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Factorization of Hilbert operators
Journal of Inequalities and Applications volume 2022, Article number: 121 (2022)
Abstract
In this research, we introduce some factorization for Hilbert operators of order n based on two important classes of Hausdorff operators, Cesàro and gamma. These factorizations lead us to some new inequalities, one is a generalized version of Hilbert’s inequality. Moreover, as an application of our factorization, we compute the norm of Hilbert operators on some matrix domains.
1 Introduction
Let ω denote the set of all real-valued sequences. Any linear subspace of ω is called a sequence space. The Banach space \(\ell _{p}\) is the set of all real sequences \(x=(x_{k})_{k=0}^{\infty}\in \omega \) such that
We consider infinite matrices \(\mathfrak{M}=(m_{j,k})\), where all the indices j and k are nonnegative. The matrix domain associated with \(\mathfrak{M}\) is defined as
In the special case \(S=\ell _{p}\), we use the notation \(\mathfrak{M}_{p}\) instead of \(\mathfrak{M}_{\ell _{p}}\). It is rather trivial that \(I_{p}=\ell _{p}\), where I is the infinite identity matrix. This concept has inspired many researchers to seek and define new Banach spaces as the domain of an infinite matrix. See [18, 20, 22] and the textbook [1].
Let Σ be a matrix with nonnegative entries, which maps \(\ell _{p}\) into itself and satisfies the inequality
for the constant ρ not depending on x and for every \(x \in \ell _{p}\). The norm of Σ is the smallest possible value of ρ. The problem of finding the norm and the lower bound of operators on the matrix domains has been investigated in some of the references [7, 8, 10–12, 19, 21].
Hilbert matrix
For a nonnegative integer n, the Hilbert matrix of order n, \(\mathcal{H}_{n}\), is defined by
Evidently, for \(n=0\), \(\mathcal{H}_{0}=\mathcal{H}\) is the well-known Hilbert matrix
which was introduced by David Hilbert in 1894. More examples:
The Hilbert matrix is a bounded operator on \(\ell _{p}\) ([6], Theorem 323) and
where \(p^{*}\) is the conjugate of p i.e. \(\frac{1}{p}+\frac{1}{p^{*}}=1\).
Hausdorff matrices
The Hausdorff matrix \(H_{\mu}\) is one of the most important examples of summability matrices defined by
where μ is a probability measure on \([0,1]\). While obtaining the \(\ell _{p}\)-norm of operators is a hard endeavor, for Hausdorff matrices, we luckily have Hardy’s formula [5, Theorem 216] which states that this matrix is a bounded operator on \(\ell _{p}\) if and only if
In fact,
Hausdorff operators have the interesting norm separating property.
Theorem 1.1
([3], Theorem 9)
Let \(p\ge 1\) and \(H_{\mu}\), \(H_{\varphi}\), and \(H_{\nu}\) be Hausdorff matrices such that \(H_{\mu}=H_{\varphi }H_{\nu}\). Then \(H_{\mu}\) is bounded on \(\ell _{p}\) if and only if both \(H_{\varphi}\) and \(H_{\nu}\) are bounded on \(\ell _{p}\). Moreover, we have
The Hausdorff matrix contains several famous classes of matrices. For positive integer n, two of these classes are as follows.
Cesàro matrix
The measure \(d\mu (\theta )=n(1-\theta )^{n-1}\,d\theta \) gives the Cesàro matrix of order n, \(C_{n}\), which is defined by
Hence, according to (1.2), \(C_{n}\) has the \(\ell _{p}\)-norm
Note that \(C_{0}=I\), where I is the identity matrix, and
is the classical Cesàro matrix, which has \(\ell _{p}\)-norm \(\|C\|_{\ell _{p} \to \ell _{p}}=\frac{p}{p-1}\). The matrix domain associated with \(C_{n}\) is defined by
having the norm
is a Banach space. The author investigated the Cesàro matrix domain in [15, 17].
Gamma matrix
The measure \(d\mu (\theta )=n\theta ^{n-1}\,d\theta \) gives the gamma matrix of order n, \(\mathcal{G}_{n}\), for which
Hence, by Hardy’s formula, \(\mathcal{G}_{n}\) has the \(\ell _{p}\)-norm
The sequence space associated with \(\mathcal{G}_{n}\) is the set
which is called the gamma space of order n. The space \(\mathcal{G}_{n}(p)\) is a Banach space with the norm
Note that \(\mathcal{G}_{1}\) is the classical Cesàro matrix C, and we show the gamma sequence space \(\mathcal{G}_{1}(p)\) by the notation \(C(p)\). For more information about the gamma matrix domain, the eager readers can refer to [9, 16].
For finding the norm of a transpose of an operator, we use a helpful theorem also known as Hellinger–Toeplitz theorem.
Theorem 1.2
([4], Proposition 7.2)
Suppose that \(1 < p,q < \infty \). A matrix Σ with nonnegative entries maps \(\ell _{p}\) into \(\ell _{q}\) if and only if the transposed matrix \(\Sigma ^{t}\) maps \(\ell _{q^{*}}\) into \(\ell _{p^{*}}\). Then we have
Motivation
The infinite Hilbert operator is one of the most complicated operators which is used in cryptography because of its complexity. Recently the author [13, 14] has introduced some factorizations for the infinite Hilbert operator based on Cesàro and gamma operators. Through this study, the author not only has generalized its previous results to the Hilbert operators of any order, but has introduced some factorizations that result in several new inequalities.
2 Factorization of the Hilbert operator
Bennett in [2] introduced a factorization of the form \(\mathcal{H}=UC\), where C is the Cesàro operator and the matrix U is defined by
The matrix U is a bounded operator on \(\ell _{p}\) and \(\|U\|_{\ell _{p} \to \ell _{p}}=\Gamma (1/p)\Gamma (1+1/p^{*})\), ([2], Proposition 2). In the sequel, we generalize this result for all Hilbert operators, but first we need the following lemma also known as Schur’s lemma.
Schur Lemma
([6], Theorem 275)
Let \(p>1\) and Σ be a matrix with nonnegative entries. Suppose that \(\mathcal{S}\), \(\mathcal{R}\) are two positive numbers such that
Then
Theorem 2.1
Let n and m be two nonnegative integers. The Hilbert matrix of order n has a factorization of the form \(\mathcal{H}_{n}=\mathcal{R}_{n,m}C_{m}\), where the matrix \(\mathcal{R}_{n,m}\) has the entries
and is a bounded operator on \(\ell _{p}\) with
In particular,
-
(a)
the Hilbert matrix of order n has a factorization of the form \(\mathcal{H}_{n}=\mathcal{R}_{n}C_{n}\), where the matrix \(\mathcal{R}_{n}\) has the entries
$$\begin{aligned}{} [\mathcal{R}_{n}]_{j,k} =& \frac{(k+1)\cdots (k+n)}{(j+k+n+1)\cdots (j+k+2n+1)} \\ =&{\binom{n+k}{k}}\beta (j+k+n+1, n+1) \end{aligned}$$and is a bounded operator on \(\ell _{p}\) with \(\|\mathcal{R}_{n}\|_{\ell _{p} \to \ell _{p}}= \frac{\Gamma (n+1/p^{*})\Gamma (1/p)}{\Gamma (n+1)}\).
-
(b)
the Hilbert matrix has a factorization of the form \(\mathcal{H}=B_{n}C_{n}\), where the matrix \(U_{n}\) has the entries
$$\begin{aligned}{} [B_{n}]_{j,k} =&\frac{(k+1)\cdots (k+n)}{(j+k+1)\cdots (j+k+n+1)} ={\binom{n+k}{k}}\beta (j+k+1, n+1) \end{aligned}$$with \(\ell _{p}\)-norm \(\|B_{n}\|_{\ell _{p} \to \ell _{p}}= \frac{\Gamma (n+1/p^{*})\Gamma (1/p)}{\Gamma (n+1)}\).
Proof
Let \(\Delta _{n}\) be the backward difference matrix of order n. That is a lower triangle matrix with the entries
which is invertible. We use the notation \(\Delta _{n}^{-1}\) as its inverse which is defined by
Note that for \(n=0\), the backward difference matrix \(\Delta _{0}=I\), where I is the identity matrix. It can be easily seen that the Cesàro matrix of order n can be represented by the backward difference operator of the form
On the other hand,
and
By induction, we can prove that
Now, by a simple calculation, we deduce that
For computing the \(\ell _{p}\)-norm of \(\mathcal{R}_{n,m}\), we show first
For convenience, let \(\mathcal{R}_{0,m}=\mathcal{R}_{m}\). We introduce a family of matrices, \(\mathcal{R}_{m}(s)\), \(0< s \le 1\), given by
Since
and
the row sums and the column sums are \(\frac{(1-s)^{m}}{s}\) and \((1-s)^{m-1}\), respectively. Thus Schur’s lemma results in
On the other hand,
Now,
The other side of the above inequality will result from the factorization \(\mathcal{H}=\mathcal{R}_{0,m}C_{m}=\mathcal{R}_{m}C_{m}\). Now, suppose that
where f is a nonnegative function. As a result of equality (2.2), we conclude that
where \(g(n)\) is a function of n. Now, let \(m=0\). Since \(\mathcal{R}_{n,0}=\mathcal{H}_{n}\) is the Hilbert operator of order n, hence \(g(n)=1\) and
So we have the desired result. □
Corollary 2.2
The Hilbert operator of order n admits a factorization of the form \(\mathcal{H}_{n}=U_{n}C\), where C is the classical Cesàro matrix and \(U_{n}\) is defined by
and has the \(\ell _{p}\)-norm
In particular, Hilbert operator has the factorization \(\mathcal{H}=UC\), where \(\|U\|_{\ell _{p} \to \ell _{p}}=\Gamma (1/p)\times\Gamma (1+1/p^{*})\).
Corollary 2.3
The following inequalities hold:
In particular,
and
More explicitly,
and
Corollary 2.4
The Hilbert operator of order n, \(\mathcal{H}_{n}\), is a bounded operator from \(C_{m}(p)\) into \(\ell _{p}\) and
In particular, the Hilbert operator \(\mathcal{H}\) is a bounded operator from \(C(p)\) into \(\ell _{p}\) and \(\|\mathcal{H}\|_{C(p) \to \ell _{p}}=\frac{\pi}{p^{*}}\csc (\pi /p)\).
Proof
Since the map \(C_{m}(p) \rightarrow \ell _{p}\), \(x \rightarrow C_{m} x\) is an isomorphism between these two spaces, according to Theorem 2.1, we have
□
Theorem 2.5
The Hilbert operator has a factorization of the form \(\mathcal{H}=U^{\prime }C^{t}\), where \(U^{\prime}\) is a bounded operator that has the \(\ell _{p}\)-norm
Proof
By a simple calculation, \(U^{\prime}\) is an operator with the matrix representation
or \(U^{\prime }={\binom{e_{1}}{U^{t}}}\), where \(e_{1}=(1,0,0,\ldots )\) and \(U^{t}\) is the transpose of the matrix U defined in relation (2.1). Obviously, \(U^{\prime}\) has the \(\ell _{p}\)-norm same as \(U^{t}\). Hence
□
As a result of the above theorem, we have the following inequality.
Corollary 2.6
The following statement holds:
More explicitly,
Theorem 2.7
For and , a Hilbert operator of order n, \(\mathcal{H}_{n}\), has a factorization of the form \(\mathcal{H}_{n}=\mathcal{S}_{n,m} \mathcal{G}_{m}\), where \(\mathcal{S}_{n,m}\) is defined by
and is a bounded operator on \(\ell _{p}\) with
Proof
At first we prove the factorization. Let \(\alpha =(k+1)(k+2)\ldots (k+m-1)\), we have
For obtaining the norm of \(\mathcal{S}_{n,m}\), consider that \(\mathcal{S}_{n,m} = (1-1/m)U^{t}_{n} +U_{n}\), where the matrix \(U_{n}\) defined in the Corollary 2.2. Hence by applying the Hellinger–Toeplitz theorem
which completes the proof. □
Corollary 2.8
The Hilbert matrix has a factorization of the form \(\mathcal{H}=\mathcal{S}_{n}\mathcal{G}_{n}\), where the matrix \(\mathcal{S}_{n}\) has the entries
and
In particular, the Hilbert matrix has Bennett’s factorization \(\mathcal{H}=UC\), where the matrix U is a bounded operator and \(\|U\|_{\ell _{p} \to \ell _{p}}=\Gamma (1/p)\Gamma (1+1/p^{*})\).
Corollary 2.9
The following inequalities hold:
In particular,
and
More explicitly,
and
Corollary 2.10
The Hilbert operator of order n, \(\mathcal{H}_{n}\), is a bounded operator from \(\mathcal{G}_{m}(p)\) into \(\ell _{p}\) and
In particular, the Hilbert operator \(\mathcal{H}\) is a bounded operator from \(C(p)\) into \(\ell _{p}\) and \(\|\mathcal{H}\|_{C(p) \to \ell _{p}}=\Gamma (1/p)\Gamma (1+1/p^{*})\).
Proof
The map \(\mathcal{G}_{m}(p) \rightarrow \ell _{p}\), \(x \rightarrow \mathcal{G}_{m} x\) is an isomorphism between these two spaces, hence according to Theorem 2.7 we have
□
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Dedicated to Prof. Maryam Mirzakhani who, in spite of a short lifetime, left a long standing impact on mathematics.
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Roopaei, H. Factorization of Hilbert operators. J Inequal Appl 2022, 121 (2022). https://doi.org/10.1186/s13660-022-02857-2
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DOI: https://doi.org/10.1186/s13660-022-02857-2