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On a composition of two Hilbert-Hardy-type integral operators and related inequalities
Journal of Inequalities and Applications volume 2015, Article number: 100 (2015)
Abstract
By applying the way of real and functional analysis and estimating the weight functions, we build some lemmas and deduce some Hilbert-type and Hilbert-Hardy-type integral inequalities with the best possible constant factors. The equivalent forms, the reverses and the operator expressions are all considered. The composition formula of two Hilbert-Hardy-type integral operators and some examples are given.
1 Introduction
Assuming that \(f(x),g(y)\geq0\), \(f,g\in L^{2}(\mathbf{R}_{+})=\{f;\|f\| _{2}=(\int_{0}^{\infty}|f(x)|^{2}\, dx)^{\frac{1}{2}}<\infty\}\), \(\|f\|_{2},\|g\|_{2}>0\), we have the following well-known Hilbert integral inequality and the equivalent form (cf. [1]):
where the constant factor π is the best possible.
In 1925, by introducing a pair of conjugate exponents \(( p,q )\) (\(\frac{1}{p}+\frac{1}{q}=1\)), Hardy [2] gave some extensions of (1) and (2) as follows: For \(p>1\), \(f(x),g(y)\geq0\), \(f\in L^{p}(\mathbf{R}_{+})\), \(g\in L^{q}(\mathbf{R}_{+})\), \(\|f\|_{p},\|g\|_{q}>0\), we have the following Hardy-Hilbert integral inequality and the equivalent form:
where the constant factor \(\frac{\pi}{\sin(\pi/p)}\) is the best possible. For \(p=q=2\), inequalities (3) and (4) reduce respectively to (1) and (2).
Definition 1
If \(\lambda\in\mathbf{R}=(-\infty,\infty)\), \(\mathbf{R} _{+}=(0,\infty)\), \(k_{\lambda}(x,y)\) is a measurable function in \(\mathbf {R}_{+}^{2}=\mathbf{R}_{+}\times\mathbf{R}_{+}\), satisfying for any \(t,x,y\in \mathbf{R}_{+}\), \(k_{\lambda}(tx,ty)=t^{-\lambda}k_{\lambda}(x,y)\), then we call \(k_{\lambda}(x,y)\) homogeneous function of degree −λ.
In 1934, by using a general non-negative homogeneous function of degree −1 as \(k_{1}(x,y)\), Hardy et al. [3] gave some extensions of (3) and (4) as follows: For \(p>1\), \(k_{p}=\int_{0}^{\infty }k_{1}(u,1)u^{\frac{-1}{p}}\, du\in\mathbf{R}_{+}\), \(f(x),g(y)\geq0\), \(f\in L^{p}(\mathbf{R}_{+})\), \(g\in L^{q}(\mathbf{R}_{+})\), \(\|f\|_{p},\|g\|_{q}>0\), we have the following Hardy-Hilbert-type integral inequality and the equivalent form:
where the constant factor \(k_{p}\) is the best possible. Some applications of (5) and (6) are provided in [4].
In 1998, by introducing an independent parameter \(\lambda\in(0,1]\), Yang [5] gave an extension of (3) with the homogeneous kernel of degree −λ as \(\frac{1}{(x+y)^{\lambda}}\). In 2009, by using a general non-negative homogeneous function of degree −λ as \(k_{\lambda}(x,y)\) and adding another pair of conjugate exponents \(( r,s )\) (\(\frac{1}{r}+\frac{1}{s}=1\)), Yang [6] gave some extensions of (5) and (6) as follows: For \(p,r>1\), \(\Phi (x)=x^{p(1-\frac{\lambda}{r})-1}\), \(\Psi(y)=y^{q(1-\frac{\lambda}{s})-1}\) (\(x,y\in\mathbf{R}_{+}\)), \(k_{\lambda}(r)=\int_{0}^{\infty}k_{\lambda }(u,1)u^{\frac{\lambda}{r}-1}\,du\in\mathbf{R}_{+}\), \(f(x),g(y)\geq0\),
\(g\in L_{q,\Psi}(\mathbf{R}_{+})\), \(\|f\|_{p,\Phi},\|g\|_{q,\Psi }>0\), we have the following Yang-Hilbert-type integral inequality and the equivalent form:
where the constant factor \(k_{\lambda}(r)\) is the best possible.
Remark 1
(i) When \(\lambda=1\), \(r=q\), \(s=p\), (7) and (8) reduce respectively to (5) and (6). (ii) By (8), setting \(y=\frac{1}{z}\), we have the following Yang-Hilbert-type inequality with the best possible constant factor and a non-homogeneous kernel:
Using (2), we may define Hilbert’s integral operator \(T:L^{2}(\mathbf{R}_{+})\rightarrow L^{2}(\mathbf{R}_{+})\) as follows (cf. [7]): For any \(f\in L^{2}(\mathbf{R}_{+})\), there exists \(Tf\in L^{2}(\mathbf{R}_{+})\) satisfying
Then by (2) we have \(\|Tf\|_{2}\leq\pi\|f\|_{2}\), and T is a bounded linear operator satisfying \(\|T\|\leq\pi\). Since the constant factor in (2) is the best possible, we have \(\|T\|=\pi\).
About the discrete forms of (1) and (2), in 1950, Wilhelm [8] gave an operator expression. In 2002, by using the operator theory, Zhang [9] gave some improvements of (2) and the discrete form. In 2006 to 2009, [10] considered a new Hilbert-type operator and its applications, and [11] and [12] gave some multiple Hilbert-type operator expressions.
By using (8), we can define the Yang-Hilbert-type integral operator \(T:L_{p,\Phi}(\mathbf{R}_{+})\rightarrow L_{p,\Phi}(\mathbf{R}_{+})\) as follows (cf. [6]): For any \(f\in L_{p,\Phi}(\mathbf{R}_{+})\), there exists \(Tf\in L_{p,\Phi}(\mathbf{R}_{+})\) satisfying
Then by (8) we have \(\|Tf\|_{p,\Phi}\leq k_{\lambda }(r)\|f\|_{p,\Phi}\), and T is a bounded linear operator satisfying \(\|T\|\leq k_{\lambda}(r)\). Since the constant factor in (8) is the best possible, we have \(\|T\|=k_{\lambda}(r)\).
About the composition of two Hilbert-type operators, the main objective is to build the expression \(\|T_{1}T_{2}\|=\|T_{1}\|\cdot\|T_{2}\|\). Recently, [13] published a composition of two discrete Hilbert-Hardy-type operators with particular kernels. Adiyasuren et al. [14] published a composition of two half-discrete Hilbert-Hardy-type operators with some particular kernels, and [15] and [16] published some composition of two Hilbert-Hardy-type integral operators with particular kernels. These works are hard and interesting.
In this paper, applying the way of real and functional analysis and estimating the weight functions, we build some lemmas and deduce some Hilbert-type and Hilbert-Hardy-type integral inequalities with the best possible constant factors. The equivalent forms, the reverses and the operator expressions are all considered. The composition formulas of two Hilbert-Hardy-type integral operators and some examples are given, which are some extensions of the results of [15] and [16].
2 Some lemmas
In the following, we agree on that \(p>0\) (\(p\neq1\)), \(\frac{1}{p}+\frac{1}{q}=1\).
Lemma 1
(cf. [17], Lemma 2.2.5)
Suppose that \(\lambda\in A=(0,c)\) (\(0< c\leq\infty\)), \(k_{\lambda}^{(s)}(x,y)\) are non-negative homogeneous functions of degree −λ in \(\mathbf{R}_{+}^{2}\),
there exists a constant \(\delta_{0}\in(0,\frac{\lambda}{2})\) such that \(k^{(s)}(\frac{\lambda}{2}\pm\delta_{0})\in\mathbf{R}_{+}\). Then, for any \(\delta\in[0,\delta_{0})\), we have \(k^{(s)}(\frac{\lambda }{2}\pm \delta)\in\mathbf{R}_{+}\) and
With the assumptions of Lemma 1, we set the following conditions.
Condition (i)
For \(\lambda\in A\), there exist constants \(\delta _{1}\in(0,\delta_{0})\) and \(L_{1}>0\) such that
Condition (ii)
For \(\lambda\in(0,1)\cap A\), there exists a constant \(L_{2}>0\) such that
Condition (iii)
For \(\lambda\in(0,1)\cap A\), there exist constants \(a\in(0,\lambda)\) and \(L_{3}>0\) such that
Example 1
For \(\lambda\in A=(0,\infty)\), \(s=1,2,3\), the functions
satisfy Conditions (i) and (iii). In fact, for \(b=\frac{\lambda }{2}+\delta _{1}\) or \(b=a\in(0,\lambda)\), we find
In view of the continuity, \(k_{\lambda}^{(s)}(u,1)u^{b}\) (\(s=1,2,3\)) are bounded in \((0,\infty)\) and then satisfy (11) and (13).
It is evident that for \(\lambda\in A=(0,1)\), the functions
satisfy Condition (ii).
Definition 2
With the assumptions of Lemma 1 and Condition (i), we define the following two sequences of real functions:
where \(k>\max\{\frac{1}{|q|\delta_{1}},\frac{1}{p\delta_{1}}\}\) (\(k\in \mathbf{N}=\{1,2,\ldots\}\)).
Setting \(u=x/y\) (\(0< y<1\)), we find
(a) If \(k_{\lambda}^{(2)}(u,1)\) satisfies Condition (i) (for \(\lambda \in A\)), then by (11) we have
(b) If \(k_{\lambda}^{(2)}(u,1)\) satisfies Condition (ii) (for \(\lambda \in(0,1)\cap A\)), then by (12) we have
Still setting \(u=x/y\) (\(x>1\)), we obtain
(c) If \(k_{\lambda}^{(3)}(u,1)\) satisfies Condition (i) (for \(\lambda \in A\)), then by (11) we have
(d) If \(k_{\lambda}^{(3)}(u,1)\) satisfies Condition (ii) (for \(\lambda \in(0,1)\cap A\)), then by (12) we have
Remark 2
In view of the results of (a)-(d), there exists a large constant \(L>0\) such that
-
(a)
\(F(y)\leq Ly^{\frac{\lambda}{2}+\delta_{1}-1}\) (\(y\in(0,1)\); \(\lambda \in A\));
-
(b)
\(F(y)\leq L\frac{y^{\lambda-1}}{(1-y)^{\lambda}}\) (\(y\in (0,1)\); \(\lambda \in(0,1)\cap A\));
-
(c)
\(G(x)\leq Lx^{\frac{\lambda}{2}-\delta_{1}-1}\) (\(x\in(1,\infty )\); \(\lambda \in A\));
-
(d)
\(G(x)\leq L\frac{x^{\lambda-1}}{(x-1)^{\lambda}}\) (\(x\in(1,\infty )\); \(\lambda\in(0,1)\cap A\)).
Lemma 2
With the assumptions of Lemma 1, (1) \(k_{\lambda }^{(2)}(u,1)\) (\(k_{\lambda}^{(3)}(u,1)\)) satisfies Condition (i) or Condition (ii); (2) if \(k_{\lambda}^{(s)}(u,1)\) (\(s=2,3\)) only satisfy Condition (i), then \(\lambda\in A\); otherwise, \(\lambda\in(0,1)\cap A\). Then we have
Proof
In view of (15) and (16), we have
where we define
It is evident that
By Fubini’s theorem, we obtain that (cf. [18])
Since \(\{k_{\lambda}^{(1)}(u,1)u^{\frac{\lambda}{2}+\frac{1}{pk}-1}\}_{k=1}^{\infty}\) (\(u\in(0,1)\)) is increasing, by Levi’s theorem (cf. [18]), it follows that
Since \(k_{\lambda}^{(1)}(u,1)u^{\frac{\lambda}{2}-\frac{1}{qk}-1}\leq k_{\lambda}^{(1)}(u,1)u^{\mu+\delta_{1}-1}\) (\(u\in(1,\infty)\)) and
then by the Lebesgue convergence control theorem (cf. [18]), we have
Hence, by Lemma 1, we find, for \(k\rightarrow\infty\),
(1) We estimate \(I_{2}\).
(a) If \(k_{\lambda}^{(2)}(u,1)\) satisfies Condition (i) for \(\lambda \in A\), then by Remark 2(a) we have
(b) If \(k_{\lambda}^{(2)}(u,1)\) satisfies Condition (ii) for \(\lambda \in (0,1)\cap A\), then by Remark 2(b) we have
Therefore, in view of (a) and (b), we have \(I_{2}\rightarrow 0\) (\(k\rightarrow \infty\)).
(2) We estimate \(I_{3}\).
(c) If \(k_{\lambda}^{(3)}(u,1)\) satisfies Condition (i) for \(\lambda \in A\), then by Remark 2(c) we have
(d) If \(k_{\lambda}^{(3)}(u,1)\) satisfies Condition (ii) for \(\lambda \in (0,1)\cap A\), then by Remark 2(d), we have
Therefore, in view of (c) and (d), we have \(I_{3}\rightarrow 0\) (\(k\rightarrow \infty\)).
By (19) and the above results, we have (17). □
Lemma 3
Suppose that (1) \(\lambda\in A=(0,c)\) (\(0< c\leq\infty \)), \(k_{\lambda}^{(s)}(x,y)\) are non-negative homogeneous functions of degree −λ in \(\mathbf{R}_{+}^{2}\),
there exists a constant \(\delta_{0}\in(0,\frac{\lambda}{2})\) such that \(k^{(s)}(\frac{\lambda}{2}\pm\delta_{0})\in\mathbf{R}_{+}\); (2) \(k_{\lambda}^{(2)}(u,1)\) (\(k_{\lambda}^{(3)}(u,1)\)) satisfies Condition (i) or Condition (ii); (3) if both \(k_{\lambda}^{(2)}(u,1)\) and \(k_{\lambda }^{(3)}(u,1)\) satisfy Condition (ii), then \(k_{\lambda}^{(1)}(u,1)\) satisfies Condition (iii); (4) if \(k_{\lambda}^{(s)}(u,1)\) (\(s=2,3\)) only satisfy Condition (i), then \(\lambda\in A\); otherwise, \(\lambda\in (0,1)\cap A\). Then we have the reverse of (17), namely
Proof
We have four cases to show that in any case, \(I_{4}\rightarrow0\) (\(k\rightarrow\infty\)).
Case (i). \(\lambda\in A\), \(F(y)\leq Ly^{\frac{\lambda}{2}+\delta _{1}-1}\) (\(y\in (0,1)\)), \(G(x)\leq Lx^{\frac{\lambda}{2}-\delta_{1}-1}\) (\(x\in(1,\infty)\)). We have
Case (ii). \(\lambda\in(0,1)\cap A\), \(F(y)\leq Ly^{\frac{\lambda }{2}+\delta _{1}-1}\) (\(y\in(0,1)\)), \(G(x)\leq L\frac{x^{\lambda-1}}{(x-1)^{\lambda}}\) (\(x\in(1,\infty)\)). We have
Case (iii). \(\lambda\in(0,1)\cap A\), \(F(y)\leq L\frac{y^{\lambda-1}}{(1-y)^{\lambda}}\) (\(y\in(0,1)\)), \(G(x)\leq Lx^{\frac{\lambda}{2}-\delta _{1}-1}\) (\(x\in(1,\infty)\)). We have
Case (iv). \(\lambda\in(0,1)\cap A\), \(F_{k}(y)\leq L\frac{y^{\lambda-1}}{ (1-y)^{\lambda}}\) (\(y\in(0,1)\)), \(G_{k}(x)\leq L\frac{x^{\lambda-1}}{(x-1)^{\lambda}}\) (\(x\in(1,\infty)\)), \(k_{\lambda}^{(1)}(u,1)\) satisfies Condition (iii). We have
Hence, in any case, \(I_{4}=\frac{1}{k}J_{4}\rightarrow0\) (\(k\rightarrow \infty\)).
Therefore, by (19) and (20), we have the reverse of (17), and then (21) follows. □
3 Some equivalent Hilbert-type inequalities
We set functions \(\varphi(x):=x^{p(1-\frac{\lambda}{2})-1}\), \(\psi (y):=y^{q(1-\frac{\lambda}{2})-1}\) (\(x,y\in\mathbf{R}_{+}\)) in the following theorem.
Theorem 1
Suppose that (1) \(\lambda\in A=(0,c)\) (\(0< c\leq \infty\)), \(k_{\lambda}^{(s)}(x,y)\) are non-negative homogeneous functions of degree −λ in \(\mathbf{R}_{+}^{2}\),
there exists a constant \(\delta_{0}\in(0,\frac{\lambda}{2})\) such that \(k^{(s)}(\frac{\lambda}{2}\pm\delta_{0})\in\mathbf{R}_{+}\); (2) \(k_{\lambda}^{(2)}(u,1)\) (\(k_{\lambda}^{(3)}(u,1)\)) satisfies Condition (i) or Condition (ii); (3) if both \(k_{\lambda}^{(2)}(u,1)\) and \(k_{\lambda }^{(3)}(u,1)\) satisfy Condition (ii), then \(k_{\lambda}^{(1)}(u,1)\) satisfies Condition (iii); (4) if \(k_{\lambda}^{(s)}(u,1)\) (\(s=2,3\)) only satisfy Condition (i), then \(\lambda\in A\); otherwise, \(\lambda\in (0,1)\cap A\). For \(p>1\), \(f(x),G(y)\geq0\), \(f\in L_{p,\varphi}(\mathbf{R}_{+})\), \(G\in L_{q,\psi}(\mathbf{R}_{+})\), \(\|f\|_{p,\varphi},\|G\| _{q,\psi }>0\), and
we have the following equivalent inequalities:
where the constant factor \(\prod_{s=1}^{2}k^{(s)}(\frac{\lambda}{2})\) is the best possible.
In particular, for \(g(y)\geq0\), \(g\in L_{q,\psi}(\mathbf{R}_{+})\), \(\|g\|_{q,\psi}>0\), and
we have the following inequality:
where the constant factor \(\prod_{s=1}^{3}k^{(s)}(\frac{\lambda}{2})\) is still the best possible.
Proof
By (9) and (8) (for \(r=s=2\)), we have
Then we have (24).
By Hölder’s inequality (cf. [19]), we have
On the other hand, suppose that (23) is valid. Setting
we find \(\|G\|_{q,\psi}^{q}=J^{p}\). If \(J=0\), then (24) is trivially valid; if \(J=\infty\), then by (27) we have \(\|F_{\lambda }\|_{p,\varphi}=\infty\), which contradicts the fact of (28). Assuming that \(0< J<\infty\), then by (23) we have
then we have (24), which is equivalent to (23).
Since we find similar to (28) that
setting \(G(x)=G_{\lambda}(x)\) in (23), we have (26).
For any \(k>\max\{\frac{1}{|q|\delta_{1}},\frac{1}{p\delta_{1}}\}\) (\(k\in \mathbf{N}\)), we set
Then we have
If there exists a positive constant \(K\leq\prod_{s=1}^{3}k^{(s)}(\frac{ \lambda}{2})\) such that (26) is valid when replacing \(\prod_{s=1}^{3}k^{(s)}(\frac{\lambda}{2})\) by K, then, in particular, we have
By (17), we find \(\prod_{s=1}^{3}k^{(s)}(\frac{\lambda }{2})+o(1)\leq \widetilde{L}_{k}< K\), and then \(\prod_{s=1}^{3}k^{(s)}(\frac{\lambda }{2})\leq K\) (\(k\rightarrow\infty\)). Hence \(K=\prod_{s=1}^{3}k^{(s)}(\frac{\lambda}{2})\) is the best possible constant factor of (26).
The constant factor in (23) is the best possible. Otherwise, setting \(G(x)=\widetilde{G}_{\lambda}(x)\), we would reach a contradiction that the constant factor in (26) is not the best possible. By the equivalency, if the constant factor in (24) is not the best possible, then by (29) we would reach a contradiction that the constant factor in (23) is not the best possible. □
Theorem 2
Suppose that (1) \(\lambda\in A=(0,c)\) (\(0< c\leq\infty \)), \(k_{\lambda}^{(s)}(x,y)\) are non-negative homogeneous functions of degree −λ in \(\mathbf{R}_{+}^{2}\),
there exists a constant \(\delta_{0}\in(0,\frac{\lambda}{2})\) such that \(k^{(s)}(\frac{\lambda}{2}\pm\delta_{0})\in\mathbf{R}_{+}\); (2) \(k_{\lambda}^{(2)}(u,1)\) (\(k_{\lambda}^{(3)}(u,1)\)) satisfies Condition (i) or Condition (ii); (3) if both \(k_{\lambda}^{(2)}(u,1)\) and \(k_{\lambda }^{(3)}(u,1)\) satisfy Condition (ii), then \(k_{\lambda}^{(1)}(u,1)\) satisfies Condition (iii); (3) if \(k_{\lambda}^{(s)}(u,1)\) (\(s=2,3\)) only satisfy Condition (i), then \(\lambda\in A\); otherwise, \(\lambda\in (0,1)\cap A\). For \(0< p<1\), \(f(x),G(y)\geq0\), \(f\in L_{p,\varphi}(\mathbf {R}_{+})\), \(G\in L_{q,\psi}(\mathbf{R}_{+})\), \(\|f\|_{p,\varphi}\), \(\|G\|_{q,\psi}>0\), and \(F_{\lambda}(y)\) being as (22), we have the equivalent reverses of (24) and (25) with the best possible constant factor \(\prod_{s=1}^{2}k^{(s)}(\frac{\lambda}{2})\).
In particular, for \(g(y)\geq0\), \(g\in L_{q,\psi}(\mathbf{R}_{+})\), \(\|g\|_{q,\psi}>0\), and \(G(x)=G_{\lambda}(x)\) as (25), we have the reverse of (26) with the best possible constant factor \(\prod_{s=1}^{3}k^{(s)}(\frac{\lambda}{2})\).
Proof
By the reverse Hölder inequality (cf. [19]), we obtain the reverses of (27) and (28). Then we deduce the reverse of (24).
By the reverse Hölder inequality, we have
Then by the reverse of (24), we obtain the reverse of (23).
On the other hand, suppose that the reverse of (23) is valid. Setting \(G(x)\) as (25), we find \(\|G\|_{q,\psi}^{q}=J^{p}\). If \(J=\infty\), then the reverse of (24) is trivially valid; if \(J=0\), then by the reverse of (27), we have \(\|F_{\lambda}\|_{p,\varphi}=0\), which contradicts the reverse of (28). Assuming that \(0< J<\infty\), by the reverse of (23), we have
and then the reverse of (24) follows, which is equivalent to the reverse of (23).
For \(q<0\), since we find similar to the reverse of (28) that
setting \(G(x)=G_{\lambda}(x)\) in the reverse of (23), we have the reverse of (26).
For any \(k>\max\{\frac{1}{|q|\delta_{1}},\frac{1}{p\delta_{1}}\}\) (\(k\in \mathbf{N}\)), we set \(\widetilde{f}(x)\), \(\widetilde{g}(y)\) as Theorem 1. If there exists a positive constant \(K\geq\prod_{s=1}^{3}k^{(s)}(\frac {\lambda}{2})\) such that the reverse of (26) is valid when replacing \(\prod_{s=1}^{3}k^{(s)}(\frac{\lambda}{2})\) by K, then, in particular, we have
By (21), we find \(\prod_{s=1}^{3}k^{(s)}(\frac{\lambda}{2})+o(1)= \widetilde{L}_{k}>K\), and then \(\prod_{s=1}^{3}k^{(s)}(\frac{\lambda }{2})\geq K\) (\(k\rightarrow\infty\)). Hence \(K=\prod_{s=1}^{3}k^{(s)}(\frac{\lambda}{2})\) is the best possible constant factor of the reverse of (26).
The constant factor in the reverse of (23) is the best possible. Otherwise, setting \(G(x)=\widetilde{G}_{\lambda}(x)\), we would reach a contradiction that the constant factor in the reverse of (26) is not the best possible. By the equivalency, if the constant factor in the reverse of (24) is not the best possible, then by (30) we would reach a contradiction that the constant factor in the reverse of (23) is not the best possible. □
4 Some corollaries on Hilbert-Hardy-type inequalities
In the following sections, if the best possible constant factor in a Hilbert-type inequality is related to \(k_{j}^{(s)}(\frac{\lambda}{2})\) (\(s=1,2,3\), \(j=0,1,2\)) defined as follows, then we call this inequality Hilbert-Hardy-type inequality. The related operator is called Hilbert-Hardy-type operator.
Assuming that \(k_{\lambda}^{(1)}(xy,1)=0\) (\(0<\frac{1}{x}\leq y\)), we find \(k_{\lambda}^{(1)}(u,1)=0\) (\(u\geq1\)), and
By Theorem 1 and Theorem 2, we have the following.
Corollary 1
With the assumptions of Theorem 1, for \(p>1\), \(k_{1}^{(1)}(\frac{\lambda}{2})\in\mathbf{R}_{+}\), we have the following equivalent inequalities:
where the constant factor \(k_{1}^{(1)}(\frac{\lambda}{2})k^{(2)}(\frac{ \lambda}{2})\) is the best possible.
In particular, for \(g(y)\geq0\), \(g\in L_{q,\psi}(\mathbf{R}_{+})\), \(\|g\|_{q,\psi}>0\), \(G(x)=G_{\lambda}(x)\) as (25), we have the following inequality:
where the constant factor \(k_{1}^{(1)}(\frac{\lambda}{2})\prod_{s=2}^{3}k^{(s)}(\frac{\lambda}{2})\) is still the best possible.
Corollary 2
With the assumptions of Theorem 2, for \(0< p<1\), \(k_{1}^{(1)}(\frac{\lambda}{2})\in\mathbf{R}_{+}\), we have the equivalent reverses of (32) and (33), where the constant factor \(k_{1}^{(1)}(\frac{\lambda}{2})k^{(2)}(\frac{\lambda}{2})\) is the best possible.
In particular, for \(g(y)\geq0\), \(g\in L_{q,\psi}(\mathbf{R}_{+})\), \(\|g\|_{q,\psi}>0\), \(G(x)=G_{\lambda}(x)\) as (25), we have the reverse of (34) with the best value \(k_{1}^{(1)}(\frac{\lambda }{2})\prod_{s=2}^{3}k^{(s)}(\frac{\lambda}{2})\).
Assuming that \(k_{\lambda}^{(1)}(xy,1)=0\) (\(0< y\leq\frac{1}{x}\)), then we find \(k_{\lambda}^{(1)}(u,1)=0\) (\(0< u\leq1\)), and
By Theorem 1 and Theorem 2, we have the following.
Corollary 3
With the assumptions of Theorem 1, for \(p>1\), \(k_{2}^{(1)}(\frac{\lambda}{2})\in\mathbf{R}_{+}\), we have the following equivalent inequalities:
where the constant factor \(k_{2}^{(1)}(\frac{\lambda}{2})k^{(2)}(\frac{ \lambda}{2})\) is the best possible.
In particular, for \(g(y)\geq0\), \(g\in L_{q,\psi}(\mathbf{R}_{+})\), \(\|g\|_{q,\psi}>0\), and \(G(x)=G_{\lambda}(x)\) as (25), we have the following inequality:
where the constant factor \(k_{2}^{(1)}(\frac{\lambda}{2})\prod_{s=2}^{3}k^{(s)}(\frac{\lambda}{2})\) is still the best possible.
Corollary 4
With the assumptions of Theorem 2, if \(0< p<1\), \(k_{2}^{(1)}(\frac{\lambda}{2})\in\mathbf{R}_{+}\), we have the equivalent reverses of (36) and (37), where the constant factor \(k_{2}^{(1)}(\frac{\lambda}{2})k^{(2)}(\frac{\lambda}{2})\) is the best possible.
In particular, for \(g(y)\geq0\), \(g\in L_{q,\psi}(\mathbf{R}_{+})\), \(\|g\|_{q,\psi}>0\), and \(G(x)=G_{\lambda}(x)\) as (25), we have the reverse of (38) with the best value \(k_{2}^{(1)}(\frac{\lambda }{2})\prod_{s=2}^{3}k^{(s)}(\frac{\lambda}{2})\).
Remark 3
For \(x>0\), we set \(A_{x,0}:=(0,\infty )\), \(A_{x,1}:=(0,\frac{1}{x})\), \(A_{x,2}:=(\frac{1}{x},\infty)\). By (24), (33) and (37), putting \(k_{0}^{(1)}(\frac{\lambda}{2}):=k^{(1)}(\frac{\lambda }{2})\), for \(i=0,1,2\), we have the following Hilbert-Hardy-type inequalities:
where the constant factor \(k_{i}^{(1)}(\frac{\lambda}{2})k^{(2)}(\frac{ \lambda}{2})\) (\(i=0,1,2\)) is still the best possible.
For \(x>0\), we set some sets \(B_{x,0}:=(0,\infty)\), \(B_{x,1}:=(x,\infty )\), \(B_{x,2}:=(0,x)\). If \(k_{\lambda}^{(2)}(x,y)=0\) (\(y\in\mathbf{R}_{+}\backslash B_{x,1}\)), then we find \(k_{\lambda}^{(2)}(u,1)=0\) (\(u\geq1\)), and
if \(k_{\lambda}^{(2)}(x,y)=0\) (\(y\in\mathbf{R}_{+}\backslash B_{x,2}\)), then we find \(k_{\lambda}^{(2)}(u,1)=0\) (\(0< u\leq1\)), and
Assuming that \(k_{0}^{(2)}(\frac{\lambda}{2}):=k^{(2)}(\frac{\lambda}{2})\), \(k_{i}^{(1)}(\frac{\lambda}{2}),k_{j}^{(2)}(\frac{\lambda}{2})\in \mathbf{R}_{+}\), for \(i,j=0,1,2\), setting
then it follows that \(F_{\lambda,0}(y)=F_{\lambda}(y)\), and by (39) we have the following united expression of Hilbert-Hardy-type inequalities:
where the constant factor \(k_{i}^{(1)}(\frac{\lambda }{2})k_{j}^{(2)}(\frac{\lambda}{2})\) (\(i,j=0,1,2\)) is the best possible.
In the same way, we still can find by (27) and (28) that
where the constant factors \(k_{i}^{(1)}(\frac{\lambda}{2})\) and \(k_{j}^{(2)}(\frac{\lambda}{2})\) are the best possible.
Example 2
(i) For \(k_{\lambda}^{(s)}(x,y)=\frac{|\ln x/y|^{\beta -1}}{(\max\{x,y\})^{\lambda}}\) (\(\lambda>0,\beta\geq1\); \(s=1,2,3\)), we find
(ii) For \(k_{\lambda}^{(s)}(x,y)=\frac{1}{|x-y|^{\lambda}}\) (\(0<\lambda <1\); \(s=1,2,3\)), we find
5 A composition of two Hilbert-Hardy-type operators
For \(F\in L_{p,\varphi}(\mathbf{R}_{+})\), we set \(h_{i}(x):=x^{\lambda -1}\int_{A_{x,i}}k_{\lambda}^{(1)}(xy,1)F(y)\,dy\) (\(x\in\mathbf{R}_{+}\); \(i=0,1,2\)). Then by (41) we have
Definition 3
With the assumptions of Theorem 1, for any \(i=0,1,2\), \(k_{i}^{(1)}(\frac{\lambda}{2})\in\mathbf{R}_{+}\), we define a Hilbert-Hardy-type operator \(T_{1}^{(i)}:L_{p,\varphi}(\mathbf{R}_{+})\rightarrow L_{p,\varphi}(\mathbf{R}_{+})\) as follows: For any \(F\in L_{p,\varphi}(\mathbf{R}_{+})\), there exists a unified expression \(T_{1}^{(i)}F=h_{i}\in L_{p,\varphi}(\mathbf{R}_{+})\) such that for any \(x\in\mathbf{R}_{+}\), \(T_{1}^{(i)}F(x)=h_{i}(x)\).
By (43), we have \(\|T_{1}^{(i)}F\|_{p,\varphi}\leq k_{i}^{(1)}(\frac{\lambda}{2})\|F\|_{p,\varphi}\). Hence, \(T_{1}^{(i)}\) is a bounded linear operator with
Since the constant factor in (43) is the best possible, we have \(\|T_{1}^{(i)}\|=k_{i}^{(1)}(\frac{\lambda}{2})\).
Definition 4
With the assumptions of Theorem 1, for any \(j=0,1,2\), \(k_{j}^{(2)}(\frac{\lambda}{2})\in\mathbf{R}_{+}\), we define a Hilbert-Hardy-type operator \(T_{2}^{(j)}:L_{p,\varphi}(\mathbf{R}_{+})\rightarrow L_{p,\varphi}(\mathbf{R}_{+})\) as follows: For any \(f\in L_{p,\varphi}(\mathbf{R}_{+})\), there exists a unified expression \(T_{2}^{(j)}f=F_{\lambda,j}\in L_{p,\varphi}(\mathbf{R}_{+})\) such that for any \(y\in\mathbf{R}_{+}\), \(T_{2}^{(j)}f(y)=F_{\lambda,j}(y)\).
By (42), we have \(\|T_{2}^{(j)}f\|_{p,\varphi}=\|F_{\lambda ,j}\|_{p,\varphi}\leq k_{j}^{(2)}(\frac{\lambda}{2})\|f\|_{p,\varphi}\). Hence, \(T_{2}^{(j)}\) is a bounded linear operator with
Since the constant in (42) is the best possible, we have \(\|T_{2}^{(j)}\|=k_{j}^{(2)}(\frac{\lambda}{2})\).
Definition 5
With the assumptions of Theorem 1, for any \(i,j\in \{0,1,2\}\), \(k_{i}^{(1)}(\frac{\lambda}{2}),k_{j}^{(2)}(\frac{\lambda }{2})\in\mathbf{R}_{+}\), we define a Hilbert-Hardy-type operator \(T_{i,j}:L_{p,\varphi}(\mathbf{R}_{+})\rightarrow L_{p,\varphi }(\mathbf{R}_{+})\) as follows: For any \(f\in L_{p,\varphi}(\mathbf{R}_{+})\), there exists a unified expression \(T_{i,j}f=T_{1}^{(i)}F_{\lambda,j}\in L_{p,\varphi}(\mathbf{R}_{+})\) such that for any \(x\in\mathbf{R}_{+}\),
It is evident that \(T_{i,j}f=T_{1}^{(i)}F_{\lambda ,j}=T_{1}^{(i)}(T_{2}^{(j)}f)=(T_{1}^{(i)}T_{2}^{(j)})f\), and then \(T_{i,j}=T_{1}^{(i)}T_{2}^{(j)}\). Hence, \(T_{i,j}\) is the composition of \(T_{1}^{(i)}\) and \(T_{2}^{(j)}\), and (cf. [20])
By (40), we have
Since the constant factor in (40) is the best possible, then the theorem follows.
Theorem 3
With the assumptions of Theorem 1, if for any \(i,j\in \{0,1,2\}\), \(k_{i}^{(1)}(\frac{\lambda}{2}),k_{j}^{(2)}(\frac{\lambda }{2})\in\mathbf{R}_{+}\), then we have the composition formula of two Hilbert-Hardy-type operators as follows:
Example 3
For \(k_{\lambda}^{(1)}(xy,1)=\frac {1}{|xy-1|^{\lambda}}\), \(k_{\lambda}^{(2)}(x,y)=\frac{|\ln(x/y)|^{\beta-1}}{(\max \{x,y\})^{\lambda}}\) (\(\beta\geq1\)), \(\lambda\in(0,1)\), by Example 2 and (44), we have
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Acknowledgements
This work is supported by the National Natural Science Foundation of China (No. 61370186), and 2013 Knowledge Construction Special Foundation Item of Guangdong Institution of Higher Learning College and University (No. 2013KJCX0140).
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BY carried out the mathematical studies, participated in the sequence alignment and drafted the manuscript. QC participated in the design of the study and performed the numerical analysis. All authors read and approved the final manuscript.
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Yang, B., Chen, Q. On a composition of two Hilbert-Hardy-type integral operators and related inequalities. J Inequal Appl 2015, 100 (2015). https://doi.org/10.1186/s13660-015-0619-2
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DOI: https://doi.org/10.1186/s13660-015-0619-2