Abstract
Here, we consider the balayage operator in the setting of \(H^p\) spaces and its Bergman space version (B-balayage) introduced by Wulan et al. (Complex Var Ellipt Equ 59(12):1775–1782, 2014), and extend some known results on these operators.
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1 Introduction
Let \({\mathbb {D}}\) denote the unit disk \(\{z:{\mathbb {C}}:|z|<1\}\) and \({\mathbb {T}}\) the unit circle. For \(0<p<\infty \), the Hardy space \(H^p\) consists of all functions f which are holomorphic on \({\mathbb {D}}\) and satisfy
It is known that each function \(f\in H^p\) has the radial limit \(f(\mathrm{e}^{it})=\lim _{r\rightarrow 1^-}f(r\mathrm{e}^{it})\) a.e. on \({\mathbb {T}}\) and \(f(\mathrm{e}^{it})\in L^p({\mathbb {T}})\).
For \(\phi \in L^1({\mathbb {T}})\), we say that \(\phi \in {\text {BMO}}({\mathbb {T}})\) if
where I denotes any arc of \({\mathbb {T}}\), |I| is its arc length and
In [7], the authors have recently considered Campanato spaces \({\mathcal {L}}^{p,\lambda }({\mathbb {T}})\) defined as follows. For \(\lambda \ge 0\) and \(1\le p<\infty \), the space \({\mathcal {L}}^{p,\lambda }({\mathbb {T}})\) consists of all functions \(\phi \in L^p({\mathbb {T}})\) for which
We note that \(\mathrm{BMO}({\mathbb {T}})={\mathcal {L}}^{p,1},\ 1 \le p<\infty ,\) (see [3, pp. 222-235]).
For a finite positive Borel measure \(\mu \) on \({\mathbb {D}}\), the function
is called the balayage of\(\mu \). It follows from Fubini’s theorem that \(S_\mu (\mathrm{e}^{it})\in L^1({\mathbb {T}})\) (see [3, p. 229]).
If I is an arc of \({\mathbb {T}}\), the Carleson square S(I) is defined as
A positive Borel measure \(\mu \) is called an s-Carleson measure, \(0<s<\infty \), if there exists a positive constant \(C=C(\mu )\), such that
A 1-Carleson measure is simply called a Carleson measure. In [1], Carleson proved that if \(\mu \) is a positive Borel measure in \({\mathbb {D}}\), then, for \(0<p<\infty \), \(H^p\subset L^p(\mathrm{d}\mu )\) if and only if \(\mu \) is a Carleson measure.
It has been proved in [3, p. 229] that if \(\mu \) is the Carleson measure, then \(S_\mu \) belongs to \(\mathrm{BMO}({\mathbb {T}})\). However, the Carleson property of measure \(\mu \) is not a necessary condition for \(S_\mu \) being a \(\mathrm{BMO}({\mathbb {T}})\) function [5].
In the next section, we obtain an extension of the result mentioned above. More precisely, we prove that if \(\mu \) is an s-Carleson measure, \(0<s\le 1\), then \(S_\mu \) belongs to \({\mathcal {L}}^{1,s}\).
In [6], H. Wulan, J. Yang, and K. Zhu introduced the Bergman space version of the balayage operator on the unit disk that was called B-balayage. The B-balayage of a finite complex measure \(\mu \) on \({\mathbb {D}}\) is given by
It has been proved in [6] that if \(\mu \) is a 2-Carleson measure, then there exists a constant \(C>0\), such that
where \(\beta \) is the hyperbolic metric on \({\mathbb {D}}\). Here, applying a similar idea to that used in the proof of this result, we prove the following theorem.
Theorem 1
Assume that \(1< p<\infty \) and \(\mu \) is a positive Borel measure on \({\mathbb {D}}\). If \(\mu \) is a 2p-Carleson measure, then there exists a positive constant \(C=C(p)\), such that
for all \(z,w\in {\mathbb {D}}\).
Actually, this theorem is a special case of a more general theorem stated in Sect. 3.
Here, C will denote a positive constant which can vary from line to line.
2 Balayage Operators and Campanato Spaces \({\mathcal {L}}^{1,s}\)
We start with the following result.
Theorem 2
If \(\mu \) is an s-Carleson measure, \(0<s\le 1\), \(S_\mu \) is given by (1) and \(0\le \gamma <1\), then there exists a positive constant C, such that for any \(I\subset {\mathbb {T}}\):
Proof
Without loss of generality, we can assume that \(|I|<1\).
Let, for \(z\in {\mathbb {D}}\) and \(\theta \in {\mathbb {R}}\):
be the Poisson kernel for the disk \({\mathbb {D}}\). By the Fubini theorem:
For a subarc I of \( {\mathbb {T}}\), let \(2^nI,\ n\in {\mathbb {N}}\) denote the subarc of \({\mathbb {T}}\) with the same center as I and the length \(2^n|I|\).
In view of the equality
we have
Consequently
Since \(P_z(\theta )\le 4\) for \(|z|\le \frac{1}{2}\), we get
Now, we assume that \(\frac{1}{2}\le |z|<1\) and \(z=|z|\mathrm{e}^{i\omega }\in S(2^{n+1}I){\setminus } S(2^nI)\). We consider two cases: (i) \(\mathrm{e}^{i\omega } \in 2^nI\) and (ii) \(\mathrm{e}^{i\omega } \in 2^{n+1}I{\setminus } 2^nI\).
In case (i), we have
Thus
Therefore, if \(\mathrm{e}^{i\theta },\mathrm{e}^{i\varphi }\in I\), then
Now, we turn to case (ii). Then, for \(\mathrm{e}^{i\psi }\in I\),
Consequently, for \(\mathrm{e}^{i\theta },\mathrm{e}^{i\varphi }\in I\), we get
Now, we put \(Q_n=S(2^nI),\ n=1,2,\ldots \) Then, by (5) and (6),
The above inequality and (4) imply
\(\square \)
The next theorem shows that if \(\mu \) is an s-Carleson measure, \(0<s\le 1\), then \(S_\mu \) is in the Campanato space \(\mathcal {L}^{1,s}\).
Theorem 3
If \(\mu \) is an s-Carleson measure on \({\mathbb {D}}\), \(0<s\le 1\) and \(S_\mu (t)=S_\mu (\mathrm{e}^{it})\) is the balayage operator of \(\mu \) given by (1), then there exists a positive constant C, such that for any \(I\subset {\mathbb {T}}\)
Proof
It is enough to observe that
and the inequality follows from Theorem 2 with \(\gamma = 0\). \(\square \)
3 B-Balayage for Weighted Bergman Spaces \(A^p_\alpha \)
Recall that, for \(0<p<\infty \), \(-1<\alpha <\infty \), the weighted Bergman space \(A_\alpha ^p\) is the space of all holomorphic functions in \(L^p({\mathbb {D}},\mathrm{d}A_\alpha )\), where
and \(\mathrm{d}A\) is the normalized Lebesgue measure on \({\mathbb {D}}\); that is, \(\int _{\mathbb {D}} \mathrm{d}A=1\). If f is in \(L^p({\mathbb {D}}, \mathrm{d}A_\alpha )\), we write
It is well known that, for \(1<p<\infty \), the Bergman projection \(P_\alpha \) given by
is a bounded operator from \(L^p({\mathbb {D}}, \mathrm{d}A_\alpha )\) onto \(A_\alpha ^p\).
Let for \(z,w\in {\mathbb {D}}\), the function
denote the automorphism of the unit disk \({\mathbb {D}}\). The hyperbolic metric on \({\mathbb {D}}\) is given by
For \(z\in {\mathbb {D}}\) and \(r>0\), the hyperbolic disk with center z and radius r is
For \(s>1\), the condition for an s-Carleson measure given in Introduction is equivalent to the condition where Carleson squares are replaced by hyperbolic disks. More exactly, the following result is known.
Proposition
[2, 10] Let \(\mu \) be a positive Borel measure on \({\mathbb {D}}\) and \(1<s<\infty \). Then, the following statements are equivalent
- (i)
\(\mu \) is an s-Carleson measure,
- (ii)
\(\mu (D(z,r))\le C(1-|z|^2)^s\) for some constant C depending only on r for all hyperbolic disk D(z, r), \(z\in {\mathbb {D}}\).
A positive Borel measure \(\mu \) on \({\mathbb {D}}\) is called an \(A^p_\alpha \)-Carleson measure if there exists a positive constant C, such that
for all \(f\in A_\alpha ^p\).
It is well known that \(\mu \) is an \(A^p_\alpha \)-Carleson measure if and only if \(\mu \) is \((2+\alpha )\)-Carleson measure (see [10, p. 133]). This means that \(A^p_\alpha \)-Carleson measures are independent of p.
The next corollary is an immediate consequence of the last proposition.
Corollary
[6] For \(\alpha>-1,\ \sigma >0\), let \(\mu ,\nu \) be positive Borel measures on \({\mathbb {D}}\), such that
Then, \(\mu \) is an \(A^p_\alpha \)-Carleson measure if and only if \(\nu \) is an \(A^p_{\alpha +\sigma }\)-Carleson measure.
Recall that, for \(1<p<\infty \), the Besov space \(B_p\) is the space of all functions f analytic on \({\mathbb {D}}\), such that
where
is the Möbius invariant measure on \({\mathbb {D}}\).
We will use the fact that the Besov space \( B_p=P_\alpha (L^p,\mathrm{d}\tau ).\) The proof of this equality is given in [9, p. 119]. Moreover, if \(f=P_\alpha g\), where \(g\in L^p(\mathrm{d}\tau )\), then
It then follows from [4, Thm. 1.9] that
The next theorem gives a Lipschitz type estimate for functions in the analytic Besov space.
Theorem 4
[8] For any \(1<p<\infty \), there exists a constant \(C_p>0\), such that
for all \(f\in B_p\) and \(z,w\in {\mathbb {D}}\), where \(\frac{1}{p}+\frac{1}{q} =1\).
In [6], the authors also consider a version of the balayage of a measure \(\mu \) on \({\mathbb {D}}\) defined by
They have proved the following generalization of inequality (2).
If \(\mu \) is an \(A^p_\alpha \)-Carleson measure, then the generalized balayage \(G_{\mu ,\alpha }\) satisfies the Lipschitz condition:
where C is independent of z and w.
It is worth noting here that the balayage given by (1) is in a certain sense a limit case of \(G_{\mu ,\alpha }\) as \(\alpha \rightarrow -1\). Since an \(A_\alpha ^p\)-Carleson measure is actually a \((2+\alpha )\)-Carleson measure, the last inequality gives a necessary condition for a measure \(\mu \) to be an s-Carleson measure, as \(1<s<\infty \).
Theorem 1 is a special case of the following more general theorem.
Theorem 5
Assume that \(1< p<\infty \), \(-1<\alpha <\infty \), and \(\mu \) is a positive Borel measure on \({\mathbb {D}}\). If \(\mu \) is a \(p(2+\alpha )\)-Carleson measure, then there exists a positive constant \(C=C(p,\alpha )\), such that
for all \(z,w\in {\mathbb {D}}\).
Proof
For z, w, we have
Since \(\mu \) is a finite measure on \({\mathbb {D}}\), the Jensen’s inequality yields
By the Corollary, \((1-|a|^2)^{p(2+\alpha )}\mathrm{d}\mu (a)\) is an \(A^p_{2p(2+\alpha )-2}\)-Carleson measure, because \(\mu \) is an \(A^p_{p(2+\alpha )-2}\)-Carleson measure. Consequently,
where q is the conjugate index for p, that is, \(\frac{1}{p} +\frac{1}{q} =1\).
Now, set \(\beta =\frac{2}{q-1}\) and note that
Put \(g=(\alpha +1)^\frac{1}{q}(1-|a|^2)^{\frac{\beta +2}{q}}f\) and observe that \(\Vert f\Vert _{q,\beta }\le 1\) if and only if \( \Vert g\Vert _{L^q(\mathrm{d}\tau )}\le 1\). Moreover, since \(\beta =\frac{2}{q-1}\) satisfies \(\frac{\beta +2}{q}=\beta \), we get
where the last inequality follows from Theorem 4 and inequality (7). \(\square \)
Change history
15 February 2020
In the original publication, article title was incorrectly published as.
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Acknowledgements
The authors are grateful to the referee for suggesting Theorem 5.
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Nowak, M., Sobolewski, P. On Balayaga and B-Balayage Operators. Comput. Methods Funct. Theory 19, 509–518 (2019). https://doi.org/10.1007/s40315-019-00277-w
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DOI: https://doi.org/10.1007/s40315-019-00277-w