int_{0}^{\infty } g(y) \biggl[ \int_{0}^{\frac{1}{y}} \frac{ (\min \{ xy,1 \} ) ^{\alpha } \vert \ln xy \vert ^{\beta }}{ ( \max \{ xy,1 \} ) ^{\lambda + \alpha }} f(x)\,dx \biggr] \,dy \\ &< M_{1} \biggl[ \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f ^{p}(x) \,dx \biggr] ^{\frac{1}{p}} \biggl[ \int_{0}^{\infty } y^{q ( 1 - \sigma_{1} ) - 1}g^{q}(y)\,dy \biggr] ^{\frac{1}{q}}. \end{aligned}$$(22) -
(iii)
\(\sigma_{1} = \sigma \).
If condition (iii) holds true, then \(M_{1} \ge k_{1}(\sigma)\) and the constant factor
in (21) and (22) is the best possible.
Proof
(i)⇒(ii). By Hölder’s inequality (cf. [21]), we have
(ii)⇒(iii). By Lemma 1, we have \(\sigma_{1} = \sigma \).
(iii)⇒(i). Setting \(u = xy\), we obtain the following weight function:
By Hölder’s inequality with weight and (24), for \(y \in (0,\infty)\), we have
If (25) obtains the form of equality for a \(y \in (0,\infty)\), then (cf. [21]), there exist constants A and B, such that they are not all zero, and
We suppose that \(A \ne 0\) (otherwise \(B = A = 0\)). It follows that
which contradicts the fact that \(0 < \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f^{p}(x)\,dx < \infty \). Hence, (25) assumes the form of strict inequality. Hence, for \(\sigma_{1} = \sigma \), by (25) and by the Fubini theorem (cf. [20]), we obtain
Setting \(M_{1} \ge k_{1}(\sigma)\), (21) follows.
Therefore, conditions (i), (ii) and (iii) are equivalent.
When condition (iii) follows, if there exists a constant factor \(M_{1} \ge k_{1}(\sigma)\), such that (22) is valid, then by Lemma 1, we have \(M_{1} \ge k_{1}(\sigma)\). Hence, the constant factor \(M_{1} = k_{1}(\sigma)\) in (22) is the best possible. The constant factor \(M_{1} = k_{1}(\sigma)\) in (21) is still the best possible. Otherwise, by (23) (for \(\sigma_{1} = \sigma \)), we can conclude that the constant factor \(M_{1} = k_{1}(\sigma)\) in (22) is not the best possible. □
Setting \(y = \frac{1}{Y}\), \(G(Y) = Y^{\lambda - 2}g(\frac{1}{Y})\), \(\mu_{1} = \lambda - \sigma_{1}\) in Theorem 1, then replacing Y by y and \(G(Y)\) by \(g(y)\), we have
Corollary 1
If \(p > 1\), \(\frac{1}{p} + \frac{1}{q} = 1\), \(\mu_{1} \in R\), \(\beta > - 1\), \(\sigma = \lambda - \mu > - \alpha \), then the following conditions are equivalent:
-
(i)
There exists a constant \(M_{1}\), such that, for any \(f(x) \ge 0\), satisfying
$$ 0 < \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f^{p}(x)\,dx < \infty , $$we have the following Hardy-type inequality of the first kind with the homogeneous kernel:
$$\begin{aligned}& \biggl\{ \int_{0}^{\infty } y^{p\mu_{1} - 1} \biggl[ \int_{0}^{y} \frac{ (\min \{ x,y \} ) ^{\alpha } \vert \ln ( x / y ) \vert ^{\beta }}{ ( \max \{ x,y \} ) ^{\lambda + \alpha }} f(x)\,dx \biggr] ^{p}\,dy \biggr\} ^{\frac{1}{p}} \\& \quad < M_{1} \biggl[ \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f ^{p}(x) \,dx \biggr] ^{\frac{1}{p}}. \end{aligned}$$(26) -
(ii)
There exists a constant \(M_{1}\), such that, for any \(f(x)\), \(g(y) \ge 0\), satisfying \(0 < \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f^{p}(x)\,dx < \infty \), and \(0 < \int_{0}^{\infty } y^{q ( 1 - \mu_{1} ) - 1}g^{q}(y)\,dy < \infty \), we have the following inequality:
$$\begin{aligned}& \int_{0}^{\infty } g(y) \biggl[ \int_{0}^{y} \frac{ ( \min \{ x,y \} ) ^{\alpha } \vert \ln ( x / y ) \vert ^{\beta }}{ ( \max \{ x,y \} ) ^{\lambda + \alpha }} f(x)\,dx \biggr] \,dy \\& \quad< M_{1} \biggl[ \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f ^{p}(x) \,dx \biggr] ^{\frac{1}{p}} \biggl[ \int_{0}^{\infty } y^{q ( 1 - \mu_{1} ) - 1}g^{q}(y) \,dy \biggr] ^{\frac{1}{q}}. \end{aligned}$$(27) -
(iii)
\(\mu_{1} = \mu \).
If condition (iii) holds true, then we have \(M_{1} \ge k_{1}(\sigma)\), and the constant \(M_{1} = k_{1}(\sigma)\) in (26) and (27) is the best possible.
Remark 2
On the other hand, setting \(y = \frac{1}{Y}\), \(G(Y) = Y^{\lambda - 2}g(\frac{1}{Y})\), \(\sigma_{1} = \lambda - \mu_{1}\) in Corollary 1, then replacing Y by y and \(G(Y)\) by \(g(y)\), we have Theorem 1. Hence, Theorem 1 and Corollary 1 are equivalent.
Similarly, we obtain the following weight function:
and then in view of Lemma 2 and in the same way, we have
Theorem 2
If \(p > 1\), \(\frac{1}{p} + \frac{1}{q} = 1\), \(\sigma _{1} \in R\), \(\beta > - 1\), \(\mu = \lambda - \sigma > - \alpha \), then the following conditions are equivalent:
-
(i)
There exists a constant \(M_{2}\), such that, for any \(f(x) \ge 0\), satisfying
$$ 0 < \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f^{p}(x)\,dx < \infty , $$we have the following Hardy-type inequality of the second kind with the non-homogeneous kernel:
$$\begin{aligned}& \biggl\{ \int_{0}^{\infty } y^{p\sigma_{1} - 1} \biggl[ \int_{ \frac{1}{y}}^{\infty } \frac{ ( \min \{ xy,1 \} ) ^{\alpha } \vert \ln xy \vert ^{\beta }}{ ( \max \{ xy,1 \} ) ^{\lambda + \alpha }} f(x)\,dx \biggr] ^{p}\,dy \biggr\} ^{\frac{1}{p}} \\& \quad < M_{2} \biggl[ \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f ^{p}(x) \,dx \biggr] ^{\frac{1}{p}}. \end{aligned}$$(28) -
(ii)
There exists a constant \(M_{2}\), such that, for any \(f(x)\), \(g(y) \ge 0\), satisfying \(0 < \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f^{p}(x)\,dx < \infty \), and \(0 < \int_{0}^{\infty } y^{q ( 1 - \sigma_{1} ) - 1}g^{q}(y)\,dy < \infty \), we have the following inequality:
$$\begin{aligned}& \int_{0}^{\infty } g(y) \biggl[ \int_{\frac{1}{y}}^{\infty } \frac{ ( \min \{ xy,1 \} ) ^{\alpha } \vert \ln xy \vert ^{\beta }}{ ( \max \{ xy,1 \} ) ^{\lambda + \alpha }} f(x)\,dx \biggr]\,dy \\& \quad < M_{2} \biggl[ \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f ^{p}(x) \,dx \biggr] ^{\frac{1}{p}} \biggl[ \int_{0}^{\infty } y^{q ( 1 - \sigma_{1} ) - 1}g^{q}(y)\,dy \biggr] ^{\frac{1}{q}}. \end{aligned}$$(29) -
(iii)
\(\sigma_{1} = \sigma \).
If condition (iii) holds true, then we have \(M_{2} \ge k_{1}(\mu)\) and the constant factor
in (28) and (29) is the best possible.
Setting \(y = \frac{1}{Y}\), \(G(Y) = Y^{\lambda - 2}g(\frac{1}{Y})\), \(\mu_{1} = \lambda - \sigma_{1}\) in Theorem 2, then replacing Y by y and \(G(Y)\) by \(g(y)\), we have
Corollary 2
If \(p > 1\), \(\frac{1}{p} + \frac{1}{q} = 1\), \(\mu_{1} \in R\), \(\beta > - 1\), \(\mu = \lambda - \sigma > - \alpha \), then the following conditions are equivalent:
-
(i)
There exists a constant \(M_{2}\), such that, for any \(f(x) \ge 0\), satisfying
$$ 0 < \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f^{p}(x)\,dx < \infty , $$we have the following Hardy-type inequality of the second kind with the homogeneous kernel:
$$\begin{aligned}& \biggl\{ \int_{0}^{\infty } y^{p\mu_{1} - 1} \biggl[ \int_{y}^{\infty } \frac{ ( \min \{ x,y \} ) ^{\alpha } \vert \ln ( x / y ) \vert ^{\beta }}{ ( \max \{ x,y \} ) ^{\lambda + \alpha }} f(x)\,dx \biggr] ^{p}\,dy \biggr\} ^{\frac{1}{p}} \\& \quad< M_{2} \biggl[ \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f ^{p}(x) \,dx \biggr] ^{\frac{1}{p}}. \end{aligned}$$(30) -
(ii)
There exists a constant \(M_{2}\), such that, for any \(f(x)\), \(g(y) \ge 0\), satisfying \(0 < \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f^{p}(x)\,dx < \infty \), and \(0 < \int_{0}^{\infty } y^{q ( 1 - \mu_{1} ) - 1}g^{q}(y)\,dy < \infty \), we have the following inequality:
$$\begin{aligned}& \int_{0}^{\infty } g(y) \biggl[ \int_{y}^{\infty } \frac{ ( \min \{ x,y \} ) ^{\alpha } \vert \ln ( x / y ) \vert ^{\beta }}{ ( \max \{ x,y \} ) ^{\lambda + \alpha }} f(x)\,dx \biggr] \,dy \\& \quad< M_{2} \biggl[ \int_{0}^{\infty } x^{p ( 1 - \sigma ) - 1}f ^{p}(x) \,dx \biggr] ^{\frac{1}{p}} \biggl[ \int_{0}^{\infty } y^{q ( 1 - \mu_{1} ) - 1}g^{q}(y) \,dy \biggr] ^{\frac{1}{q}}. \end{aligned}$$(31) -
(iii)
\(\mu_{1} = \mu \).
If condition (iii) holds true, then we have \(M_{2} \ge k_{1}(\mu)\), and the constant \(M_{2} = k_{1}(\mu)\) in (30) and (31) is the best possible.
Remark 3
4 Operator expressions
For \(p > 1\), \(\frac{1}{p} + \frac{1}{q} = 1\), σ, \(\lambda > 0\), \(\mu = \lambda - \sigma \), we set the following functions: \(\varphi (x): = x ^{p ( 1 - \sigma ) - 1}\), \(\psi (y): = y^{q ( 1 - \sigma ) - 1}\), \(\phi (y): = y^{q ( 1 - \mu ) - 1}\), wherefrom, \(\psi^{1 - p}(y) = y^{p \sigma - 1}\), \(\phi^{1 - p}(y) = y^{p\mu - 1}\) (\(x,y \in \mathbf{R}_{ +}\)). Define the following real normed linear spaces:
wherefrom
(a) In view of Theorem 1 (\(\sigma_{1} = \sigma\)), for \(f \in L_{p, \varphi} (\mathbf{R}_{ +})\), setting
by (21), we have
Definition 1
Define a Hardy-type integral operator of the first kind with the non-homogeneous kernel \(T_{1}^{(1)}:L_{p,\varphi} (\mathbf{R}_{ +}) \to L_{p,\psi^{1 - p}}(\mathbf{R}_{ +})\) as follows: For any \(f \in L_{p,\varphi} (\mathbf{R}_{ +})\), there exists a unique representation \(T_{1}^{(1)}f = h_{1} \in L_{p,\psi ^{1 - p}}(\mathbf{R}_{ +})\), satisfying for any \(y \in \mathbf{R}_{ +}\), \(T_{1}^{(1)}f(y) = h_{1}(y)\).
In view of (32), it follows that \(\Vert T_{1}^{(1)}f \Vert _{p, \psi^{1 - p}} = \Vert h_{1} \Vert _{p,\psi^{1 - p}} \le M_{1} \Vert f \Vert _{p,\varphi}\), and then the operator \(T_{1}^{(1)}\) is bounded satisfying
If we define the formal inner product of \(T_{1}^{(1)}f\) and g as follows:
then we can rewrite Theorem 1 (for \(\sigma_{1} = \sigma \)) as follows.
Theorem 3
If \(p > 1\), \(\frac{1}{p} + \frac{1}{q} = 1\), \(\beta > - 1\), \(\sigma > - \alpha \), then the following conditions are equivalent:
-
(i)
There exists a constant \(M_{1}\), such that, for any \(f(x) \ge 0,f \in L_{p,\varphi} (\mathbf{R}_{ +}),\Vert f \Vert _{p,\varphi} > 0\), we have the following inequality:
$$ \bigl\Vert T_{1}^{(1)}f \bigr\Vert _{p,\psi^{1 - p}} < M_{1}\Vert f \Vert _{p,\varphi}. $$(33) -
(ii)
There exists a constant \(M_{1}\), such that, for any \(f(x)\), \(g(y)\ge 0\), \(f \in L_{p,\varphi} (\mathbf{R}_{ +})\), \(g \in L_{q,\psi } (\mathbf{R}_{ +}),\Vert f \Vert _{p,\varphi}, \Vert g \Vert _{q,\psi } > 0\), we have the following inequality:
$$ \bigl( T_{1}^{(1)}f,g \bigr) < M_{1}\Vert f \Vert _{p,\varphi} \Vert g \Vert _{q,\psi }. $$(34)
We still have \(\Vert T_{1}^{(1)} \Vert = k_{1}(\sigma) \le M _{1}\).
(b) In view of Corollary 1 (\(\mu_{1} = \mu\)), for \(f \in L_{p,\varphi} (\mathbf{R}_{ +})\), setting
by (26), we have
Definition 2
Define a Hardy-type integral operator of the first kind with the homogeneous kernel \(T_{1}^{(2)}:L_{p,\varphi} (\mathbf{R}_{ +}) \to L_{p,\phi^{1 - p}}(\mathbf{R}_{ +})\) as follows: For any \(f \in L_{p,\varphi} (\mathbf{R}_{ +})\), there exists a unique representation \(T_{1}^{(2)}f = h_{2} \in L_{p, \phi^{1 - p}}(\mathbf{R}_{ +})\), satisfying for any \(y \in \mathbf{R}_{ +}\), \(T_{1}^{(2)}f(y) = h_{2}(y)\).
In view of (35), it follows that \(\Vert T_{1}^{(2)}f \Vert _{p, \phi^{1 - p}} = \Vert h_{2} \Vert _{p,\phi^{1 - p}} \le M _{1}\Vert f \Vert _{p,\varphi}\), and then the operator \(T_{1}^{(2)}\) is bounded satisfying
If we define the formal inner product of \(T_{1}^{(2)}f\) and g as follows:
then we can rewrite Corollary 1 (for \(\mu_{1} = \mu \)) as follows.
Corollary 3
If \(p > 1,\frac{1}{p} + \frac{1}{q} = 1,\beta > - 1,\sigma = \lambda - \mu > - \alpha \), then the following conditions are equivalent:
-
(i)
There exists a constant \(M_{1}\), such that, for any \(f(x) \ge 0,f \in L_{p,\varphi} (\mathbf{R}_{ +}),\Vert f \Vert _{p,\varphi} > 0\), we have the following inequality:
$$ \bigl\Vert T_{1}^{(2)}f \bigr\Vert _{p,\phi^{1 - p}} < M_{1}\Vert f \Vert _{p,\varphi}. $$(36) -
(ii)
There exists a constant \(M_{1}\), such that, for any \(f(x),g(y) \ge 0,f \in L_{p,\varphi} (\mathbf{R}_{ +}),g \in \)
\(L_{q,\phi } (\mathbf{R}_{ +}),\Vert f \Vert _{p,\varphi}, \Vert g \Vert _{q,\phi } > 0\), we have the following inequality:
$$ \bigl( T_{1}^{(2)}f,g \bigr) < M_{1}\Vert f \Vert _{p,\varphi} \Vert g \Vert _{q,\phi }. $$(37)
We still have \(\Vert T_{1}^{(2)} \Vert = k_{1}(\sigma) \le M _{1}\).
Remark 4
Theorem 3 and Corollary 3 are equivalent.
(c) In view of Theorem 2 (\(\sigma_{1} = \sigma\)), for \(f \in L_{p, \varphi} (\mathbf{R}_{ +})\), setting
by (28), we have
Definition 3
Define a Hardy-type integral operator of the second kind with the non-homogeneous kernel \(T_{2}^{(1)}:L_{p,\varphi} (\mathbf{R}_{ +}) \to L_{p,\psi^{1 - p}}(\mathbf{R}_{ +})\) as follows: For any \(f \in L_{p,\varphi} (\mathbf{R}_{ +})\), there exists a unique representation \(T_{2}^{(1)}f = H_{1} \in L_{p,\psi ^{1 - p}}(\mathbf{R}_{ +})\), satisfying for any \(y \in \mathbf{R}_{ +}\), \(T_{2}^{(1)}f(y) = H_{1}(y)\).
In view of (38), it follows that \(\Vert T_{2}^{(1)}f \Vert _{p, \psi^{1 - p}} = \Vert H_{1} \Vert _{p,\psi^{1 - p}} \le M_{2} \Vert f \Vert _{p,\varphi}\), and then the operator \(T_{2}^{(1)}\) is bounded satisfying
If we define the formal inner product of \(T_{2}^{(1)}f\) and g as follows:
then we can rewrite Theorem 2 (for \(\sigma_{1} = \sigma \)) as follows.
Theorem 4
If \(p > 1\), \(\frac{1}{p} + \frac{1}{q} = 1\), \(\beta > - 1\), \(\mu = \lambda - \sigma > - \alpha \), then the following conditions are equivalent:
-
(i)
There exists a constant \(M_{2}\), such that, for any \(f(x) \ge 0\), \(f \in L_{p,\varphi} (\mathbf{R}_{ +})\), \(\Vert f \Vert _{p,\varphi} > 0\), we have the following inequality:
$$ \bigl\Vert T_{2}^{(1)}f \bigr\Vert _{p,\psi^{1 - p}} < M_{2}\Vert f \Vert _{p,\varphi}. $$(39) -
(ii)
There exists a constant \(M_{2}\), such that, for any \(f(x)\), \(g(y) \ge 0\), \(f \in L_{p,\varphi} (\mathbf{R}_{ +})\), \(g \in L_{q,\psi } (\mathbf{R}_{ +})\), \(\Vert f \Vert _{p,\varphi}, \Vert g \Vert _{q,\psi } > 0\), we have the following inequality:
$$ \bigl( T_{2}^{(1)}f,g \bigr) < M_{2} \Vert f \Vert _{p,\varphi} \Vert g \Vert _{q,\psi }. $$(40)
We still have \(\Vert T_{2}^{(1)} \Vert = k_{1}(\mu) \le M_{2}\).
(d) In view of Corollary 2 (\(\mu_{1} = \mu\)), for \(f \in L_{p,\varphi} (\mathbf{R}_{ +})\), setting
by (30), we have
Definition 4
Define a Hardy-type integral operator of the second kind with the homogeneous kernel \(T_{2}^{(2)}:L_{p,\varphi} (\mathbf{R}_{ +}) \to L_{p,\phi^{1 - p}}(\mathbf{R}_{ +})\) as follows: For any \(f \in L_{p,\varphi} (\mathbf{R}_{ +})\), there exists a unique representation \(T_{2}^{(2)}f = H_{2} \in L_{p, \phi^{1 - p}}(\mathbf{R}_{ +})\), satisfying for any \(y \in \mathbf{R}_{ +}\), \(T_{2}^{(2)}f(y) = H_{2}(y)\).
In view of (41), it follows that \(\Vert T_{2}^{(2)}f \Vert _{p, \phi^{1 - p}} = \Vert H_{2} \Vert _{p,\phi^{1 - p}} \le M _{2}\Vert f \Vert _{p,\varphi}\), and then the operator \(T_{2}^{(2)}\) is bounded satisfying
If we define the formal inner product of \(T_{1}^{(2)}f\) and g as follows:
then we can rewrite Corollary 2 (for \(\mu_{1} = \mu \)) as follows.
Corollary 4
If \(p > 1\), \(\frac{1}{p} + \frac{1}{q} = 1\), \(\beta > - 1\), \(\mu = \lambda - \sigma > - \alpha \), then the following conditions are equivalent:
-
(i)
There exists a constant \(M_{2}\), such that, for any \(f(x) \ge 0\), \(f \in L_{p,\varphi} (\mathbf{R}_{ +})\), \(\Vert f \Vert _{p,\varphi} > 0\), we have the following inequality:
$$ \bigl\Vert T_{2}^{(2)}f \bigr\Vert _{p,\phi^{1 - p}} < M_{2}\Vert f \Vert _{p,\varphi}. $$(42) -
(ii)
There exists a constant \(M_{2}\), such that, for any \(f(x)\), \(g(y) \ge 0\), \(f \in L_{p,\varphi} (\mathbf{R}_{ +})\), \(g \in L_{q,\phi } (\mathbf{R}_{ +})\), \(\Vert f \Vert _{p,\varphi}\), \(\Vert g \Vert _{q,\phi } > 0\), we have the following inequality:
$$ \bigl( T_{2}^{(2)}f,g \bigr) < M_{2} \Vert f \Vert _{p,\varphi} \Vert g \Vert _{q,\phi }. $$(43)
We still have \(\Vert T_{2}^{(2)} \Vert = k_{1}(\mu) \le M_{2}\).
Remark 5
5 Conclusions
In this paper, by means of real analysis and weight functions a few equivalent conditions of two kinds of Hardy-type integral inequalities with the non-homogeneous kernel and parameters are obtained by Theorem 1, 2. The constant factors related to the gamma function are proved to be the best possible. We also consider the operator expressions in Theorem 3, 4. The dependent cases of homogeneous kernel are assumed by Corollary 1-4. The method of weight functions is very important, it is the key to help us proving the main inequalities with the best possible constant factor. The lemmas provide an extensive account of this type of inequalities.
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Acknowledgements
This work is supported by the National Natural Science Foundation (No. 11401113), and Appropriative Researching Fund for Professors and Doctors, Guangdong University of Education (No. 2015ARF25). We are grateful for their help.
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BY carried out the mathematical studies, participated in the sequence alignment and drafted the manuscript. JL participated in the design of the study and performed the numerical analysis. All authors read and approved the final manuscript.
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Liao, J., Yang, B. On Hardy-type integral inequalities with the gamma function. J Inequal Appl 2017, 131 (2017). https://doi.org/10.1186/s13660-017-1404-1
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DOI: https://doi.org/10.1186/s13660-017-1404-1